Tools providing new insight into coastal anoxygenic purple bacterial mats: review and perspectives

Coastal photosynthetic microbial mats are highly structured microbial communities that populate a variety of shallow environments such as estuaries, sheltered sandy beaches, intertidal flats, salt marshes and hypersaline salterns. In soft sediments, most of these microbial mats are formed of vertically stratified, multicolored cohesive thin layers, of several functional groups of microorganisms, such as cyanobacteria, colorless sulfur bacteria, purple sulfur bacteria and sulfate-reducing bacteria, distributed along vertical microgradients of oxygen, sulfide and light. These microbial communities are highly productive and significant contributors to carbon, nitrogen and sulfur cycles and to sediment stability in shallow-water habitats. Many examples of these communities have been cited in the past, but comparatively few microbial mats have been presented for which mass developments of anoxygenic purple bacteria have been observed. Yet, application of molecular approaches has provided fresh insight into the ecology, diversity and evolution of microbial mats. In situ measurements using electrochemical and optical microprobes led to detailed characterization of their physical and chemical environment, whereas reflectance measurements revealed the spatial and temporal heterogeneity of microbial mat surfaces. We hereby report the main discoveries due to introduction of these powerful techniques and we point out the potential insight to be gained from the study of anoxygenic purple bacterial mats.