Intact membrane lipids as tracers for microbial life in the marine deep biosphere

Abstract

The main objective of this thesis was to elucidate the structure and carbon metabolism of microbial communities living in deeply buried sediments by using intact polar lipids (IPL) as markers for active microbial cells. A globally distributed sample set obtained during ODP Legs 201, 204, and 207, IODP Expeditions 301 and 311, and cruises Sonne SO147, Karei KY04-11, and Professor Logatchev TTR15 was analyzed to constrain the composition and quantity of deep marine subseafloor life on a global scale. Surface sediments contained abundant bacterial phospholipids having phosphatidylethanolamine, phosphatidylglycerol, and phosphatidylcholine as head groups with C16 and C18 acyl side chains, whereas the deeper sediment layers were dominated by archaeal glycolipids of tetraether and diether type with diglycosidic head groups. The transition from bacterial to archaeal lipids occurred within the top 0.1 mbsf of the sediment column. These results contrast previous studies based on microbiological methods which have identified a dominance of viable bacteria in deeply buried sediments. Supporting evidence comes from (i) "traditional- analysis of phospholipidderived fatty acids which confirmed the low contribution of bacteria to the total population, and (ii) improved microbiological methods which detected a higher proportion of archaeal cells compared to previous studies that possibly discriminated against archaeal phylogenetic lineages. Furthermore a decrease in IPL concentrations with depth was observed, following a similar relationship as previously seen for directly counted cells. IPL concentration was found to be dependent on concentration of sedimentary organic carbon, reflecting the inherent heterotrophic nature of this ecosystem. Modeling of organic carbon concentrations allowed to estimate the magnitude of the deep biosphere as 90 Pg of cellular C-units. This estimate is independent of microscopic cell counts and underlines the global importance of the deep biosphere. Carbon isotopic analysis indicated that the majority of the archaeal biomass is indeed heterotrophic and utilizes carbon derived from degradation of sedimentary organic matter to synthesize biomass. It could also be shown that methane oxidation in deeply buried sulfate-methane transition zones is mediated differently compared to seep-sites in surface sediments. Calculation of community turnover rates supported previous values on the order of hundreds to thousands of years. Such low rates point to maintenance energy requirements much lower than known from laboratory cultures and challenge our understanding of life.