Publications

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2007
King, AL, Barbeau K.  2007.  Evidence for phytoplankton iron limitation in the southern California Current System. Marine Ecology-Progress Series. 342:91-103.   10.3354/meps342091   AbstractWebsite

Observations of phytoplankton iron limitation in the world's oceans have primarily been confined to high-nutrient, low-chlorophyll (HNLC) regimes, found in the western equatorial and subarctic Pacific, Southern Ocean, and coastal upwelling zones off California and Peru. We investigated the potential for phytoplankton iron limitation in coastal transition zones (50 to 200 km offshore) of the southern California Current System, a weak upwelling regime that is relatively low in nutrients (< 4 mu mol nitrate 1(-1)) and low in chlorophyll (< 1 mu g chl a 1(-1)). In grow-out incubation experiments conducted during summer, July 2003 and 2004, phytoplankton responded to nanomolar iron additions, despite the non-HNLC initial conditions, Observed changes in phytoplankton and nutrient parameters upon iron addition were significant, although markedly lower in amplitude relative to typical grow-out experiments in HNLC regimes. While we cannot disprove alternate explanations for the observed limitation of phytoplankton growth, such as a proximate grazing control, our results indicate that phytoplankton growth in the southern California Current System is, at times, limited by the supply of iron. Based on our findings and the results of previous studies in this region, we suggest that phytoplankton biomass is generally limited by the supply of nitrate, while iron, directly or indirectly, influences macronutrient utilization, community species composition, and phytoplankton spatial and temporal distribution.

2008
Hopkinson, BM, Barbeau KA.  2008.  Interactive influences of iron and light limitation on phytoplankton at subsurface chlorophyll maxima in the eastern North Pacific. Limnology and Oceanography. 53:1303-1318.   10.4319/lo.2008.53.4.1303   AbstractWebsite

The roles of iron and light as limiting and colimiting factors for phytoplankton growth in subsurface chlorophyll maxima (SCMs) were investigated in mesotrophic to oligotrophic waters of the Southern California Bight and the eastern tropical North Pacific using microcosm manipulation experiments. Phytoplankton responses indicative of iron-light colimitation were found at several SCMs underlying macronutrient-limited surface waters in the eastern Pacific. Iron additions led to a shift in the size and taxonomic structure of the phytoplankton community, where large diatoms dominated what was formerly a diverse community of relatively small phytoplankton. The strongest and most ubiquitous responses of diatoms to iron addition were found under elevated light conditions, indicating that iron availability may have the greatest potential to affect SCM phytoplankton communities when light levels increase rapidly, such as during eddy events or with strong internal waves. The results show that iron influences phytoplankton community structure at SCMs, which would have consequences for nutrient cycling and carbon export within the lower euphotic zone.

2011
King, AL, Barbeau KA.  2011.  Dissolved iron and macronutrient distributions in the southern California Current System. Journal of Geophysical Research-Oceans. 116   10.1029/2010jc006324   AbstractWebsite

The distribution of dissolved iron in the southern California Current System (sCCS) is presented from seven research cruises between 2002 and 2006. Dissolved iron concentrations were generally low in most of the study area (<0.5 nM), although high mixed layer and water column dissolved iron concentrations (up to 8 nM) were found to be associated with coastal upwelling, both along the continental margin and some island platforms. A significant supply of iron was probably not from a deep remineralized source but rather from the continental shelf and bottom boundary layer as identified in previous studies along the central and northern California coast. With distance offshore, dissolved iron decreased more rapidly relative to nitrate in a transition zone 10-250 km offshore during spring and summer, resulting in relatively high ratios of nitrate: dissolved iron. Higher nitrate: dissolved iron ratios could be the result of utilization and scavenging in addition to an overall lower supply of iron relative to nitrate in the offshore transition zones. The low supply of iron leads to phytoplankton iron limitation and a depletion in silicic acid relative to nitrate in the coastal upwelling and transition zones of the sCCS.

