Molecular Investigations and Experimental Manipulations of Microbial Mats: A View to Paleomicrobial Ecosystems

Abstract

Four decades of research into the biogeochemistry of microbial mats has revealed miniature ecosystems, as complex as any on Earth, with an astounding diversity of microbial life. Microbial mats from hypersaline environments are probably the best studied (with apologies to hot spring microbial mats), as in these environments the densest, most cohesive, and extensive mats are formed. Hypersaline microbial mats have been under much attention as they serve as (a) model ecosystems with complex biogeochemistry, (b) analogs of the most ancient forms of communal life on Earth, (c) a dominant form of shallow marine life during the Precambrian, influencing oceanic and atmospheric conditions on a global scale, (d) a natural environment to explore the physiochemical limits of microbial life, and (e) a highly complex, dynamic model microbial community conducive to intense ecological interactions between microbial functional groups. Incidentally, the microbial mats from moderately hypersaline environments (e.g., 80–120%) have the additional benefit of being transportable. Few scientists studying natural ecosystems are able to excise a chunk of a self-supporting environment and transport it to the laboratory. This feature of thick microbial mats has also made these systems invaluable as experimental ecosystems, in which external environmental conditions can be modulated, and the effect on the entire microbial community and biogeochemistry studied. This chapter explores the types of experimental research that has been conducted on hypersaline microbial mats, with a particular eye toward the exploration of paleomicrobial ecosystems through such studies. These experiments have included salinity modification, sulfate depletion, addition of exogenous carbon substrates, nitrogen levels (dinitrogen fixation), and light levels. In this chapter, we explicitly consider experimental manipulations of sulfate and salinity, the effect of these manipulations on geochemical cycling and microbial community, the implication of these data for understanding Earth’s earliest laminated microbial communities, and the limitations of such studies for gazing into the Precambrian.