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Stukel, MR, Aluwihare LI, Barbeau KA, Chekalyuk AM, Goericke R, Miller AJ, Ohman MD, Ruacho A, Song H, Stephens BM, Landry MR.  2017.  Mesoscale ocean fronts enhance carbon export due to gravitational sinking and subduction. Proceedings of the National Academy of Sciences of the United States of America. 114:1252-1257.   10.1073/pnas.1609435114   AbstractWebsite

Enhanced vertical carbon transport (gravitational sinking and subduction) at mesoscale ocean fronts may explain the demonstrated imbalance of new production and sinking particle export in coastal upwelling ecosystems. Based on flux assessments from U-238:Th-234 disequilibrium and sediment traps, we found 2 to 3 times higher rates of gravitational particle export near a deep-water front (305 mg C.m(-2).d(-1)) compared with adjacent water or to mean (nonfrontal) regional conditions. Elevated particle flux at the front wasmechanistically linked to Fe-stressed diatoms and high-mesozooplankton fecal pellet production. Using a data assimilative regional ocean model fit to measured conditions, we estimate that an additional similar to 225 mg C.m(-2).d(-1) was exported as subduction of particle-rich water at the front, highlighting a transport mechanism that is not captured by sediment traps and is poorly quantified by most models and in situ measurements. Mesoscale fronts may be responsible for over a quarter of total organic carbon sequestration in the California Current and other coastal upwelling ecosystems.

Hansman, RL, Thurber AR, Levin LA, Aluwihare LI.  2017.  Methane fates in the benthos and water column at cold seep sites along the continental margin of Central and North America. Deep-Sea Research Part I-Oceanographic Research Papers. 120:122-131.   10.1016/j.dsr.2016.12.016   AbstractWebsite

The potential influence of methane seeps on carbon cycling is a key question for global assessments, but the study of carbon cycling in surface sediments and the water column of cold seep environments is complicated by the high temporal and spatial variability of fluid and gas fluxes at these sites. In this study we directly examined carbon sources supporting benthic and planktonic food webs at venting methane seeps using isotopic and molecular approaches that integrate this variability. At four seep environments located along North and Central America, microorganisms from two size fractions were collected over several days from 2800 to 90501 of seawater to provide a time-integrated measure of key microbial groups and the carbon sources supporting the overall planktonic microbial community. In addition to water column measurements, the extent of seafloor methane release was estimated at two of the sites by examining the stable carbon isotopic signature (delta C-13) of benthic metazoan infauna. This signature reveals carbon sources fueling the base of the food chain and thus provides a metric that represents a time-integrated view of the dominant microbial processes within the sediment. The stable carbon isotopic composition of microbial DNA (delta C-13-DNA), which had values between -17.0 and -19.5%(0), indicated that bulk planktonic microbial production was not ultimately linked to methane or other C-13-depleted seep-derived carbon sources. Instead these data support the importance of organic carbon derived from either photo- or chemoautotrophic CO2 fixation to the planktonic food web. Results of qPCR of microbial DNA sequences coding for a subunit of the particulate methane monooxygenase gene (pmoA) showed that only a small percentage of the planktonic microbial community were potential methane oxidizers possessing pmoA (< 5% of 16S rRNA gene copies). There was an overall decrease of C-13-depleted carbon fueling the benthic metazoan community from 3 to 5 cm below the seafloor to the sediment surface, reflecting limited use of isotopically depleted carbon at the sediment surface. Rare methane emission as indicated by limited aerobic methane oxidation acts to corroborate our findings for the planktonic microbial community.

Kumar, A, Borgen M, Aluwihare LI, Fenical W.  2017.  Ozone-activated halogenation of mono- and dimethylbipyrrole in seawater. Environmental Science & Technology. 51:589-595.   10.1021/acs.est.6b03601   AbstractWebsite

Polyhalogenated N-methylbipyrroles of two different structure classes have been detected worldwide in over 100 environmental samples including seawater, bird eggs, fish, dolphin blubber, and in the breast milk of humans that consume seafood. These molecules are concentrated in the fatty tissues in comparable abundance to some of the most important anthropogenic contaminants, such as the halogenated flame-retardants and pesticides. Although the origin of these compounds is still unknown, we present evidence that the production of these materials can involve the direct ozone activated seawater halogenation of N-methylbipyrrole precursors. This observation shows that environmental polyhalogenated bipyrroles can be produced via an abiotic process, and implies that the ozone activated halogenation of a variety of natural and anthropogenic seawater organics may be a significant process occurring in surface ocean waters.

