Ecophysiology of sulfate-reducing bacteria and syntrophic communities in marine anoxic sediments

Propionate, butyrate, acetate, hydrogen and formate are the major intermediates of organic matter degradation. Sulfate-reducing bacteria (SRB) contribute significantly to the consumption of these substrates in sulfate-rich marine sediments. In sulfate-depleted sediments, however, complete degradation of propionate or butyrate is only possible via syntrophic cooperation of acetogenic bacteria and methanogenic archaea. Despite that the predominance of SRB in sulfate-rich and methanogens in sulfate-depleted sediments was reported, recent studies showed that both types of microorganism could be present in upper and lower parts of marine sediments. In this thesis, propionate and butyrate conversions and the involved microbial community in sulfate, sulfate-methane transition and methane zone sediment of Aarhus Bay, Denmark were studied using sediment slurry incubations. Interspecies hydrogen transfer and coexistence during acetate degradation were investigated in mixed pure cultures. In <strong>Chapter 2</strong>, interspecies hydrogen transfer between aceticlastic <em>Methanosaeta concilii</em> and hydrogenotrophic microorganisms, <em>Desulfovibrio vulgaris</em> or <em>Methanococcus maripaludis</em>, was investigated. Additionally, coexistence of <em>M. concilii</em> and <em>Desulfobacter latus</em> growing on acetate under sulfidogenic conditions was studied. The results of <strong>Chapter 2 </strong>showed that <em>D. vulgaris </em>could reduce sulfate and grow on leaked hydrogen from <em>M. concilii</em>. Hydrogen leakage from <em>M. concilii</em> provides an explanation for biogeochemical zonation both for competitive (e.g. acetate) and non-competitive substrates (methyl compounds), and this indicates the possible coexistence of SRB and methanogens in sulfate-rich environments. In <strong>chapter 3 and 4</strong>, long term incubations were examined focusing on butyrate and propionate conversion and the microbial community dynamics in sediment slurry enrichments at different sulfate (o, 3 and 20 mM) concentrations and incubation temperatures (10°C and 25°C). Sulfate reduction is the dominant process for butyrate and propionate conversion in Aarhus Bay sediments. In the absence of sulfate, both substrates can be converted efficiently, indicating the presence of syntrophic communities throughout the sediment. The fluctuating methane concentrations and the enrichment of anaerobic methanotrophic archaea (ANME) during butyrate and propionate conversion at 10°C suggest the occurrence of anaerobic oxidation of methane (AOM) in sulfate-methane transition zone (SMTZ) of Aarhus Bay. The microbial community involved in butyrate and propionate conversions were investigated using next generation sequencing (NGS) of the 16S rRNA amplicon sequencing. The enriched sulfate-reducing bacteria at high sulfate concentration (20 mM) were different when butyrate and propionate were used as substrate. <em>Desulfosarcina</em> and <em>Desulfobacterium</em> dominate the butyrate-converting slurries (<strong>Chapter 3</strong>), whereas <em>Desulfosarcina</em>, <em>Desulfobulbus</em> and <em>Desulforhopalus</em> are the main SRB in propionate-converting slurries (<strong>Chapter 4</strong>). The increase in the relative abundance of <em>Desulfobacteraceae</em> and <em>Desulfobulbaceae</em> in SZ, SMTZ and MZ sediment slurries suggests the presence of sulfate reducers throughout the anoxic sediment column. In the absence of sulfate, <em>Syntrophomonas</em> and <em>Cyrptanaerobacter</em> become dominant which suggests their role in syntrophic butyrate and propionate conversion, respectively. These results were further supported in <strong>Chapter 6</strong>. The increase in the relative abundance of <em>Syntrophomonas</em> in the presence of sulfate (<strong>Chapter 3</strong>) and some members of <em>Desulfobacteraceae</em> (<strong>Chapter 4</strong>) in the absence of sulfate shows the metabolic flexibility of the microorganisms at different sulfate concentrations. Temperature has an impact on the microbial community (<strong>Chapter 4</strong>) and IPL composition (<strong>Chapter 5</strong>) in enrichment slurries. <em>Cryptanaerobacter</em> is dominant at 25°C, and<em>, Desulfobacteraceae</em> (<em>Desulfofaba</em>), especially <em>Desulfobulbaceae</em> members (<em>Desulfobulbus</em>, <em>Desulforhopalus</em>) become dominant at 10°C at 0 and 3 mM sulfate concentrations in propionate-amended enrichment slurries. In butyrate-amended slurries, <em>Clostridiales</em> have higher relative abundance at 10°C regardless of the sulfate concentration and the sediment depth which supports important role of <em>Clostridiales</em> in butyrate conversion in marine sediments. Archaeal community analyses revealed the dominance of hydrogenotrophic methanogens belonging to <em>Methanomicrobiales</em> in both butyrate- and propionate-converting slurries (<strong>Chapter 3 and 4)</strong> and enrichment cultures (<strong>Chapter 6</strong>) regardless of the sediment depth, the incubation temperature and the presence of sulfate, which indicate that they are the main syntrophic partners of butyrate and propionate degraders. The other syntrophic partner organisms are the aceticlastic methanogenic families: <em>Methanosarcinaceae</em> and <em>Methanosaetaeceae</em>. The presence of methane-oxidizing archaea (ANME-1b) in low temperature SMTZ slurries together with <em>Desulfobacteraceae</em> (<strong>Chapter 3 and 4</strong>) suggests the occurrence of anaerobic oxidation of methane (AOM) in SMTZ of Aarhus Bay. In conclusion, this thesis confirms the presence and activity of methanogens in sulfate-rich, and SRB in sulfate-depleted marine sediments; and their involvement in butyrate, propionate and acetate conversion. Novel bacterial and archaeal members enriched in the sediment slurries are likely involved in propionate, butyrate and acetate conversions at different depths of marine sediments in addition to known the cultured species.