Microbes have central roles in ocean food webs and global biogeochemical processes, yet specific ecological relationships among these taxa are largely unknown. This is in part due to the dilute, microscopic nature of the planktonic microbial community, which prevents direct observation of their interactions. Here, we use a holistic (that is, microbial system-wide) approach to investigate time-dependent variations among taxa from all three domains of life in a marine microbial community. We investigated the community composition of bacteria, archaea and protists through cultivation-independent methods, along with total bacterial and viral abundance, and physicochemical observations. Samples and observations were collected monthly over 3 years at a welldescribed ocean time-series site of southern California. To find associations among these organisms, we calculated time-dependent rank correlations (that is, local similarity correlations) among relative abundances of bacteria, archaea, protists, total abundance of bacteria and viruses and physico-chemical parameters. We used a network generated from these statistical correlations to visualize and identify time-dependent associations among ecologically important taxa, for example, the SAR11 cluster, stramenopiles, alveolates, cyanobacteria and ammonia-oxidizing archaea. Negative correlations, perhaps suggesting competition or predation, were also common. The analysis revealed a progression of microbial communities through time, and also a group of unknown eukaryotes that were highly correlated with dinoflagellates, indicating possible symbioses or parasitism. Possible 'keystone' species were evident. The network has statistical features similar to previously described ecological networks, and in network parlance has non-random, small world properties (that is, highly interconnected nodes). This approach provides new insights into the natural history of microbes.

Literature review and synthesis of growth rates of aquatic protists focused on the role of temperature in the formation of massive annual algal blooms in high-latitude ecosystems. Maximal growth rates of herbivorous protists equaled or exceeded maximal growth rates of phototrophic protists at temperatures above 15uC. Maximal growth rates of herbivorous protists declined more rapidly with decreasing temperature than did those of phototrophic protists, and at the very low temperatures common to high-latitude ecosystems, the maximal growth rates of herbivorous protists were less than half the maximal growth rates of phototrophic protists. Growth rates of herbivorous protists were consistently lower than those of bacterivorous protists and were unrelated to differences in cell volume between the two groups. Linear equations describing the relationship of the natural log of maximal growth rates of bacterivorous and herbivorous protists to temperature were generated and compared to published information for maximal growth rates of phototrophic protists and copepods. The three heterotrophic groups had similar slopes (0.12 for bacterivorous protists, 0.10 for herbivorous protists, and 0.13 for copepods) that were approximately double that of phototrophic protists (0.06). The massive annual algal blooms observed in high latitudes are due in part to a fundamental difference in the relationship between growth and temperature for phototrophic protists and their grazers.

We examined the effects of increased temperature, pCO 2 , and irradiance on a calcifying strain of the marine coccolithophore Emiliania huxleyi in semi-continuous laboratory cultures. Emiliania huxleyi CCMP 371 was cultured in four temperature and pCO 2 treatments at both low and high irradiance (50 and 400 mmol photons m À2 s À1): (i) 20 C and 375 ppm CO 2 (ambient control); (ii) 20 C and 750 ppm CO 2 (high pCO 2 ); (iii) 24 C and 375 ppm CO 2 (high temperature); and (iv) 24 C and 750 ppm CO 2 ('greenhouse'). The growth of E. huxleyi was greatly accelerated by elevated temperature at low irradiance. Photosynthesis was significantly promoted by increases in both pCO 2 and temperature at both irradiances. Higher cellular C/P ratios were found in the higher CO 2 treatments at high irradiance, indicating a reduced requirement for P. The PIC/POC (particulate inorganic to organic carbon) ratio remained constant at low light, regardless of CO 2 or temperature conditions. However, both the cellular PIC content and PIC/POC ratio were greatly decreased by elevated irradiance, and were further decreased by increased pCO 2 only at high light, indicating a combined effect of CO 2 and light on calcification. These results suggest that future trends of CO 2 enrichment, sea-surface warming and exposure to higher mean irradiances from intensified stratification will have a large influence on the growth of Emiliania huxleyi, and potentially on the PIC/POC 'rain ratio'. Our study demonstrates that it is possible to obtain a more complete picture of global change impacts on marine phytoplankton by designing experiments that consider multiple global change variables and their mutual interactions.

Primary production in the Ross Sea, one of the most productive areas in the Southern Ocean, has previously been shown to be seasonally limited by iron. In two of three bottle incubation experiments conducted in the austral summer, significantly higher chlorophyll a (Chl a) concentrations were measured upon the addition of iron and B 12 , relative to iron additions alone. Initial bacterial abundances were significantly lower in the two experiments that showed phytoplankton stimulation upon addition of B 12 and iron relative to the experiment that did not show this stimulation. This is consistent with the hypothesis that the bacteria and archaea in the upper water column are an important source of B 12 to marine phytoplankton. The addition of iron alone increased the growth of Phaeocystis antarctica relative to diatoms, whereas in an experiment where iron and B 12 stimulated total phytoplankton growth, the diatom Pseudonitzschia subcurvata went from comprising approximately 70% of the phytoplankton community to over 90%. Cobalt additions, with and without iron, did not alter Chl a biomass relative to controls and iron additions alone in the Ross Sea. Iron and vitamin B 12 plus iron treatments caused reductions in the DMSP (dimethyl sulfoniopropionate) : Chl a ratio relative to the control and B 12 treatments, consistent with the notion of an antioxidant function for DMSP. These results demonstrate the importance of a vitamin to phytoplankton growth and community composition in the marine environment.The nutritional controls on marine phytoplankton growth have important implications for the regulation of the global carbon cycle. Nitrogen and iron are thought to be the dominant controllers of phytoplankton growth in the oceans, and hence the discovery of a vitamin such as B 12 having an influence on marine primary productivity would be a finding of significance. The limited information about the biogeochemical cycle of this vitamin suggests that it may be in limiting quantities in seawater. B 12 is a biologically produced cobalt-containing organometallic molecule, and only select bacteria and archaea possess the capability for B 12 biosynthesis. As a result, all eukaryotic organisms, from eukaryotic phytoplankton to humans, must either acquire B 12 from the environment or possess an alternate biochemistry that does not require the vitamin. Removal of B 12 from the water column has never been directly quantified but likely includes photodegradation 1 Corresponding author (mak@whoi.edu). 2 Coauthors. AcknowledgmentsWe thank Peter Sedwick for allowing us to utilize his tracemetal-clean fish sampling system and David Hutchins for allowing us to work in his laboratory van and for helpful discussions. We also thank Bettina Sohst and Carol Pollard for nutrient analyses and Tyler Goepfert for help in Phaeocystis antarctica culture studies, and Sheila Clifford for comments on the manuscript. Special thanks to the captain, crew, and Raytheon marine and scientific technical staff of the RV N. B. Palmer. Thanks also to ...