Tolar, BB, Ross MJ, Wallsgrove NJ, Liu Q, Aluwihare LI, Popp BN, Hollibaugh JT.  2016.  Contribution of ammonia oxidation to chemoautotrophy in Antarctic coastal waters. Isme Journal. 10:2605-2619.   10.1038/ismej.2016.61   AbstractWebsite

There are few measurements of nitrification in polar regions, yet geochemical evidence suggests that it is significant, and chemoautotrophy supported by nitrification has been suggested as an important contribution to prokaryotic production during the polar winter. This study reports seasonal ammonia oxidation (AO) rates, gene and transcript abundance in continental shelf waters west of the Antarctic Peninsula, where Thaumarchaeota strongly dominate populations of ammonia-oxidizing organisms. Higher AO rates were observed in the late winter surface mixed layer compared with the same water mass sampled during summer (mwean +/- s.e.:62 +/- 16 versus 13 +/- 2.8 nM per day, t-test P<0.0005). AO rates in the circumpolar deep water did not differ between seasons (21 +/- 5.7 versus 24 +/- 6.6 nM per day; P=0.83), despite 5- to 20-fold greater Thaumarchaeota abundance during summer. AO rates correlated with concentrations of Archaea ammonia monooxygenase (amoA) genes during summer, but not with concentrations of Archaea amoA transcripts, or with ratios of Archaea amoA transcripts per gene, or with concentrations of Betaproteobacterial amoA genes or transcripts. The AO rates we report (<0.1-220 nM per day) are similar to 10-fold greater than reported previously for Antarctic waters and suggest that inclusion of Antarctic coastal waters in global estimates of oceanic nitrification could increase global rate estimates by similar to 9%. Chemoautotrophic carbon fixation supported by AO was 3-6% of annualized phytoplankton primary production and production of Thaumarchaeota biomass supported by AO could account for similar to 9% of the bacterioplankton production measured in winter. Growth rates of thaumarchaeote populations inferred from AO rates averaged 0.3 per day and ranged from 0.01 to 2.1 per day.

Mackintosh, SA, Dodder NG, Shaul NJ, Aluwihare LI, Maruya KA, Chivers SJ, Danil K, Weller DW, Hoh E.  2016.  Newly identified DDT-related compounds accumulating in Southern California bottlenose dolphins. Environmental Science & Technology. 50:12129-12137.   10.1021/acs.est.6b03150   AbstractWebsite

Nontargeted GCxGC-TOF/MS analysis of blubber from 8 common bottlenose dolphins (Tursiops truncatus) inhabiting the Southern California Bight was performed to identify novel, bioaccumulative DDT-related compounds and to determine their abundance relative to the commonly studied DDT-related compounds. We identified 45 bioaccumulative DDT-related compounds of which the majority (80%) is not typically monitored in environmental media. Identified compounds include transformation products, technical mixture impurities such as tris(chlorophenyl)methane (TCPM), the presumed TCPM metabolite tris(chlorophenyl)methanol (TCPMOH), and structurally related compounds with unknown sources, such as hexa- to octachlorinated diphenylethene. To investigate impurities in pesticide mixtures as possible sources of these compounds, we analyzed technical DDT, the primary source of historical contamination in the region, and technical Dicofol, a current use pesticide that contains DDT-related compounds. The technical mixtures contained only 33% of the compounds identified in the blubber, suggesting that transformation products contribute to the majority of the load of DDT-related contaminants in these sentinels of ocean health. Quantitative analysis revealed that TCPM was the second most abundant compound class detected in the blubber, following DDE, and TCPMOH loads were greater than DDT. QSPR estimates verified 4,4',4"-TCPM and 4,4'4,"-TCPMOH are persistent and bioaccumulative.

Kharbush, JJ, Allen AE, Moustafa A, Dorrestein PC, Aluwihare LI.  2016.  Intact polar diacylglycerol biomarker lipids isolated from suspended particulate organic matter accumulating in an ultraoligotrophic water column. Organic Geochemistry. 100:29-41.   10.1016/j.orggeochem.2016.07.008   Abstract

Intact polar diacylglycerols (IP-DAGs) are essential components of cell membranes. Because they are structurally diverse and hypothesized to represent primarily living cells, they are potential molecular markers for a recent contribution by microbial communities to various carbon reservoirs. This study employed a novel molecular networking approach to investigate the evolution of IP-DAG structural diversity with depth in an ultraoligotrophic environment of the western South Pacific Ocean to test the hypothesis that particle transport to depth is rapid enough to preserve the IP-DAG biomarker signature of the photosynthetic community. IP-DAG profiles of several cultured cyanobacteria and photosynthetic picoeukaryotes were used as templates for constructing molecular networks to compare and interpret IP-DAG signatures of suspended particles isolated from a water column depth profile. Analysis of corresponding genetic community composition data for the same field samples was used to connect IP-DAG structures with their likely biological sources. Our data show that, although most IP-DAG classes associated with photosynthetic organisms were not observed below the euphotic zone, several other IP-DAG classes in deep samples might provide interesting targets for future studies seeking to examine the in situ contribution of deep sea microbes to suspended particulate organic matter (POM). Overall, the results represent the deepest water column IP-DAG dataset to date and demonstrate the utility of molecular networking for analyzing and visualizing complex environmental datasets.

