Abstract
There is a relative absence of studies dealing with mats of purple sulphur bacteria in the intertidal zone. These bacteria display an array of metabolic pathways that allow them to disperse and develop under a wide variety of conditions, making these mats important in terms of ecosystem processes and functions. Mass blooms of purple sulphur bacteria develop during summer on sediments in the intertidal zone especially on macroalgal deposits. The microbial composition of different types of mats differentially affected by the development of purple sulphur bacteria was examined, at low tide, using a set of biochemical markers (fatty acids, pigments) and composition was assessed against their influence on ecosystem functions (sediment cohesiveness, CO2 fixation). We demonstrated that proliferation of purple sulphur bacteria has a major impact on intertidal mats diversity and functions. Indeed, assemblages dominated by purple sulphur bacteria (Chromatiaceae) were efficient exopolymer producers and their biostabilisation potential was significant. In addition, the massive growth of purple sulphur bacteria resulted in a net CO2 degassing whereas diatom dominated biofilms represented a net CO2 sink.
Highlights
Microbial mats on intertidal sediments are complex and highly organised ecosystems where opposed oxygen and sulphide gradients favour the development of vertically stratified, multicoloured and cohesive layers of different functional groups of microorganisms that display various forms of respiration, from the surface to the depth
Low-density mats (LD) corresponded to bare sediments whose surface did not display any visible patches of purple sulphur bacteria (Figure 1); medium density (MD) mats corresponded to bare sediments with several patches of purple sulphur bacteria at the surface; and high density (HD) mats corresponded to similar sediments with green macroalgae deposits covered by a thick layer of purple sulphur bacteria (100% of surface covered)
HD mats were characterised by high amounts of degraded chlorophyll (i.e pheophorbide a, 3.4 ± 1.8 μg.g-1), in comparison to MD and LD mats (0.15 ± 0.08 and 0.05 ± 0.09 μg.g-1)
Summary
Microbial mats on intertidal sediments are complex and highly organised ecosystems where opposed oxygen and sulphide gradients favour the development of vertically stratified, multicoloured and cohesive layers of different functional groups of microorganisms that display various forms of respiration, from the surface to the depth. Phototrophic purple sulphur bacteria are generally found in anoxic layers where light can penetrate Their main metabolism is anoxygenic photosynthesis (without O2 release), which uses reduced sulphur compounds as electron donors but they can be chemolithotrophs, in dark environments, by using sulphide or thiosulphate as electron donor and oxygen as the terminal electron acceptor [2]. In muddy environments, they commonly proliferate in a layer of about 1-2 mm depth that matches their needs both in terms of light and sulphide availability. Purple sulphur bacteria develop at the air/water sediment interface in these systems and are exposed to oxygen, forming large bright pink mats visible to the naked eye
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