Nitrogen fixation dynamics in microbial mats

Abstract

(N) limitation is a widespread feature of estuatine and oceanic waters (Dugdale 1967; Carpenter and Capone 1983). Despite this, benthic microbial mats frequently flourish in geographically-diverse intertidal and subtidal sand/mud flat, lagoon, reef, marsh and mangrove habitats characterizing such waters (Cohen et al. 1984; Cohen and Rosenberg 1989). Mats are particularly prolific in oligotrophic tropical and subtropical waters, where an appreciable portion of the primary production can be attributed to benthic microalgae (Whitton and Potts 1982; Bauld 1984). The remarkable success of mats in these N-limited environments has been attributed to the ability of specific groups of mat microorganisms (cyanobacteria, photosynthetic-, heterotrophic- and lithotrophic bacteria) to ``fix'' (reduce) atmospheric N (N2), thereby providing biologically-available N (NH3) to mat flora and fauna (Wiebe et al. 1975; Potts and Whitton 1977; Carpenter et al. 1978; Paerl et al. 1981; Stal et al. 1984). Nitrogen fixation is often the sole biologically-derived source of ``new'' N supporting primary production in these waters. In the face of N loss (as N2) in mats (and other ecosystem components) via denitrification (Joye and Paerl 1993) and to a lesser extent ammonification, N2 fixation is also of importance in regulating flux and mass balance of this important nutrient.