Haskell, WZ, Prokopenko MG, Hammond DE, Stanley RHR, Berelson WM, Baronas JJ, Fleming JC, Aluwihare L.  2016.  An organic carbon budget for coastal Southern California determined by estimates of vertical nutrient flux, net community production and export. Deep-Sea Research Part I-Oceanographic Research Papers. 116:49-76.   10.1016/j.dsr.2016.07.003   AbstractWebsite

Organic carbon export and burial in coastal upwelling regions is an important mechanism for oceanic uptake of atmospheric CO2. In order to understand how these complex systems will respond to future climate forcing, further studies of nutrient input, biological production and export are needed. Using a Be-7-based approach, we produced an 18-month record of upwelling velocity estimates at the San Pedro Ocean Time-series (SPOT), Southern California Bight. These upwelling rates and vertical nutrient distributions have been combined to make estimates of potential new production (PNP), which are compared to estimates of net community oxygen production (NOP) made using a one-dimensional, two-box non-steady state model of euphotic zone biological oxygen supersaturation. NOP agrees within uncertainty with PNP, suggesting that upwelling is the dominant mechanism for supplying the ecosystem with new nutrients in the spring season, but negligible in the fall and winter. Combining this data set with estimates of sinking particulate organic carbon (POC) flux from water column Th-234:U-238 disequilibrium and sediment trap deployments, and an estimate of the ratio of dissolved organic carbon (DOC):POC consumption rates, we construct a simple box model of organic carbon in the upper 200 m of our study site. This box model (with uncertainties of +/- 50%) suggests that in spring, 28% of net production leaves the euphotic zone as DOC, of this, similar to 12% as horizontal export and 16% via downward mixing. The remaining similar to 72% of net organic carbon export exits as sinking POC, with only 10% of euphotic zone export reaching 200 m. We find the metabolic requirement for the local heterotrophic community below the euphotic zone, but above 200 m, is similar to 105 +/- 50 mmol C m(-2) d(-1), or similar to 80% of net euphotic zone production in spring. (C) 2016 Elsevier Ltd. All rights reserved.

Moran, MA, Kujawinski EB, Stubbins A, Fatland R, Aluwihare LI, Buchan A, Crump BC, Dorrestein PC, Dyhrman ST, Hess NJ, Howe B, Longnecker K, Medeiros PM, Niggemann J, Obernosterer I, Repeta DJ, Waldbauer JR.  2016.  Deciphering ocean carbon in a changing world. Proceedings of the National Academy of Sciences of the United States of America. 113:3143-3151.   10.1073/pnas.1514645113   AbstractWebsite

Dissolved organic matter (DOM) in the oceans is one of the largest pools of reduced carbon on Earth, comparable in size to the atmospheric CO2 reservoir. A vast number of compounds are present in DOM, and they play important roles in all major element cycles, contribute to the storage of atmospheric CO2 in the ocean, support marine ecosystems, and facilitate interactions between organisms. At the heart of the DOM cycle lie molecular-level relationships between the individual compounds in DOM and the members of the ocean microbiome that produce and consume them. In the past, these connections have eluded clear definition because of the sheer numerical complexity of both DOM molecules and microorganisms. Emerging tools in analytical chemistry, microbiology, and informatics are breaking down the barriers to a fuller appreciation of these connections. Here we highlight questions being addressed using recent methodological and technological developments in those fields and consider how these advances are transforming our understanding of some of the most important reactions of the marine carbon cycle.

Kharbush, JJ, Kejriwal K, Aluwihare LI.  2016.  Distribution and abundance of hopanoid producers in low-oxygen environments of the Eastern Pacific Ocean. Microbial Ecology. 71:401-408.   10.1007/s00248-015-0671-y   AbstractWebsite

Hopanoids are bacterial membrane lipid biomarker molecules that feature prominently in the molecular fossil record. In the modern marine water column, recent reports implicate bacteria inhabiting low-oxygen environments as important sources of hopanoids to marine sediments. However, the preliminary biogeography reported by recent studies and the environmental conditions governing such distributions can only be confirmed when the numerical abundance of these organisms is known with more certainty. In this study, we employ two different approaches to examine the quantitative significance of phylogenetically distinct hopanoid producers in low-oxygen environments. First, we develop a novel quantitative PCR (qPCR) assay for the squalene hopene cyclase (sqhC) gene, targeting a subset of hopanoid producers previously identified to be important in the eastern North Pacific Ocean. The results represent the first quantitative gene abundance data of any kind for hopanoid producers in the marine water column and show that these putative alphaproteobacterial hopanoid producers are rare, comprising at most 0.2 % of the total bacterial community in our samples. Second, a complementary analysis of existing low-oxygen metagenomic datasets further examined the generality of the qPCR observation. We find that the dominant sqhC sequences in these metagenomic datasets are associated with phyla such as Nitrospinae rather than Proteobacteria, consistent with the qPCR finding that alphaproteobacterial hopanoid producers are not very abundant in low-oxygen environments. In fact, positive correlations between sqhC gene abundance and environmental parameters in these samples identify nitrite availability as a potentially important factor in the ecology of hopanoid producers that dominate low-oxygen environments.

