We have applied "whole-genome shotgun sequencing" to microbial populations collected en masse on tangential flow and impact filters from seawater samples collected from the Sargasso Sea near Bermuda. A total of 1.045 billion base pairs of nonredundant sequence was generated, annotated, and analyzed to elucidate the gene content, diversity, and relative abundance of the organisms within these environmental samples. These data are estimated to derive from at least 1800 genomic species based on sequence relatedness, including 148 previously unknown bacterial phylotypes. We have identified over 1.2 million previously unknown genes represented in these samples, including more than 782 new rhodopsin-like photoreceptors. Variation in species present and stoichiometry suggests substantial oceanic microbial diversity.

The Cyanobacteria Prochlorococcus and Synechococcus account for a substantial fraction of marine primary production. Here, we present quantitative niche models for these lineages that assess present and future global abundances and distributions. These niche models are the result of neural network, nonparametric, and parametric analyses, and they rely on >35,000 discrete observations from all major ocean regions. The models assess cell abundance based on temperature and photosynthetically active radiation, but the individual responses to these environmental variables differ for each lineage. The models estimate global biogeographic patterns and seasonal variability of cell abundance, with maxima in the warm oligotrophic gyres of the Indian and the western Pacific Oceans and minima at higher latitudes. The annual mean global abundances of Prochlorococcus and Synechococcus are 2.9 ± 0.1 × 10 27 and 7.0 ± 0.3 × 10 26 cells, respectively. Using projections of sea surface temperature as a result of increased concentration of greenhouse gases at the end of the 21st century, our niche models projected increases in cell numbers of 29% and 14% for Prochlorococcus and Synechococcus, respectively. The changes are geographically uneven but include an increase in area. Thus, our global niche models suggest that oceanic microbial communities will experience complex changes as a result of projected future climate conditions. Because of the high abundances and contributions to primary production of Prochlorococcus and Synechococcus, these changes may have large impacts on ocean ecosystems and biogeochemical cycles.climate change | marine biogeochemistry | microbial biogeography M arine phytoplankton contribute roughly one-half of the global net primary production and play a key role in regulating global biogeochemical cycles (1). Marine phytoplankton are very diverse (2), including phylogenetic, biochemical, metabolic, and ecological variability (3-6). Thus, understanding the contribution of different phytoplankton groups to ecosystem functioning is central to predicting the biogeochemical impact of future environmental changes (7). However, our limited quantitative understanding of the global distribution and abundance of most phytoplankton groups constrains our ability to incorporate phytoplankton diversity into ocean biogeochemical models.The marine Cyanobacteria Prochlorococcus and Synechococcus are abundant in many ocean regions. The known geographical distributions of the two lineages are based primarily on individual cruises or time series observations and secondarily on macroecological descriptions, indicating central tendencies and boundary constraints related to light, temperature, nutrients, and chlorophyll a concentration (8-10). Prochlorococcus is present from the surface to a depth of ∼150 m in the open ocean between 40°N and 40°S. The population size declines beyond these latitudinal limits, and Prochlorococcus is thought to be absent at temperatures below 15°C (11). Furthermore, the lineage is believed to be out...

Phosphorus is an obligate requirement for the growth of all organisms; major biochemical reservoirs of phosphorus in marine plankton include nucleic acids and phospholipids. However, eukaryotic phytoplankton and cyanobacteria (that is, 'phytoplankton' collectively) have the ability to decrease their cellular phosphorus content when phosphorus in their environment is scarce. The biochemical mechanisms that allow phytoplankton to limit their phosphorus demand and still maintain growth are largely unknown. Here we show that phytoplankton, in regions of oligotrophic ocean where phosphate is scarce, reduce their cellular phosphorus requirements by substituting non-phosphorus membrane lipids for phospholipids. In the Sargasso Sea, where phosphate concentrations were less than 10 nmol l-1, we found that only 1.3 +/- 0.6% of phosphate uptake was used for phospholipid synthesis; in contrast, in the South Pacific subtropical gyre, where phosphate was greater than 100 nmol l-1, plankton used 17 6% (ref. 6). Examination of the planktonic membrane lipids at these two locations showed that classes of sulphur- and nitrogen-containing membrane lipids, which are devoid of phosphorus, were more abundant in the Sargasso Sea than in the South Pacific. Furthermore, these non-phosphorus, 'substitute lipids' were dominant in phosphorus-limited cultures of all of the phytoplankton species we examined. In contrast, the marine heterotrophic bacteria we examined contained no substitute lipids and only phospholipids. Thus heterotrophic bacteria, which compete with phytoplankton for nutrients in oligotrophic regions like the Sargasso Sea, appear to have a biochemical phosphorus requirement that phytoplankton avoid by using substitute lipids. Our results suggest that phospholipid substitutions are fundamental biochemical mechanisms that allow phytoplankton to maintain growth in the face of phosphorus limitation.

Nearly 75 years ago, Alfred C. Redfield observed a similarity between the elemental composition of marine plankton in the surface ocean and dissolved nutrients in the ocean interior 1 . This stoichiometry, referred to as the Redfield ratio, continues to be a central tenet in ocean biogeochemistry, and is used to infer a variety of ecosystem processes, such as phytoplankton productivity and rates of nitrogen fixation and loss [2][3][4] . Model, field and laboratory studies have shown that different mechanisms can explain both constant and variable ratios of carbon to nitrogen and phosphorus among ocean plankton communities. The range of C/N/P ratios in the ocean, and their predictability, are the subject of much active research [5][6][7][8][9][10][11][12] . Here we assess global patterns in the elemental composition of phytoplankton and particulate organic matter in the upper ocean, using published and unpublished observations of particulate phosphorus, nitrogen and carbon from a broad latitudinal range, supplemented with elemental data for surface plankton populations. We show that the elemental ratios of marine organic matter exhibit large spatial variations, with a global average that differs substantially from the canonical Redfield ratio. However, elemental ratios exhibit a clear latitudinal trend. Specifically, we observed a ratio of 195:28:1 in the warm nutrient-depleted low-latitude gyres, 137:18:1 in warm, nutrient-rich upwelling zones, and 78:13:1 in cold, nutrient-rich high-latitude regions. We suggest that the coupling between oceanic carbon, nitrogen and phosphorus cycles may vary systematically by ecosystem.A. C. Redfield first noted the similarity between the particulate nutrient ratios of plankton living in the surface ocean and that of dissolved nutrients in the deep ocean 1 . He predicted that deep-ocean nutrient ratios were controlled by the uniform nutrient requirements of sinking, and subsequently remineralized, surface plankton. This concept remains a central tenet in our understanding of ocean biogeochemistry. However, model, field and laboratory studies have shown that different mechanisms can explain both constant and variable C/N/P ratios among ocean plankton communities, but the range of ratios in the ocean and the relative importance of each mechanism are subject to much research [5][6][7][8][9][10]13,14 . Despite this continued lack of understanding of global patterns, the N/P ratio is routinely used to draw conclusions about ocean ecosystem processes, particularly related to nutrient limitation of phytoplankton production and the net magnitude of N 2 fixation and denitrification. 2,3 . Alas, these ratios are of critical importance to study, model and predict ocean biogeochemical cycles. Equally important is the C/P ratio of ocean plankton