The focus of this proposal is the role of bacterioplankton microbial community stratification in the ocean carbon cycle. Complex biological, chemical and physical processes control the efficiency of carbon transfer from the euphotic zone ocean to the deep sea, where sequestration is a possibility. Most organic carbon exported from the euphotic zone never leaves the surface 500 m, with approximately ninety percent of the exported organic matter being remineralized in the mesopelagic zone (140 - 1000 m). Microbial communities are vertically stratified in the oceans, particularly in the surface layer (0-300 m), which spans the region of deep mixing events and transition from the euphotic zone to the upper mesopelagic - the region of highest carbon remineralization activity.
The premise of this proposal is that stratified bacterioplankton clades engage in specialized biogeochemical activities that can be identified by integrated oceanographic and microbiological approaches. Specifically, the objective of this proposal is to assess if the mesopelagic microbial community rely on diagenetically altered organic matter and subcellular fragments that are produced by microbial processes in the euphotic zone and delivered into the upper mesopelagic by sinking or mixing. In past efforts this microbial observatory had greater success cultivating members of the euphotic zone microbial community, and revealed an unanticipated growth requirement for reduced sulfur compounds in alphaproteobacteria of the SAR11 clade. Genomic information showed that intense competition for substrates imposes trade-offs on bacterioplankton - there are regions of N dimensional nutrient space where specialists win. We postulate that specific growth requirements may explain some the regular spatial and temporal patterns that have been observed in upper mesopelagic bacterioplankton communities, and the difficulties of culturing some of these organisms.
The specific objectives of this project are: 1) to produce 13C and 15N labeled subcellular (e.g., soluble, cell wall, and membrane) and DOM fractions from photosynthetic plankton cultures and use stable isotope probing to identify specific clades in the surface and upper mesopelagic microbial community that assimilate fractions of varying composition and lability. 2) to use fluorescence in situ hybridization approaches to monitor temporal and spatial variability of specific microbial populations identified from the SIP and HTC experiments. To increase resolution we will use CARD-FISH protocols. 3) to measure the proteomes of bacterioplankton communities to identify highly translated genes in the surface layer and upper mesopelagic, and community responses to seasonal nutrient limitation. 4) and, to cultivate these organisms via high throughput culturing (HTC) by pursuing the hypothesis that they require specific nutrient factors and/or diagenetically altered organic substrates. Complete genome sequences from key organisms will be sought and used as queries to study patterns of natural variation in genes and populations that have been associated with biogeochemically important functions.
This project will make cultures of novel bacterioplankton and genome sequences available to the scientific community. Findings from this research may be used directly in foodweb and ocean carbon cycle models. The educational component of this research brings microbial oceanography training to students from many disciplines, through a summer course, and specialized training to graduate and undergraduate students involved directly in research.
