Lawrence Berkeley National Laboratory

Sulfate-reducing bacteria and methanogens are found to coexist in a variety of anoxic marine sediments. In these systems they either compete for substrates or engage in successful syntrophic relationships. In our experimental setup, Desulfovibrio vulgaris Hildenborough ferments lactate, producing acetate and hydrogen. Methanococcus maripaludis, a hydrogenotrophic methanogen, then utilizes hydrogen while also incorporating limited amounts of acetate as a carbon source. Mid-log growth phase of this co-culture is achieved in 3 days growing at 37oC at which point, nearly 50percent of the initial lactate was depleted. In this study we investigate the stress response of this coculture and compare it to the D. vulgaris monoculture. Minimum Inhibitory Concentration (MIC) determinations of two environmentally relevant stressors (NO3- and NaCl) on the coculture and monoculture suggest nitrate predominantly affects M. maripaludis with a MIC of 25mM while sodium stress affects D. vulgaris with a MIC of 100mM. The response of the coculture to stressors like nitrate, nitrite, salt and peroxide was monitored by several methods. The fate of metabolites was tracked in the cultures and rates of gas evolution/utilization were measured with the Micro-Oxymax. Total biomass was measured over time with direct cell counts (including ratios of SRB: methanogen), cell protein and optical density. Metal reducing capability of log phase co-culture under NO3 stress was investigated and compared to that of under NaCl stress. Phenotype Microarray substrate utilization profiles generated by the Omnilog technology for a variety of metabolic substrates showed differential profiles for the coculture and the monoculture. Whole-genome transcriptional analysis of NaCl stressed coculture indicates up-regulation of genes coding for numerous transmembrane electron transfer enzymes.