The increases in atmospheric pCO 2 over the last century are accompanied by higher concentrations of CO 2 (aq) in the surface oceans. This acidification of the surface ocean is expected to influence aquatic primary productivity and may also affect cyanobacterial nitrogen (N)-fixers (diazotrophs). No data is currently available showing the response of diazotrophs to enhanced oceanic CO 2 (aq). We examined the influence of pCO 2 [preindustrial $ 250 ppmv (low), ambient $ 400, future $ 900 ppmv (high)] on the photosynthesis, N fixation, and growth of Trichodesmium IMS101. Trichodesmium spp. is a bloom-forming cyanobacterium contributing substantial inputs of 'new N' to the oligotrophic subtropical and tropical oceans. High pCO 2 enhanced N fixation, C : N ratios, filament length, and biomass of Trichodesmium in comparison with both ambient and low pCO 2 cultures. Photosynthesis and respiration did not change significantly between the treatments. We suggest that enhanced N fixation and growth in the high pCO 2 cultures occurs due to reallocation of energy and resources from carbon concentrating mechanisms (CCM) required under low and ambient pCO 2 . Thus, in oceanic regions, where light and nutrients such as P and Fe are not limiting, we expect the projected concentrations of CO 2 to increase N fixation and growth of Trichodesmium. Other diazotrophs may be similarly affected, thereby enhancing inputs of new N and increasing primary productivity in the oceans.

Phytoplankton form the foundation of the marine food web and regulate key biogeochemical processes. These organisms face multiple environmental changes 1 , including the decline in ocean pH (ocean acidification) caused by rising atmospheric p CO 2 (ref. 2). A meta-analysis of published experimental data assessing growth rates of di erent phytoplankton taxa under both ambient and elevated p CO 2 conditions revealed a significant range of responses. This e ect of ocean acidification was incorporated into a global marine ecosystem model to explore how marine phytoplankton communities might be impacted over the course of a hypothetical twenty-first century. Results emphasized that the di ering responses to elevated p CO 2 caused su cient changes in competitive fitness between phytoplankton types to significantly alter community structure. At the level of ecological function of the phytoplankton community, acidification had a greater impact than warming or reduced nutrient supply. The model suggested that longer timescales of competition-and transport-mediated adjustments are essential for predicting changes to phytoplankton community structure.The world's oceans have absorbed about 30% of anthropogenic carbon emissions, causing a significant decrease in surface ocean pH (ref. 2). Concerns over the impacts of ocean acidification (OA) on marine life have led to a number of laboratory and field experiments examining the response of marine biota to acidification.OA is not the only driver that is affecting marine ecosystems 1,3 . The oceans are warming, and nutrient and light environments are changing. Numerical models (for example, refs 4-6) have explored how these other drivers impact primary productivity, although less emphasis has been placed on changes in community structure. Phytoplankton types are not physiologically interchangeable, and the specific taxa in a community can impact the cycling of elements and the flow of nutrients and energy through the marine food web. In this study we employed a meta-analysis of OA experiments as input for a numerical model to explore how OA, relative to other drivers, may change phytoplankton community composition.We compiled data from 49 papers (Methods and Supplementary Table 1) in which direct comparisons were made between the growth rates of marine phytoplankton cultures exposed to ambient p CO 2 (∼380 µatm) versus elevated p CO 2 within the range predicted by 2100 (refs 2,7; ∼700-1,000 µatm). The tested organisms were 0.0 0.5 1.0 1.5 * * * * * * GRR C o c c o l i t h o p h o r e D i a t o m O t h e r l a r g e D i a z o t r o p h S y n e c h o c o c c u s P r o c h l o r o c o c c u s 2.0 2.5 Figure 1 | Meta-analysis of GRR of phytoplankton in p CO2 manipulation experiments. Circles represent observations comparing laboratory cultures at high and ambient p CO2 ; triangles indicate long-term experiments; squares represent data from mixed community field incubations. Grey boxes span the 25th-75th percentiles; central lines indicate median values; whiskers extend from the 10th ...

