Award: MCB-0703159

There is an ever-increasing realization of the importance of microbial processes in global biogeochemical cycles in marine ecosystems. Our work has focused on understanding the ecological interactions of the huge diversity of species that comprise the natural microbial assemblage of a coastal oceanic ecosystem. This research project has entailed a study of the diversity all microorganisms (bacteria, archaea and protists) at an oceanographic station in the San Pedro Channel off southern California. The overarching goal of this research project has been the discovery of new microbial life, and the characterization of food web interactions, nutrient uptake and release, and energy flow among microorganisms within the pelagic food web at this site. The San Pedro Basin combines a number of unique environmental features including a deep-water environment (≈890 m) in close proximity to shore, very low oxygen concentration in deep waters (<1 ml L-1 below 150 m), natural inputs from oceanic regions, as well as significant anthropogenic inputs from a major metoropolitan area (greater Los Angeles region). Our objective has been to develop a more complete view of microbial interactions in the region by considering a more complete collection of all the interacting microorganisms than has been attempted in most microbial ecological studies to date. Our guiding hypothesis is that the microbial community is organized into æguildsÆ of microbes (self-organizing groups of interacting microbial species) that form functional subunits within the huge diversity of taxa that comprise planktonic microbial communities. We speculate that these functional subunits conduct the major biogeochemical processes in the water, and that changes in environmental conditions result in shifts, or substitutions, in the specific guild of species dominating at any given time. This hypothesis has been tested by examining changes in the composition of the microbial community on a monthly basis (sampling started in 2000, as part of a previously-funded project) carried out approximately mid-channel between Long Beach, CA and the USC Wrigley Marine Science Center on Santa Catalina island, in conjunction with the San Pedro Ocean Time Series (SPOT) program. The composition of the microbial community has been carried out using state-of-the-art DNA sequencing approaches and analyses, and standard oceanographic approaches, to characterize the species of microbes present at various depths and on dates of sampling. Trophic linkages between microbial groups during monthly, seasonal and annual changes in community composition have been examined by comparisons of microbial community structure using network analysis, which aids in establishing relationships among microbial species (e.g. predator-prey relationships, parasitic relationships, etc.) and with environmental variables (temperature, nutrients, etc.). This study has also included experimental studies to examine the response of microbial community composition to environmental perturbations. The primary intellectual merit of our research program is that we have uncovered significant relationships among the various microbial species, as consequence of the unique goal of our project to characterize the complete spectrum of microorganisms (from viruses to the largest protists) in the water column at our study site. Few research programs have attempted to simultaneously follow and relate these disparate but highly interdependent assemblages. Through analysis of our extensive monthly time-series (10 years of data), which incorporates both biological and environmental parameters, we have provided new insights into the short-term responses of microbial communities to environmental change, as well as established a dataset from which we can begin to examine the impact of long-term changes (e.g. decadal climate shifts such as El Nino, global warming, oceanic acidification). Genetic fingerprinting of the total ...