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Abstract Nitrogen (N) is a limiting nutrient in vast regions of the world’s oceans, yet the sources of N available to various phytoplankton groups remain poorly understood. In this study, we investigated inorganic carbon (C) fixation rates and nitrate (NO3−), ammonium (NH4+) and urea uptake rates at the single cell level in photosynthetic pico-eukaryotes (PPE) and the cyanobacteria Prochlorococcus and Synechococcus. To that end, we used dual 15N and 13C-labeled incubation assays coupled to flow cytometry cell sorting and nanoSIMS analysis on samples collected in the North Pacific Subtropical Gyre (NPSG) and in the California Current System (CCS). Based on these analyses, we found that photosynthetic growth rates (based on C fixation) of PPE were higher in the CCS than in the NSPG, while the opposite was observed for Prochlorococcus. Reduced forms of N (NH4+ and urea) accounted for the majority of N acquisition for all the groups studied. NO3− represented a reduced fraction of total N uptake in all groups but was higher in PPE (17.4 ± 11.2% on average) than in Prochlorococcus and Synechococcus (4.5 ± 6.5 and 2.9 ± 2.1% on average, respectively). This may in part explain the contrasting biogeography of these picoplankton groups. Moreover, single cell analyses reveal that cell-to-cell heterogeneity within picoplankton groups was significantly greater for NO3− uptake than for C fixation and NH4+ uptake. We hypothesize that cellular heterogeneity in NO3− uptake within groups facilitates adaptation to the fluctuating availability of NO3− in the environment.
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Genomic adaptation of marine phytoplankton populations regulates phosphate uptake
Abstract In this study, we combined “reciprocal transplant experiments,” cell‐sorting, and metagenomics to understand how phytoplankton adapt to differences in phosphate availability and the implications for nutrient uptake rates. Reciprocal transplant experiments were conducted on six stations ranging from cold, nutrient‐rich water in the Labrador Sea to warm, extremely P‐deplete water in the Sargasso Sea. In most cases, the direct impact of environmental conditions and likely P availability was the strongest control on phosphate uptake. However, especially the transplant experiments between the northern and southern stations revealed that there are situations where changes in community composition and functional genes have an important effect on uptake rates. Phytoplankton lineages responded uniquely to changing environmental conditions. The picoeukaryotic phytoplankton P uptake response was strongly regulated by the phosphate concentration, whereas the effect of community composition was larger forProchlorococcusandSynechococcus. In support, we found a tight negative relationship between ambient phosphate concentration and the frequency of P acquisition genes in bothProchlorococcusandSynechococcus, and such differences in genome content could be linked to lineage‐specific shifts in uptake rates. Linking genes with ocean biogeochemistry is a major scientific and technical challenge and most studies rely on correlations between genotypes and environmental conditions. However, our study demonstrates how reciprocal transplant experiments are a possible tool for understanding the relative role of environmental condition vs. plankton diversity in regulating important open ocean ecosystem processes.
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- PAR ID:
- 10457797
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Limnology and Oceanography
- Volume:
- 65
- Issue:
- S1
- ISSN:
- 0024-3590
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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