Diatoms are unicellular algae that accumulate significant amounts of triacylglycerols as storage lipids when their growth is limited by nutrients. Using biochemical, physiological, bioinformatics, and reverse genetic approaches, we analyzed how the flux of carbon into lipids is influenced by nitrogen stress in a model diatom, Phaeodactylum tricornutum. Our results reveal that the accumulation of lipids is a consequence of remodeling of intermediate metabolism, especially reactions in the tricarboxylic acid and the urea cycles. Specifically, approximately one-half of the cellular proteins are cannibalized; whereas the nitrogen is scavenged by the urea and glutamine synthetase/glutamine 2-oxoglutarate aminotransferase pathways and redirected to the de novo synthesis of nitrogen assimilation machinery, simultaneously, the photobiological flux of carbon and reductants is used to synthesize lipids. To further examine how nitrogen stress triggers the remodeling process, we knocked down the gene encoding for nitrate reductase, a key enzyme required for the assimilation of nitrate. The strain exhibits 40-50% of the mRNA copy numbers, protein content, and enzymatic activity of the wild type, concomitant with a 43% increase in cellular lipid content. We suggest a negative feedback sensor that couples photosynthetic carbon fixation to lipid biosynthesis and is regulated by the nitrogen assimilation pathway. This metabolic feedback enables diatoms to rapidly respond to fluctuations in environmental nitrogen availability.lipid | metabolism | stress | NR | RNAi

Recent studies on the diazotrophic cyanobacterium Trichodesmium erythraeum (IMS101) showed that increasing CO 2 partial pressure (pCO 2 ) enhances N 2 fixation and growth. Significant uncertainties remain as to the degree of the sensitivity to pCO 2 , its modification by other environmental factors, and underlying processes causing these responses. To address these questions, we examined the responses of Trichodesmium IMS101 grown under a matrix of low and high levels of pCO 2 (150 and 900 matm) and irradiance (50 and 200 mmol photons m 22 s 21 ). Growth rates as well as cellular carbon and nitrogen contents increased with increasing pCO 2 and light levels in the cultures. The pCO 2 -dependent stimulation in organic carbon and nitrogen production was highest under low light. High pCO 2 stimulated rates of N 2 fixation and prolonged the duration, while high light affected maximum rates only. Gross photosynthesis increased with light but did not change with pCO 2 . HCO 3 2 was identified as the predominant carbon source taken up in all treatments. Inorganic carbon uptake increased with light, but only gross CO 2 uptake was enhanced under high pCO 2 . A comparison between carbon fluxes in vivo and those derived from 13 C fractionation indicates high internal carbon cycling, especially in the low-pCO 2 treatment under high light. Light-dependent oxygen uptake was only detected under low pCO 2 combined with high light or when low-light-acclimated cells were exposed to high light, indicating that the Mehler reaction functions also as a photoprotective mechanism in Trichodesmium. Our data confirm the pronounced pCO 2 effect on N 2 fixation and growth in Trichodesmium and further show a strong modulation of these effects by light intensity. We attribute these responses to changes in the allocation of photosynthetic energy between carbon acquisition and the assimilation of carbon and nitrogen under elevated pCO 2 . These findings are supported by a complementary study looking at photosynthetic fluorescence parameters of photosystem II, photosynthetic unit stoichiometry (photosystem I:photosystem II), and pool sizes of key proteins in carbon and nitrogen acquisition.

Extracellular polysaccharide aggregates, operationally defined as transparent exopolymeric particles (TEP), are recognized as an important conduit for carbon recycling and export in aquatic systems. Yet, the factors controlling the build-up of the TEP pool are not well characterized. Here we show that increased TEP production by Trichodesmium, an oceanic bloom-forming nitrogen-fixing (diazotrophic) cyanobacterium, is coupled with autocatalytic programmed cell death (PCD) process. We demonstrate that PCD induction, in both laboratory cultures and natural populations, is characterized by high caspase-like activity, correlates with enhanced TEP production, and occurs under iron and phosphorus starvation, as well as under high irradiance and oxidative stress. Enhanced TEP production was not observed in actively growing populations. We provide further evidence that iron is a key trigger for the induction of PCD. We demonstrate, for the first time, the concomitant enhanced build-up of the TEP pool when Trichodesmium is Fe-stressed. These results suggest a functional linkage between activation of caspases and PCD in Trichodesmium and regulation of vertical carbon and nitrogen fluxes. We hypothesize that modulation of TEP formation and its qualities by different mortality pathways could regulate the fate of phytoplankton blooms and particulate organic matter in aquatic ecosystems.