Structural and functional responses of microbial mats to reductions in nutrient and salinity stressors in a Bahamian hypersaline lagoon

Abstract

Microbial mats in Bahamian hypersaline lagoons are affected by the combination of salinity fluctuations and external nutrient inputs, both of which are seasonally variable. The purpose of this study was to examine the singular and combined effects of salinity and nutrient (N+P) stress on primary production, extracellular enzyme activity, and the composition of the photoautotroph community in this episodically varying extreme environment. Anoxygenic phototrophic bacteria were able to increase their relative abundance when nutrients were supplied under hypersaline con- ditions (300 g l -1 ). When salinities were lowered (38 g l -1 ) and nutrients added, extracellular enzyme activity (aminopeptidase, α-glucosidase, and β-glucosidase), rates of oxygenic photosynthesis, and phototroph biomass increased in the oxic surface layers of the mat. Once salinity stress had been low- ered, oxygenic photosynthesis allowed the proliferation of Cyanobacteria, heterotrophic activity, and a corresponding reduction in the abundance of anoxygenic phototrophic bacteria. On reduction of nutrient stress, mat phototrophs responded by increasing biomass (using either anoxygenic or oxy- genic photosynthesis, or both). In this hypersaline system, seasonal as well as short-term (days) vari- ations in environmental conditions may promote structural changes in the mat community which alter the rates of major processes such as oxygenic photosynthesis and heterotrophy, and illustrate the cyclic behavior of microbial dormancy and proliferation in this extreme environment. Cycles in nutrient input and salinity are primary forcing factors for the maintenance of a dynamic and diverse (both structurally and functionally) benthic microbial community in a small hypersaline lagoon, Salt Pond, on San Salvador Island, Bahamas.