Test of viability measures in starved, sedimentary, anaerobic bacterial isolates and in a temperature stressed estuarine sedimentary microbial community : insights for deep biosphere studies
Marine sediments harbour vast and diverse prokaryotic communities. With ongoing burial and ageing of respective sediment layers, however, available organic matter becomes more recalcitrant. Thus, sedimentary microorganisms face starvation and ultimately death. Nonetheless, live and active cells are present in old and deeply buried sediments, up to 111 Ma (Roussel et al., 2008). During IODP Leg 307 an organic-matter poor, cold-water, buried coral carbonate mound was sampled. Nineteen isolates, mainly Proteobacteria, were obtained from the mound and surrounding sediments. Additionally, one putative new species belonging to the genus Ornithinimicrobium (Actinobacteria) was isolated. Strains were subsequently phylogenetically and phenotypically characterised. Selected isolates and other sedimentary bacteria were subsequently subjected to anaerobic starvation-survival experiments and their responses to substrate limitation were compared to those of near-surface relatives. All strains survived long periods of starvation (incubated up to 3 years). This was confirmed by constant total cell counts and only slowly increasing proportions of dead cells (20% after one year). Culturability and FISH detectability decreased with time but radiotracer experiments conducted after starvation confirmed viability and potential metabolic activity of many strains. No significant correlations between FISH detectability and other viability measures occurred. Instead starvation time was significantly positively correlated with percentages of dead cells and inversely with culturability. Pure culture starvation experiments were complemented by a study on an estuarine, surface-sediment microbial community, which was stressed in sediment slurry sequential heating experiments. This mimicked burial and resulted in decreasing total counts, culturability, and FISH detcctability but these were still present even after heating to 90 °C. Temperatures above 42 °C were significantly correlated with the reduction of total cells and FISH detectability This project showed that marine sedimentary microbes maintain high levels of viability and culturability during long-term anaerobic starvation and during sequential heating to mimic burial this is consistent with the large cell population in sub-seafloor sediments.
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