Sherwood, BP, Shaffer EA, Reyes K, Longnecker K, Aluwihare LI, Azam F.  2015.  Metabolic characterization of a model heterotrophic bacterium capable of significant chemical alteration of marine dissolved organic matter. Marine Chemistry. 177:357-365.   10.1016/j.marchem.2015.06.027   AbstractWebsite

The marine bacterium Alteromonas sp. AltSIO was previously found to consume an equivalent magnitude of surface coastal marine dissolved organic carbon (DOC) as diverse bacterial assemblages (Pedler et al., 2014). In this study, we sought to investigate the potential of AltSIO to alter the chemical composition of marine DOC by characterizing its capacity to metabolize a broad suite of environmentally relevant model substrates. Results showed that AltSIO had a particularly broad capacity to degrade carbohydrates relative to other marine bacteria characterized as generalist heterotrophs. Growth in seawater incubations amended with model neutral sugars and radiolabeled substrates showed that AltSIO preferentially utilized la-galactose and disaccharides, but showed little to no biomass incorporation or respiration of D-glucose. Lastly, analysis of ambient dissolved organic matter (DOM) from time-course mesocosms by ultrahigh resolution mass spectrometry showed that both AltSIO grown in pure culture and a mixed bacterial community significantly altered ambient DOM, yet the alteration appeared uniform across chemical classes. for both treatments. This study provides insight into the physiological mechanisms of a globally distributed generalist bacterial taxon that has the capacity to significantly alter the geochemistry of marine DOM. (C) 2015 Elsevier B.V. All rights reserved.

Arakawa, N, Aluwihare L.  2015.  Direct identification of diverse alicyclic terpenoids in Suwannee River fulvic acid. Environmental Science & Technology. 49:4097-4105.   10.1021/es5055176   AbstractWebsite

The chemical complexity of dissolved organic matter (DOM) obstructs our ability to definitively recover source compounds from within DOM, an objective which has the capacity to alter our understanding, of carbon sequestration on a. global scale. To advance compositional studies of DOM we have applied a previously published reduction method to an environmental standard, Suwannee River Fulvic Acid (SRFA). The reduction products, comprising 12% of the prereduced carbon, were then separated by comprehensive two-dimensional gas chromatography time-of-flight mass spectrometry (GCXGC-TOF-MS). estilts indicate that the majority of obsetved reduced compounds corresponded to alicyclic hydrocarbons in the size range C-10 to C-17. Cyclic terpenoids are the only biomolecule class with contiguous, alicyclic carbon backbones of this size. These terpenoid reduction products contain series offset by CH2 and exhibit great isomeric diversity, features previously inferred from ultrahigh resolution mass spectrometry and NMR studies of unreduced SRFA. Reduction of Taxodium leaf litter as a Source material to SRFA confirmed the prevalence of tetpenoids in SRFA and provided insight into the parent compounds that must be diagenetically modified on relatively short time scales. These data corroborate several recent studies that suggest alicydic hydrocarbons to be important components of longer-lived DOM.

Agarwal, V, Li J, Rahman I, Borgen M, Aluwihare LI, Biggs JS, Paul VJ, Moore BS.  2015.  Complexity of naturally produced polybrominated diphenyl ethers revealed via mass spectrometry. Environmental Science & Technology. 49:1339-1346.   10.1021/es505440j   AbstractWebsite

Polybrominated diphenyl ethers (PBDEs) are persistent and bioaccumulative anthropogenic and natural chemicals that are broadly distributed in the marine environment. PBDEs are potentially toxic due to inhibition of various mammalian signaling pathways and enzymatic reactions. PBDE isoforms vary in toxicity in accordance with structural differences, primarily in the number and pattern of hydroxyl moieties afforded upon a conserved core structure. Over four decades of isolation and discovery-based efforts have established an impressive repertoire of natural PBDEs. Based on our recent reports describing the bacterial biosyntheses of PBDEs, we predicted the presence of additional classes of PBDEs to those previously identified from marine sources. Using mass spectrometry and NMR spectroscopy, we now establish the existence of new structural classes of PBDEs in marine sponges. Our findings expand the chemical space explored by naturally produced PBDEs, which may inform future environmental toxicology studies. Furthermore, we provide evidence for iodinated PBDEs and direct attention toward the contribution of promiscuous halogenating enzymes in further expanding the diversity of these polyhalogenated marine natural products.

