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  1. Abstract

    Throughout the open ocean, a minimum in dissolved iron concentration (dFe) overlaps with the deep chlorophyll maximum (DCM), which marks the lower limit of the euphotic zone. Maximizing light capture in these dim waters is expected to require upregulation of Fe-bearing photosystems, further depleting dFe and possibly leading to co-limitation by both iron and light. However, this effect has not been quantified for important phytoplankton groups likeProchlorococcus, which contributes most of the productivity in the oligotrophic DCM. Here, we present culture experiments withProchlorococcusstrain MIT1214, a member of the Low Light 1 ecotype isolated from the DCM in the North Pacific subtropical gyre. Under a matrix of iron and irradiance matching those found at the DCM, the ratio of Fe to carbon inProchlorococcusMIT1214 cells ranged from 10–40 × 10−6 mol Fe:mol C and increased with light intensity and growth rate. These results challenge theoretical models predicting highest Fe:C at lowest light intensity, and are best explained by a large photosynthetic Fe demand that is not downregulated at higher light. To sustain primary production in the DCM with the rigid Fe requirements of low-light-adaptedProchlorococcus, dFe must be recycled rapidly and at high efficiency.

  2. Free, publicly-accessible full text available October 1, 2023
  3. Free, publicly-accessible full text available May 26, 2023
  4. Free, publicly-accessible full text available April 1, 2023
  5. Stewart, Frank J. (Ed.)
    ABSTRACT Cluster 5 Synechococcus species are widely acknowledged for their broad distribution and biogeochemical importance. In particular, subcluster 5.2 strains inhabit freshwater, estuarine, and marine environments but are understudied, compared to other subclusters. Here, we present the genome for Synechococcus sp. strain LA31, a strain that was recently isolated from Narragansett Bay, Rhode Island, USA.
    Free, publicly-accessible full text available February 23, 2023
  6. York, A (Ed.)
  7. A small subset of marine microbial enzymes and surface transporters have a disproportionately important influence on the cycling of carbon and nutrients in the global ocean. As a result, they largely determine marine biological productivity and have been the focus of considerable research attention from microbial oceanographers. Like all biological catalysts, the activity of these keystone biomolecules is subject to control by temperature and pH, leaving the crucial ecosystem functions they support potentially vulnerable to anthropogenic environmental change. We summarize and discuss both consensus and conflicting evidence on the effects of sea surface warming and ocean acidification for five of these critical enzymes [carbonic anhydrase, ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), nitrogenase, nitrate reductase, and ammonia monooxygenase] and one important transporter (proteorhodopsin). Finally, we forecast how the responses of these few but essential biocatalysts to ongoing global change processes may ultimately help to shape the microbial communities and biogeochemical cycles of the future greenhouse ocean.
  8. The Southern Ocean (SO) harbors some of the most intense phytoplankton blooms on Earth. Changes in temperature and iron availability are expected to alter the intensity of SO phytoplankton blooms, but little is known about how these changes will influence community composition and downstream biogeochemical processes. We performed light-saturated experimental manipulations on surface ocean microbial communities from McMurdo Sound in the Ross Sea to examine the effects of increased iron availability (+2 nM) and warming (+3 and +6 °C) on nutrient uptake, as well as the growth and transcriptional responses of two dominant diatoms, Fragilariopsis and Pseudo-nitzschia . We found that community nutrient uptake and primary productivity were elevated under both warming conditions without iron addition (relative to ambient −0.5 °C). This effect was greater than additive under concurrent iron addition and warming. Pseudo-nitzschia became more abundant under warming without added iron (especially at 6 °C), while Fragilariopsis only became more abundant under warming in the iron-added treatments. We attribute the apparent advantage Pseudo-nitzschia shows under warming to up-regulation of iron-conserving photosynthetic processes, utilization of iron-economic nitrogen assimilation mechanisms, and increased iron uptake and storage. These data identify important molecular and physiological differences between dominant diatom groups and add tomore »the growing body of evidence for Pseudo-nitzschia ’s increasingly important role in warming SO ecosystems. This study also suggests that temperature-driven shifts in SO phytoplankton assemblages may increase utilization of the vast pool of excess nutrients in iron-limited SO surface waters and thereby influence global nutrient distribution and carbon cycling.« less