Abstract
We investigated the impact of temperature on the microbial turnover of organic matter (OM) in a hydrothermal vent system in Guaymas Basin, by calculating microbial bio- and necromass turnover times based on the culture-independent D:L-amino acid model. Sediments were recovered from two stations near hydrothermal mounds (<74°C) and from one cold station (<9°C). Cell abundance at the two hydrothermal stations dropped from 108 to 106 cells cm-3 within ∼5 m of sediment depth resulting in a 100-fold lower cell number at this depth than at the cold site where numbers remained constant at 108 cells cm-3 throughout the recovered sediment. There were strong indications that the drop in cell abundance was controlled by decreasing OM quality. The quality of the sedimentary OM was determined by the diagenetic indicators %TAAC (percentage of total organic carbon present as amino acid carbon), %TAAN (percentage of total nitrogen present as amino acid nitrogen), aspartic acid:β-alanine ratios, and glutamic acid:γ-amino butyric acid ratios. All parameters indicated that the OM became progressively degraded with increasing sediment depth, and the OM in the hydrothermal sediment was more degraded than in the uniformly cold sediment. Nonetheless, the small community of microorganisms in the hydrothermal sediment demonstrated short turnover times. The modeled turnover times of microbial bio- and necromass in the hydrothermal sediments were notably faster (biomass: days to months; necromass: up to a few hundred years) than in the cold sediments (biomass: tens of years; necromass: thousands of years), suggesting that temperature has a significant influence on the microbial turnover rates. We suggest that short biomass turnover times are necessary for maintance of essential cell funtions and to overcome potential damage caused by the increased temperature.The reduced OM quality at the hyrothemal sites might thus only allow for a small population size of microorganisms.
Highlights
Microorganisms buried in the seabed play a crucial role in the degradation of organic matter (OM) in the sediments
The Gly:Ser and the GlcN:GalN ratios, indicate that the amino acids and amino sugars in the sediment were primarily of prokaryotic origin. This indicates that microorganisms have reworked the amino acids and amino sugars in the original OM from the water column by forming new microbial biomass (Keil and Fogel, 2001) during OM sedimentation in the water column and after its incorporation into the sediment
This observation is consistent with previous studies that have found similar GlcN:GalN ratios (Benner and Kaiser, 2003; Lomstein et al, 2006; Niggemann and Schubert, 2006) and Gly:Ser ratios (Langerhuus et al, 2012), and related these to the bacterial origin of the amino sugars and the amino acids
Summary
Microorganisms buried in the seabed play a crucial role in the degradation of organic matter (OM) in the sediments. Guaymas Basin offers a unique opportunity to study deeply buried organisms in high temperature environments because the active seafloor spreading in this marginal rift basin co-occurs with the rapid deposition of OM rich sediments from the overlaying highly productive waters (Calvert, 1966). In Guaymas Baisn, the high productivity of the water column together with significant terrestrial sediment input result in a several hundred meter thick sediment cover (Einsele et al, 1980). Basaltic magma intrudes as sills into the sediment, heating the pore water and creating hydrothermal fluids that percolate through the thick OM rich sediment and expose the sediment to temperatures of >300◦C (Einsele et al, 1980; Lonsdale and Becker, 1985; Von Damm et al, 1985)
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