Goldberg, SJ, Ball GI, Allen BC, Schladow SG, Simpson AJ, Masoom H, Soong R, Graven HD, Aluwihare LI.  2015.  Refractory dissolved organic nitrogen accumulation in high-elevation lakes. Nature Communications. 6   10.1038/ncomms7347   AbstractWebsite

The role of dissolved organic matter (DOM) as either a sink for inorganic nutrients or an additional nutrient source is an often-neglected component of nutrient budgets in aquatic environments. Here, we examined the role of DOM in reactive nitrogen (N) storage in Sierra Nevada (California, USA) lakes where atmospheric deposition of N has shifted the lakes toward seasonal phosphorus (P)-limitation. Nuclear magnetic resonance (NMR) spectroscopy and isotope analyses performed on DOM isolated from Lake Tahoe reveal the accumulation of refractory proteinaceous material with a 100-200-year residence time. In contrast, smaller lakes in the same watershed contain DOM with typical terrestrial characteristics, indicating that proteins in Lake Tahoe are autochthonously produced. These data support the role of DOM as a possible sink for reactive N in these lake ecosystems and identify a potential role for DOM in affecting the inorganic nutrient stoichiometry of these environments.

Meador, TB, Aluwihare LI.  2014.  Production of dissolved organic carbon enriched in deoxy sugars representing an additional sink for biological C drawdown in the Amazon River plume. Global Biogeochemical Cycles. 28:1149-1161.   10.1002/2013gb004778   Abstract

In North Atlantic waters impacted by discharges from the Amazon and Orinoco Rivers, where planktonic diatom-diazotroph associations (DDA) were active, we observed that an average ( standard deviation) of 61 +- 12% of the biological drawdown of dissolved inorganic carbon (DIC) was partitioned into the accumulating total organic carbon pool, representing a flux of up to 94 Tg C yr(-1). This drawdown corresponded with chemical alteration of ultrafiltered dissolved organic matter (UDOM), including increases in stable C isotopic composition (C-13) and C:N. The dissolved carbohydrate component of UDOM also increased with biological DIC drawdown and diatom-associated diazotroph (i.e., Richelia) abundance. New carbohydrates could be distinguished by distinctively high relative abundances of deoxy sugars (up to 55% of monosaccharides), which may promote aggregate formation and enhance vertical carbon export. The identified production of non-Redfieldian, C-enriched UDOM thus suggests a mechanism to explain enhanced C sequestration associated with DDA N-2 fixation, which may be widespread in mesohaline environments.

Pedler, BE, Aluwihare LI, Azam F.  2014.  Single bacterial strain capable of significant contribution to carbon cycling in the surface ocean. Proceedings of the National Academy of Sciences of the United States of America. 111:7202-7207.   10.1073/pnas.1401887111   AbstractWebsite

Marine dissolved organic carbon (DOC) encompasses one of the largest reservoirs of carbon on Earth. Heterotrophic bacteria are the primary biotic force regulating the fate of this material, yet the capacity of individual strains to significantly contribute to carbon cycling is unknown. Here we quantified the ability of a single Alteromonas strain [Alteromonas sp. strain Scripps Institution of Oceanography (AltSIO)] to drawdown ambient DOC in a coastal ecosystem. In three experiments, AltSIO alone consumed the entire pool of labile DOC, defined here as the quantity consumed by the submicron size fraction of ambient microbial assemblages within 5 d. These findings demonstrate that complete removal of the labile DOC pool in coastal surface seawater can be achieved by a single taxon. During long-term incubations (>1 y) testing semilabile DOC consumption, AltSIO entered dormancy but remained viable, while the diverse assemblages continued to consume carbon. Given that AltSIO is a large bacterium and thus subject to increased grazing pressure, we sought to determine the ecological relevance of this phenotype. Growth dynamics in natural seawater revealed that AltSIO rapidly outgrew the native bacteria, and despite intense grazing pressure, was never eliminated from the population. A survey in the California Current Ecosystem revealed that large bacteria (>= 40 fg C.cell(-1)) were persistent, accounting for up to 12% of total bacterial abundance and 24% of total bacterial biomass. We conclude that large, rapidly growing bacteria have the potential to disproportionately alter the fate of carbon in the mesotrophic ocean and play an important role in ecosystem function.

Collins, DB, Ault AP, Moffet RC, Ruppel MJ, Cuadra-Rodriguez LA, Guasco TL, Corrigan CE, Pedler BE, Azam F, Aluwihare LI, Bertram TH, Roberts GC, Grassian VH, Prather KA.  2013.  Impact of marine biogeochemistry on the chemical mixing state and cloud forming ability of nascent sea spray aerosol. Journal of Geophysical Research-Atmospheres. 118:8553-8565.   10.1002/jgrd.50598   AbstractWebsite

The composition and properties of sea spray aerosol, a major component of the atmosphere, are often controlled by marine biological activity; however, the scope of impacts that ocean chemistry has on the ability for sea spray aerosol to act as cloud condensation nuclei (CCN) is not well understood. In this study, we utilize a mesocosm experiment to investigate the impact of marine biogeochemical processes on the composition and mixing state of sea spray aerosol particles with diameters<0.2 mu m produced by controlled breaking waves in a unique ocean-atmosphere facility. An increase in relative abundance of a distinct, insoluble organic particle type was observed after concentrations of heterotrophic bacteria increased in the seawater, leading to an 86 +/- 5% reduction in the hygroscopicity parameter () at 0.2% supersaturation. Aerosol size distributions showed very little change and the submicron organic mass fraction increased by less than 15% throughout the experiment; as such, neither of these typical metrics can explain the observed reduction in hygroscopicity. Predictions of the hygroscopicity parameter that make the common assumption that all particles have the same bulk organic volume fractions lead to overpredictions of CCN concentrations by 25% in these experiments. Importantly, key changes in sea spray aerosol mixing state that ultimately influenced CCN activity were driven by bacteria-mediated alterations to the organic composition of seawater.