2012
Roe, KL, Barbeau K, Mann EL, Haygood MG.  2012.  Acquisition of iron by Trichodesmium and associated bacteria in culture. Environmental Microbiology. 14:1681-1695.   10.1111/j.1462-2920.2011.02653.x   AbstractWebsite

Trichodesmium colonies contain an abundant microbial consortium that is likely to play a role in nutrient cycling within the colony. This study used laboratory cultures of Trichodesmium and two genome-sequenced strains of bacteria typical of Trichodesmium-associated microbes to develop an understanding of the cycling of iron, a potentially limiting micronutrient, within Trichodesmium colonies. We found that the ferric siderophores desferrioxamine B and aerobactin were not readily bioavailable to Trichodesmium, relative to ferric chloride or citrate-associated iron. In contrast, the representative bacterial strains we studied were able to acquire iron from all of the iron sources, implying that naturally occurring Trichodesmium-associated bacteria may be capable of utilizing a more diverse array of iron sources than Trichodesmium. From the organism-specific uptake data collected in this study, a theoretical Trichodesmium colony was designed to model whole colony iron uptake. The bacteria accounted for most (> 70%) of the iron acquired by the colony, highlighting the importance of determining organism-specific uptake in a complex environment. Our findings suggest that, although they may share the same micro-environment, Trichodesmium and its colony-associated microbial cohort may differ substantially in terms of iron acquisition strategy.

2013
Hopkinson, BM, Seegers B, Hatta M, Measures CI, Mitchell BG, Barbeau KA.  2013.  Planktonic C:Fe ratios and carrying capacity in the southern Drake Passage. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 90:102-111.   10.1016/j.dsr2.2012.09.001   AbstractWebsite

The carbon to iron (C:Fe) ratio of planktonic biomass constrains net production in iron-limited regions of the ocean and is an important parameter for predicting biomass production from iron inputs. On a cruise to the southern Drake Passage in July-August 2006, we used two approaches to determine the C:Fe ratio of planktonic material: dual-radiotracer labeling and net biomass production in iron-limited grow-out experiments. There was variability in C:Fe ratios among experiments, but values from the two methods overlapped with average values of 1.4 x 10(5) (mol:mol) for the radiotracer method and 1.7 x 10(5) for the net biomass production method. This is notable since the net biomass production method is a new approach to determine C:Fe ratios. Although it has potential issues related to bottle effects and sensitivity to trace contamination, the method avoids some of the questions associated with iron speciation and bioavailability since ambient iron supports production. Because light intensity is known to affect C:Fe ratios in phytoplankton through photosynthetic iron demands, we tested the effect of light level on C:Fe in Antarctic assemblages. In contrast to what is seen in many phytoplankton cultures, C:Fe ratios increased at low-light, but we suspect that this is due to initial photoinhibition of the low-light adapted winter assemblages at higher light levels. (c) 2012 Elsevier Ltd. All rights reserved.

2016
Semeniuk, DM, Taylor RL, Bundy RM, Johnson WK, Cullen JT, Robert M, Barbeau KA, Maldonado MT.  2016.  Iron-copper interactions in iron-limited phytoplankton in the northeast subarctic Pacific Ocean. Limnology and Oceanography. 61:279-297.   10.1002/lno.10210   AbstractWebsite

In August 2010, iron (Fe) and Fe and copper (Cu) addition incubation experiments were conducted at two low Fe stations (P20 and P26) along Line P, off the western coast of British Columbia, to investigate Cu physiology in Fe- and Fe-light co-limited phytoplankton. Chlorophyll a concentrations ([Chl a]), maximum variable fluorescence yield (F-v/F-m), and Fe uptake rates by the Cu-dependent high-affinity Fe transport system (HAFeTS) were measured. Additions of Fe resulted in an increase in [Chl a] and F-v/F-m at both stations compared with the controls, regardless of light availability, and confirmed that the phytoplankton communities were Fe-limited. Uptake of Fe by the HAFeTS in both incubations increased with the addition of Fe, and likely reflects luxury Fe uptake and storage. While the in situ inorganic Cu concentrations were similar to those that can induce Cu-limitation in laboratory cultures, increasing Cu availability had no effect on biomass accumulation during both incubations, regardless of Fe availability or light regime. At P26, additions of 1 nmol L-1 CuSO4 resulted in a short-term increase in F-v/F-m of the phytoplankton community, and an increase in Fe uptake rates by large phytoplankton (>5 mu m), but only when light was not limiting. These data confirm a complex interaction between light, Fe and Cu physiology in indigenous phytoplankton communities, and suggest that these interactions may be both spatially heterogeneous and different for different phytoplankton size classes.