Microbial activity is a fundamental component of oceanic nutrient cycles. Photosynthetic microbes, collectively termed phytoplankton, are responsible for the vast majority of primary production in marine waters. The availability of nutrients in the upper ocean frequently limits the activity and abundance of these organisms. Experimental data have revealed two broad regimes of phytoplankton nutrient limitation in the modern upper ocean. Nitrogen availability tends to limit productivity throughout much of the surface low-latitude ocean, where the supply of nutrients from the subsurface is relatively slow. In contrast, iron often limits productivity where subsurface nutrient supply is enhanced, including within the main oceanic upwelling regions of the Southern Ocean and the eastern equatorial Pacific. Phosphorus, vitamins and micronutrients other than iron may also (co-)limit marine phytoplankton. The spatial patterns and importance of co-limitation, however, remain unclear. Variability in the stoichiometries of nutrient supply and biological demand are key determinants of oceanic nutrient limitation. Deciphering the mechanisms that underpin this variability, and the consequences for marine microbes, will be a challenge. But such knowledge will be crucial for accurately predicting the consequences of ongoing anthropogenic perturbations to oceanic nutrient biogeochemistry

Abstract. Marine N2 fixing microorganisms, termed diazotrophs, are a key functional group in marine pelagic ecosystems. The biological fixation of dinitrogen (N2) to bioavailable nitrogen provides an important new source of nitrogen for pelagic marine ecosystems and influences primary productivity and organic matter export to the deep ocean. As one of a series of efforts to collect biomass and rates specific to different phytoplankton functional groups, we have constructed a database on diazotrophic organisms in the global pelagic upper ocean by compiling about 12 000 direct field measurements of cyanobacterial diazotroph abundances (based on microscopic cell counts or qPCR assays targeting the nifH genes) and N2 fixation rates. Biomass conversion factors are estimated based on cell sizes to convert abundance data to diazotrophic biomass. The database is limited spatially, lacking large regions of the ocean especially in the Indian Ocean. The data are approximately log-normal distributed, and large variances exist in most sub-databases with non-zero values differing 5 to 8 orders of magnitude. Reporting the geometric mean and the range of one geometric standard error below and above the geometric mean, the pelagic N2 fixation rate in the global ocean is estimated to be 62 (52–73) Tg N yr−1 and the pelagic diazotrophic biomass in the global ocean is estimated to be 2.1 (1.4–3.1) Tg C from cell counts and to 89 (43–150) Tg C from nifH-based abundances. Reporting the arithmetic mean and one standard error instead, these three global estimates are 140 ± 9.2 Tg N yr−1, 18 ± 1.8 Tg C and 590 ± 70 Tg C, respectively. Uncertainties related to biomass conversion factors can change the estimate of geometric mean pelagic diazotrophic biomass in the global ocean by about ±70%. It was recently established that the most commonly applied method used to measure N2 fixation has underestimated the true rates. As a result, one can expect that future rate measurements will shift the mean N2 fixation rate upward and may result in significantly higher estimates for the global N2 fixation. The evolving database can nevertheless be used to study spatial and temporal distributions and variations of marine N2 fixation, to validate geochemical estimates and to parameterize and validate biogeochemical models, keeping in mind that future rate measurements may rise in the future. The database is stored in PANGAEA (doi:10.1594/PANGAEA.774851).

In the modern ocean, a significant amount of nitrogen fixation is attributed to filamentous, nonheterocystous cyanobacteria of the genus Trichodesmium. In these organisms, nitrogen fixation is confined to the photoperiod and occurs simultaneously with oxygenic photosynthesis. Nitrogenase, the enzyme responsible for biological N 2 fixation, is irreversibly inhibited by oxygen in vitro. How nitrogenase is protected from damage by photosynthetically produced O 2 was once an enigma. Using fast repetition rate fluorometry and fluorescence kinetic microscopy, we show that there is both temporal and spatial segregation of N 2 fixation and photosynthesis within the photoperiod. Linear photosynthetic electron transport protects nitrogenase by reducing photosynthetically evolved O 2 in photosystem I (PSI). We postulate that in the early evolutionary phase of oxygenic photosynthesis, nitrogenase served as an electron acceptor for anaerobic heterotrophic metabolism and that PSI was favored by selection because it provided a micro-anaerobic environment for N 2 fixation in cyanobacteria.

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We present experimental laboratory evidence and field observations of an autocatalyzed, programmed cell death (PCD) pathway in the nitrogen-fixing cyanobacterium Trichodesmium spp., which forms massive blooms in the subtropical and tropical oceans. The PCD pathway was induced in response to phosphorus and iron starvation as well as high irradiance and oxidative stress. Transmission electron microscopy revealed morphological degradation of internal components including thylakoids, carboxysomes, and gas vesicles, whereas the plasma membranes remained intact. Physiologically stressed cells displayed significantly elevated endonuclease activity and terminal d-UTP nick-end labeling. Nucleic acid degradation was concordant with increased immunoreactivity to human caspase-3 polyclonal antisera and enhanced cleavage of a caspase-specific substrate, DEVD. Caspase activity was positively correlated with mortality and was inhibited by the irreversible caspase inhibitor Z-VAD-FMK. A search of the Trichodesmium erythraeum genome identified several protein sequences containing a conserved caspase domain structure, including the histidine-and cysteine-containing catalytic diad found in true caspases, paracaspases, and metacaspases. Induction of PCD by caspase-like proteases in a bacterial photoautotroph with an ancient evolutionary history requires a reassessment about the origins and roles of cell death cascades. This process is a previously unappriciated mortality mechanism that can lead to the termination of natural Trichodesmium blooms and that can influence the fluxes of organic matter in the ocean.Planktonic marine cyanobacteria of the genus Trichodesmium form extensive blooms in the oligotrophic tropical and subtropical oceans, where they make significant contributions to global nitrogen fixation (Capone et al. 1997). Natural blooms and laboratory cultures of Trichodesmium often terminate abruptly, with cell lysis and biomass degradation occurring within 1-2 d (Ohki 1999). The mechanisms controlling the dramatic and abrupt termination of Trichodesmium blooms are not well understood, even though this termination drives nutrient flow and biogeochemical cycling of organic and inorganic matter produced by these organisms, including the redistribution of fixed nitrogen in the upper ocean and the flow of organic matter through ecosystem pathways like the grazer food chain, the microbial loop, and vertical sinking flux (Azam 1998).

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.

The phytoplankton assemblage in Lake Kinneret is dominated in spring by a bloom of the dinoflagellate Peridinium gatunense, which terminates sharply in summer [1]. The pH in Peridinium patches rises during the bloom to values higher than pH9 [2] and results in CO(2) limitation. Here we show that depletion of dissolved CO(2) (CO(2(dis))) stimulated formation of reactive oxygen species (ROS) and induced cell death in both natural and cultured Peridinium populations. In contrast, addition of CO(2) prevented ROS formation. Catalase inhibited cell death in culture, implicating hydrogen peroxide (H(2)O(2)) as the active ROS. Cell death was also blocked by a cysteine protease inhibitor, E-64, a treatment which stimulated cyst formation. Intracellular ROS accumulation induced protoplast shrinkage and DNA fragmentation prior to cell death. We propose that CO(2) limitation resulted in the generation of ROS to a level that induced programmed cell death, which resembles apoptosis in animal and plant cells. Our results also indicate that cysteine protease(s) are involved in processes that determine whether a cell is destined to die or to form a cyst.

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 ...