Kharbush, JJ, Ugalde JA, Hogle SL, Allen EE, Aluwihare LI.  2013.  Composite bacterial hopanoids and their microbial producers across oxygen gradients in the water column of the California Current. Applied and Environmental Microbiology.   10.1128/aem.02367-13   AbstractWebsite

Hopanoids are pentacyclic triterpenoid lipids produced by many prokaryotes as cell membrane components. The structural variations of composite hopanoids, or bacteriohopanepolyols (BHPs), produced by various bacterial genera makes them potentially useful molecular biomarkers of bacterial communities and metabolic processes, in both modern and ancient environments. Building on previous work suggesting that organisms in low-oxygen environments are important contributors to BHP production in the marine water column and that there may be physiological roles for BHPs specific to these environments, this study investigated the relationship between trends in BHP structural diversity and abundance and the genetic diversity of BHP producers for the first time in a low-oxygen environment of the Eastern Tropical North Pacific. Amplification of the hopanoid biosynthesis gene, squalene hopene cyclase (sqhC), indicated far greater genetic diversity than would be predicted by examining BHP structural diversity alone, and that greater sqhC genetic diversity exists in the marine environment than is represented by cultured representatives and most marine metagenomes. In addition, the genetic relationships in this dataset suggest microaerophilic environments as potential “hot spots” of BHP production. Finally, structural analysis of BHPs showed that an isomer of the commonly-observed BHP bacteriohopanetetrol (BHT) may be linked to a producer that is more abundant in low-oxygen environments. Results of this study increase the known diversity of BHP producers and provide a detailed phylogeny with implications for the role of hopanoids in modern bacteria as well as the evolutionary history of hopanoid biosynthesis, both of which are important considerations for future interpretations of the marine sedimentary record.

Close, HG, Shah SR, Ingalls AE, Diefendorf AF, Brodie EL, Hansman RL, Freeman KH, Aluwihare LI, Pearson A.  2013.  Export of submicron particulate organic matter to mesopelagic depth in an oligotrophic gyre. Proceedings of the National Academy of Sciences of the United States of America. 110:12565-12570.   10.1073/pnas.1217514110   AbstractWebsite

Sixty percent of the world ocean by area is contained in oligotrophic gyres [Longhurst A (1995) Prog Oceanog 36:77-16], the biomass of which is dominated by picophytoplankton, including cyanobacteria and picoeukaryotic algae, as well as picoheterotrophs. Despite their recognized importance in carbon cycling in the surface ocean, the role of small cells and their detrital remains in the transfer of particulate organic matter (POM) to the deep ocean remains disputed. Because oligotrophic marine conditions are projected to expand under current climate trends, a better understanding of the role of small particles in the global carbon cycle is a timely goal. Here we use the lipid profiles, radiocarbon, and stable carbon isotopic signatures of lipids from the North Pacific Subtropical Gyre to show that in the surface ocean, lipids from submicron POM (here called extra-small POM) are distinct from larger classes of suspended POM. Remarkably, this distinct extra-small POM signature dominates the total lipids collected at mesopelagic depth, suggesting that the lipid component of mesopelagic POM primarily contains the exported remains of small particles. Transfer of submicron material to mesopelagic depths in this location is consistent with model results that claim the biological origin of exported carbon should be proportional to the distribution of cell types in the surface community, irrespective of cell size [Richardson TL, Jackson GA (2007) Science 315:838-840]. Our data suggest that the submicron component of exported POM is an important contributor to the global biological pump, especially in oligotrophic waters.

