The macroecological relationships among marine phytoplankton total cell density, community size structure and temperature have lacked a theoretical explanation. The tiniest members of this planktonic group comprise cyanobacteria and eukaryotic algae smaller than 2 lm in diameter, collectively known as picophytoplankton. We combine here two ecological rules, the temperature-size relationship with the allometric sizescaling of population abundance to explain a remarkably consistent pattern of increasing picophytoplankton biomass with temperature over the À0.6 to 22 1C range in a merged dataset obtained in the eastern and western temperate North Atlantic Ocean across a diverse range of environmental conditions. Our results show that temperature alone was able to explain 73% of the variance in the relative contribution of small cells to total phytoplankton biomass regardless of differences in trophic status or inorganic nutrient loading. Our analysis predicts a gradual shift toward smaller primary producers in a warmer ocean. Because the fate of photosynthesized organic carbon largely depends on phytoplankton size, we anticipate future alterations in the functioning of oceanic ecosystems.

Heterotrophic bacteria are the most abundant organisms on the planet and dominate oceanic biogeochemical cycles, including that of carbon. Their role in polar waters has been enigmatic, however, because of conflicting reports about how temperature and the supply of organic carbon control bacterial growth. In this Analysis article, we attempt to resolve this controversy by reviewing previous reports in light of new data on microbial processes in the western Arctic Ocean and by comparing polar waters with low-latitude oceans. Understanding the regulation of in situ microbial activity may help us understand the response of the Arctic Ocean and Antarctic coastal waters over the coming decades as they warm and ice coverage declines.

To determine the effects of Saharan dust on the abundance, biomass, community structure, and metabolic activity of oceanic microbial plankton, we conducted eight bioassay experiments between ca. 30uN and 30uS in the central Atlantic Ocean. We found that, although bulk abundance and biomass tended to remain unchanged, different groups of phytoplankton and bacterioplankton responded differently to Saharan dust addition. The predominant type of metabolic response depended on the ecosystem's degree of oligotrophy and was modulated by competition for nutrients between phytoplankton and heterotrophic bacteria. The relative increase in bacterial production, which was the dominant response to dust addition in ultraoligotrophic environments, became larger with increasing oligotrophy. In contrast, primary production, which was stimulated only in the least oligotrophic waters, became less responsive to dust as the ecosystem's degree of oligotrophy increased. Given the divergent consequences of a predominantly bacterial vs. phytoplanktonic response, dust inputs can, depending on the ecosystem's degree of oligotrophy, stimulate or weaken biological CO 2 drawdown. Thus, the biogeochemical implications of changing dust fluxes might not be universal, but variable through both space and time.

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Our view of the effects of temperature on bacterial carbon fluxes in the ocean has been confounded by the interplay of resource availability. Using an extensive compilation of cell-specific bacterial respiration (BRi) and production (BPi), we show that both physiological rates respond to changing temperature in a similar manner and follow the predictions of the metabolic theory of ecology. Their apparently different temperature dependence under warm, oligotrophic conditions is due to strong resource limitation of BP, but not of BRi. Thus, and despite previous preconception, bacterial growth efficiency (BGE = BPi/[BPi + BRi]) is not directly regulated by temperature, but by the availability of substrates for growth. We develop simple equations that can be used for the estimation of bacterial community metabolism from temperature, chlorophyll concentration, and bacterial abundance. Since bacteria are the greatest living planktonic biomass, our results challenge current understanding of how warming and shifts in ecosystem trophic state will modify oceanic carbon cycle feedbacks to climate change.

We analyzed the strength of phytoplankton-bacterioplankton coupling by comparing the rate of particulate (PPP) and dissolved primary production (DPP) with bacterial carbon demand (BCD) in four contrasting marine regions: offshore and coastal waters of the Southern Ocean, a coastal area of the NE Atlantic, and a coastal-offshore transect in the NW Mediterranean. We measured bacterial heterotrophic production (BHP) and estimated BCD from a literature model. Average phytoplanktonic percent extracellular release [PER = DPP/(DPP + PPP)] was 18-20% in the Antarctic (offshore and coastal, respectively), 16% in the NW Mediterranean, and 7% in the NE Atlantic. A significant inverse relationship was found between PER and total system productivity with pooled data. On average BHP amounted to <5% of total primary production in all regions. However, the strength of phytoplankton-bacterioplankton coupling, estimated as the potential importance of DPP in meeting BCD, differed greatly in the four regions. DPP was highly correlated to BCD in offshore Antarctic waters and was sufficient to meet BCD. In contrast, BCD exceeded DPP and bore no significant relationship in the remaining regions. The data suggest that a strong dependence of bacteria on algal extracellular production is only expected in open-ocean environments isolated from coastal inputs of DOC.

Rare microbial taxa are increasingly recognized to play key ecological roles, but knowledge of their spatio-temporal dynamics is lacking. In a time-series study in coastal waters, we detected 83 bacterial lineages with significant seasonality, including environmentally relevant taxa where little ecological information was available. For example, Verrucomicrobia had recurrent maxima in summer, while the Flavobacteria NS4, NS5 and NS2b clades had contrasting seasonal niches. Among the seasonal taxa, only 4 were abundant and persistent, 20 cycled between rare and abundant and, remarkably, most of them (59) were always rare (contributing < 1% of total reads). We thus demonstrate that seasonal patterns in marine bacterioplankton are largely driven by lineages that never sustain abundant populations. A fewer number of rare taxa (20) also produced episodic 'blooms', and these events were highly synchronized, mostly occurring on a single month. The recurrent seasonal growth and loss of rare bacteria opens new perspectives on the temporal dynamics of the rare biosphere, hitherto mainly characterized by dormancy and episodes of 'boom and bust', as envisioned by the seed-bank hypothesis. The predictable patterns of seasonal reoccurrence are relevant for understanding the ecology of rare bacteria, which may include key players for the functioning of marine ecosystems.

We used flow cytometry of autofluorescent and Sytol3-stained marine bacteria and the uptake of tritiated leucine to assess the effects of different filter types on picoplankton abundance, community structure and bacterial activity in the filtrate. Coastal and oceanic samples from the NW and SW Mediterranean and the Atlantic coast of Galicia were size-fractionated using polycarbonate (PC), mixed cellulose esters (CE), aluminum oxide (IM) and glass fiber (GF) filters of 0.2 to 1 2 pm nominal pore size from Mferent brands. Flow cytometry of Sytol3-stained marine bactena and autofluorescent photosynthetic prokaryotes was used to analyze picoplankton abundance, size structure and community composition before and after filtration. We combined this capability with the detection of the changes in cell-specific heterotrophic activity in the filtrates. We found that the CE filters retained picoplankton better than the PC filters. The PC filters did not discriminate prokaryotes according to size as much as the GF and the CE filters did. In our hands the I M filters were no better than the CE filters.Bacterial activity in the filtrates increased in the PC and in the CE filtrates and this stimulation of bacteiial activity was more lrnportant in the less productive environments. We conclude that care must be taken when PC filters are used for generating bacteria-free water, and that the use of CE 0.22 pm filters is the best way of creating picoplankton-free water. However, the picoplankters that will go through the filters may encounter increased nutrient levels.