Participants:

Project Summary

The nutrient elements, such as carbon , nitrogen, phosphorous and several important metals, occur in ecosystems in many different forms (e.g., organic carbon and carbon dioxide; nitrate, nitrite, nitrous oxide, organic nitrogen and nitrogen gas, etc.). The transformations between different forms, and the distributions of the various compounds, are largely controlled by microbes. Thus the physiology of bacteria and phytoplankton is largely responsible for the chemistry of natural systems, through what we call microbial biogeochemical cycling.

Our present understanding of elemental cycling is partly derived from measurements and modeling of the distribution of chemical compounds, and the measurement of the rates of transfer of compounds from one form to another. This approach has led to an appreciation of the overwhelming importance of microbes in regulating ecosystem biogeochemistry. But they still represent a great oversimplification of the complexities of microbial processes.

Recent advances in molecular biological studies of microbes involved in biogeochemical processes have shown that this microbial world contains immense diversity and complexity at every level: There is redundancy and duplication of functional genes (i.e., genes responsible for the individual steps in the biogeochemical cycles) within a single organism. Different organisms, which are capable of the same process, may use different kinds of enzymes to carry out essentially the same process. Even among different organisms that use the same enzyme to perform a particular process, there may be many different versions of the gene that encodes that essential enzyme. Different microbial communities that appear to function equally well may be composed of quite different groups of species, yet essentially the same processes go on in the different communities.

The goal of this project is to investigate the functional relationship between complexity in microbial communities and the biogeochemical cycles of natural ecosystems. It seems likely that microbial community complexity is related to the physical/chemical complexity of the environment, although just what is mean t by complexity is difficult to define and quantify. Our study will include sites in the Chesapeake Bay, one of its branches, the Choptank River, and the open ocean of the Sargasso Sea, which is the major ocean basin into which water from the Chesapeake Bay flows. We will characterize the physical/chemical complexity of these systems in terms of chemical and hydrographical variables , using data collected by our program and other ongoing programs in the region.

Into this oceanographic context, we will set our investigation of the complexity of microbial guilds. The microbial processes and the diversity of functional genes and organisms involved in these processes, will be investigated two parallel ways.

1. Diversity of functional genes: Previously, such analysis was technologically limited by the inability to assay large numbers of samples simultaneously for a large number of genes and organisms. Using gene array technology , we will be able to detect the distribution and differential expression of functional genes in natural systems. The results of this study will constitute the first step towards application of DNA chip technology for gene expression of processes and organisms in the natural environment.
2. Rates of biogeochemical processes: Studies will focus on key transformations in the carbon and nitrogen cycles (C fixation, N fixation, nitrification, denitrification, urea assimilation). The diversity of microbial guilds will be interpreted in terms of ecosystem function, assessed using the physical/chemical data mentioned above and tracer experiments to estimate actual transforma-tion rates. In addition to field studies designed to investigate and dissect the natural system, we will also perform perturbation experiments using mesocosms, to investigate the response of natural systems to perturbations, and the manipulate the complexity of systems to look for key community members or processes. We will focus on the Nitrogen cycle and the bacterial processes that are important in nitrogen transformations in aquatic systems.

The goal of these experiments is to determine how microbial species diversity affects the major energy and nutrient flows within ecosystems, and to assess the degree of stability or instability associated with changes in redundancy within guilds of microorganisms responsible for major nitrogen and carbon pathways.