Identification and activity of acetate-assimilating microorganisms in diffuse hydrothermal fluids

Sampling

In diffuse hydrothermal fluids concentrations of organic compounds such as acetate can be significant. To date knowledge about mixo- and heterotrophic microorganisms in hydrothermal systems is derived from pure cultures only. We set out to identify acetate-consuming microorganisms in diffuse fluids from two distinct hydrothermal systems using cultivation-independent approaches. For this purpose we combined a characterization of the microbial community in fluids with short-term incubations (8-12 h) using 13C-labeled acetate at low concentrations (10 or 30 µM). We followed cell growth and assimilation of 13C into single cells by nanoSIMS combined with fluorescence in situ hybridization (FISH). In 55°C fluids from the Menez Gwen system, Mid-Atlantic Ridge, a novel epsilonproteobacterial group related to Nautiliales accounted for nearly all acetate-assimilating cells. In contrast, in 4°C and 37°C fluids from the Manus Basin (Papua New Guinea) Gammaproteobacteria dominated the 13C-acetate-assimilating community, which was supported by 16S rRNA sequences related to Marinobacter and Alteromonas. We also detected yet unidentified, weakly acetate assimilating cells in 72°C fluids (Manus Basin) that were presumably related to Acinetobacter. In particular in the 37°C and 55°C incubations the microbial communities differed from those in native fluids indicating rapid growth of heterotrophic organisms. The instant response suggests that acetate-consumers in diffuse fluids are r-strategists, which quickly exploit their food sources whenever available under the spatially and temporally highly fluctuating conditions at hydrothermal vents. Our data provide first insights into a largely under-investigated part of microbial carbon cycling at hydrothermal vents and reveals potential roles of known and yet unknown heterotrophic microorganisms in these systems.

Method steps

1. Pipeline used: https://www.ebi.ac.uk/metagenomics/pipelines/4.1