Prather, KA, Bertram TH, Grassian VH, Deane GB, Stokes MD, DeMott PJ, Aluwihare LI, Palenik BP, Azam F, Seinfeld JH, Moffet RC, Molina MJ, Cappa CD, Geiger FM, Roberts GC, Russell LM, Ault AP, Baltrusaitis J, Collins DB, Corrigan CE, Cuadra-Rodriguez LA, Ebben CJ, Forestieri SD, Guasco TL, Hersey SP, Kim MJ, Lambert WF, Modini RL, Mui W, Pedler BE, Ruppel MJ, Ryder OS, Schoepp NG, Sullivan RC, Zhao DF.  2013.  Bringing the ocean into the laboratory to probe the chemical complexity of sea spray aerosol. Proceedings of the National Academy of Sciences of the United States of America. 110:7550-7555.   10.1073/pnas.1300262110   AbstractWebsite

The production, size, and chemical composition of sea spray aerosol (SSA) particles strongly depend on seawater chemistry, which is controlled by physical, chemical, and biological processes. Despite decades of studies in marine environments, a direct relationship has yet to be established between ocean biology and the physicochemical properties of SSA. The ability to establish such relationships is hindered by the fact that SSA measurements are typically dominated by overwhelming background aerosol concentrations even in remote marine environments. Herein, we describe a newly developed approach for reproducing the chemical complexity of SSA in a laboratory setting, comprising a unique ocean-atmosphere facility equipped with actual breaking waves. A mesocosm experiment was performed in natural seawater, using controlled phytoplankton and heterotrophic bacteria concentrations, which showed SSA size and chemical mixing state are acutely sensitive to the aerosol production mechanism, as well as to the type of biological species present. The largest reduction in the hygroscopicity of SSA occurred as heterotrophic bacteria concentrations increased, whereas phytoplankton and chlorophyll-a concentrations decreased, directly corresponding to a change in mixing state in the smallest (60-180 nm) size range. Using this newly developed approach to generate realistic SSA, systematic studies can now be performed to advance our fundamental understanding of the impact of ocean biology on SSA chemical mixing state, heterogeneous reactivity, and the resulting climate-relevant properties.

Samo, TJ, Pedler BE, Ball GI, Pasulka AL, Taylor AG, Aluwihare LI, Azam F, Goericke R, Landry MR.  2012.  Microbial distribution and activity across a water mass frontal zone in the California Current Ecosystem. Journal of Plankton Research. 34:802-814.   10.1093/plankt/fbs048   AbstractWebsite

Ocean fronts with accumulated biomass and organic matter may be significant sites of enhanced microbial activity. We sampled a frontal region (the A-Front) separating oligotrophic and mesotrophic water masses within the California Current Ecosystem (CCE) to assess the influence of frontal hydrography on several microbial parameters. Samples for heterotrophic bacterial, viral and flagellate abundance, dissolved and particulate carbon and nitrogen, transparent particles and bacterial carbon production were collected at 6 depths from the surface to 100 m with 59 conductivity/temperature/depth casts along a 26-km northerly transect across the front. Relative to adjacent oligotrophic and mesotrophic waters, the frontal transition displayed peaks in the mean estimates of cell-specific bacterial carbon and bulk bacterial production, particulate organic carbon and particulate organic nitrogen concentrations, and the abundance and size of transparent particles. Bacterial carbon production increased approximate to 5-fold northward from oligotrophic waters to the frontal zone, in agreement with an increase in the frequency of dividing cells, but bacterial abundance was lower than at adjacent stations. This may be partially explained by high chlorophyll, elevated virus:bacteria ratios and low nanoflagellate grazer abundance at the front. Our data suggest that CCE fronts can facilitate intense biological transformation and physical transport of organic matter, in sharp contrast to adjacent low productivity waters, and harbor dynamic microbial populations that influence nutrient cycling.

Ball, GI, Xu L, McNichol AP, Aluwihare LI.  2012.  A two-dimensional, heart-cutting preparative gas chromatograph facilitates highly resolved single-compound isolations with utility towards compound-specific natural abundance radiocarbon (C-14) analyses. Journal of Chromatography A. 1220:122-131.   10.1016/j.chroma.2011.11.021   AbstractWebsite

Motivated by the need to develop clean, high purity preparative enrichments of individual compounds for micro-scale compound-specific natural abundance isotope and radiocarbon (C-14) analyses, we describe a new, two-dimensional, heart-cutting, low-bleed, three-oven, single GC preparative system, demonstrate its resolving capabilities as applied to a typically complex environmental sample matrix, and investigate the robustness with which it preserves the authigenic C-13/C-12 and C-14/C-12 ratios of individual compounds it targets for preparative enrichment. The system is comprised of a programmable temperature vaporizing (PTV) inlet, a single GC oven, two modular, door-mounted, resistively heated low thermal mass (LTM) columns, a preparative fraction collector (PFC), and a Deans pneumatic switching device which facilitates heart-cutting between the system's 1 degrees and 2 degrees chromatographic dimensions. Further, the system's inlet and trapping parameters are optimized for the efficient preparative enrichment of the methyl ether and ester derivatives of the lignin phenol compound class. The lignin phenols include such compounds as the vanillyl and syringyl aldehydes, ethanones, and acids and are unrivaled biomarkers of terrestrial organic matter, some of which are also important components of fragrances and flavors. Using this suite of compounds, the suitability of this augmented preparative capillary GC (PCGC) system was investigated for micro-scale compound-specific (CS) stable isotope and natural abundance radiocarbon analyses (RA). Analysis of a >300 injection enrichment scheme reveals the instrument to fractionate C-13 in predictable ways and to preserve the authigenic Delta C-14 of compounds it targets for preparative enrichment to within 6.7 +/- 5.0 parts per thousand, demonstrating the promising new utility of such systems towards micro-scale CSRA investigations for which clean and high resolution separation techniques are prerequisite. (C) 2011 Elsevier B.V. All rights reserved.

Gaston, CJ, Furutani H, Guazzotti SA, Coffee KR, Bates TS, Quinn PK, Aluwihare LI, Mitchell BG, Prather KA.  2011.  Unique ocean-derived particles serve as a proxy for changes in ocean chemistry. Journal of Geophysical Research-Atmospheres. 116   10.1029/2010jd015289   AbstractWebsite

Oceans represent a significant natural source of gases and particles to the atmosphere. Relative to gas phase compounds, less is known regarding the influence of changes in biological activity in the ocean on the chemistry of sea spray aerosols produced in marine environments. To gain insight into the influence of ocean biology and chemistry on atmospheric aerosol chemistry, simultaneous real-time measurements were made of atmospheric aerosol size and chemical mixing-state, gas phase dimethyl sulfide (DMS), as well as seawater DMS and chlorophyll a. In three different marine environments with elevated chlorophyll a and DMS, unique Mg particles were detected containing Mg(2+), Ca(2+), K(+), and organic carbon. These particles were segregated from sea salt particles highlighting that two subpopulations within the sea spray were being ejected from the ocean. Strong temporal correlations were observed between these unique ocean-derived particles and freshly emitted sea salt particles (R(2) = 0.86), particularly as wind speed increased to at least 10 m/s, and atmospheric DMS (R(2) = 0.76). Time series correlations between ocean measurements and atmospheric aerosol chemistry suggest that chlorophyll a and DMS serve as indicators of changes in the chemistry of the ocean, most likely an increase in organic material, which is directly reflected in the single particle mixing-state. This is the first time such real-time correlations are shown between ocean chemistry and atmospheric aerosol mixing-state. The reasons behind these observed changes in aerosol chemistry are critical for understanding the heterogeneous reactivity, water uptake, and cloud forming potential of sea spray aerosols.

Beaupre, SR, Aluwihare L.  2010.  Constraining the 2-component model of marine dissolved organic radiocarbon. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 57:1494-1503.   10.1016/j.dsr2.2010.02.017   AbstractWebsite

Keeling plots of dissolved organic carbon (DOC) concentration and Δ14C depth profiles imply rapid, diapycnal transport of DOC to the meso- and bathypelagic zones, but do not constrain the mechanism of redistribution. We review the 2-component Keeling plot model, and present an alternative formulation explicitly based on homogenization of water parcels. Applying this new model to DOC and dissolved inorganic carbon (DIC) depth profiles suggests that the dominant controls on DOC redistribution differ throughout the water column. DOC concentration and Δ14C gradients were consistent with biogeochemical processing in the epipelagic and advection in the mesopelagic. Vertical gradients in DOC concentration and Δ14C were insufficient for further interpretation in the bathypelagic. Ultimately, additional concurrent measurements of DOC and DIC concentrations and Δ14C values throughout the water column at more locations are needed to constrain the applicability of two-component mixing models to marine DOC.

Hansman, RL, Griffin S, Watson JT, Druffel ERM, Ingalls AE, Pearson A, Aluwihare LI.  2009.  The radiocarbon signature of microorganisms in the mesopelagic ocean. Proceedings of the National Academy of Sciences of the United States of America. 106:6513-6518.   10.1073/pnas.0810871106   AbstractWebsite

Several lines of evidence indicate that microorganisms in the meso-and bathypelagic ocean are metabolically active and respiring carbon. In addition, growing evidence suggests that archaea are fixing inorganic carbon in this environment. However, direct quantification of the contribution from deep ocean carbon sources to community production in the dark ocean remains a challenge. In this study, carbon flow through the microbial community at 2 depths in the mesopelagic zone of the North Pacific Subtropical Gyre was examined by exploiting the unique radiocarbon signatures (Delta(14)C) of the 3 major carbon sources in this environment. The radiocarbon content of nucleic acids, a biomarker for viable cells, isolated from size-fractionated particles (0.2-0.5 mu m and >0.5 mu m) showed the direct incorporation of carbon delivered by rapidly sinking particles. Most significantly, at the 2 mesopelagic depths examined (670 m and 915 m), carbon derived from in situ autotrophic fixation supported a significant fraction of the free-living microbial community (0.2-0.5 mu m size fraction), but the contribution of chemoautotrophy varied markedly between the 2 depths. Results further showed that utilization of the ocean's largest reduced carbon reservoir, (14)C-depleted, dissolved organic carbon, was negligible in this environment. This isotopic portrait of carbon assimilation by the in situ, free-living microbial community, integrated over > 50,000 L of seawater, implies that recent, photosynthetic carbon is not always the major carbon source supporting microbial community production in the mesopelagic realm.