Bruchert, Volker
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Bruchert, Volker
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Bruchert, Volker
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Item type:Publication, Temperature control of acterial carbonmineralization processes in marine sediments(2009-09-18); ; ; The present work analyzes the potential impact of anticipated global warming on the bacterial carbon cycling in marine shelf sediments. Current changes of the marine biological carbon cycle in response to climate warming in different regions of the world ocean are closely coupled to the response of bacteria to environmental temperatures. We investigated the correlation between ambient temperatures and the physiological adaptations, in terms of energy metabolism, of sulfatereducing bacteria (SRB) in polar, temperate and tropical sediments. In short-term sediment incubations in a temperature gradient block, sulfate-reduction rates (SRR) were measured using 35S-sulfate. Resulting temperature response profiles were used to examine the competitiveness of SRB, in terms of relative SRR of maximal potential rates, the temperature dependence for energy metabolism of SRB and the correlation of cardinal temperatures of sulfate reduction and sediment temperatures. We observed that SRB in polar sediments are more competitive than their counterparts in warmer habitats at similar low temperatures. Although metabolic rates in warmer latitudes exhibited higher temperature dependence below 8-18Ã °C, optimal temperature conditions for sulfate reduction in these environments are closer to their ambient temperatures resulting in a higher competitiveness at in situ conditions. Together, these observations imply that biography and, consequently, environmental temperature variability play an important role in the physiological selection and divergence of microbiota in different latitudes. Over a long-term (2 year) temperature incubation experiment, we measured 35S-SRR in a temperature gradient block and used CARDFISH of sulfate-reducing bacteria to describe the temperature control of carbon mineralization rates via sulfate reduction in Arctic marine sediments in comparison to a temperate habitat. This study is innovative in that we examine the consequences of temperature shifts by investigating the activity and the population dynamics of sulfatereducing bacteria. We found that the investigated Arctic sediment hosts a sulfate-reducing bacterial community that changes rapidly and is not capable of accommodating long-term temperature upshifts as high as 20 Ã °C. Lower bulk sulfate reduction rates at 20Ã °C compared to 0Ã °C and 10Ã °C in arctic sediments are indicative of the strong temperature effect on the active SRB community. In contrast, the community in a temperate habitat appears to be largely insensitive to temperature changes, whether down or up, and appears to contain abundant psychrotolerant/mesophilic bacteria that outcompete specialized psychrophiles even during the cold season. We also investigated rates and temperature optima of extracellular enzymatic hydrolysis as well as the dynamics of key intermediates in anoxic carbon degradation pathways, and their relationship to sulfate reduction, the terminal step in carbon cycling. Following 24 months incubation at 0Ã °C, 10Ã °C, and 20Ã °C, we observed increasing concentrations of dissolved organic carbon (DOC) and total dissolved carbohydrates, particularly at higher temperatures, as well as limitation in sulfate reduction rates. Together, our results showed increasing decoupling between hydrolysis and terminal oxidation of organic matter via sulfate reduction. The decline of sulfate reduction rates, particularly in the Arctic sediments, suggests an inability of the fermentative community to transform refractory DOC to substrates suitable for sulfate reducers.doctoral thesis297 105 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Microbial ecology of anaerobic carbon mineralization in Namibian shelf sediments(2008-01-25); ; ; This PhD thesis is an essential component of the 'Namibia Gas' (NAMIBGAS) project, which aims to improve the understanding of the rate and fluxes of hydrogen sulfide and methane to the sediment surface and into the water column on the Namibian shelf, which is one of the most productive upwelling systems on Earth. Organic matter degradation in the sediment drives hydrogen sulfide production, the maintenance of anoxia, and methane formation. Therefore, the primary aims of the thesis are to study carbon transformation processes within the sediment, determine the reactivity of organic carbon in the sediment and, investigate the control of microbial community structure and activity by the amount and accessibility of carbon sources. The emphasis is on linking microbial community structure (identity) to their function (activity) to provide new insights into the microbial ecology that controls carbon turnover in these upwelling sediments. By studying the stepwise degradation of organic carbon a complete process overview of organic carbon mineralization could be obtained.Firstly, the diagenetic transformation of dissolved organic carbon (DOC), in particular dissolved carbohydrates, was studied by using both biogeochemical methods and molecular techniques. The bulk sediment composition, pore water chemistry, polysaccharide hydrolysis rates, 35S-sulfate reduction rates, and the abundance of active bacteria involved in the initial and terminal processes of organic carbon degradation within the top 15 cm of the sediment from two sampling stations were determined (Chapter 2). Secondly, the diversity of bacteria from the same two sampling stations within the top 12 cm of the sediment, using the 16S rRNA library approach were investigated (Chapter 3). The central question was whether shifts of the community structure of bacteria involved in the major carbon transformation steps of hydrolysis, fermentation, and terminal oxidation are reflected in changes of biogeochemical rates. Finally, the effect of a sudden high input of DOC into the sediment in the form of high molecular weight substances (the polysaccharide laminarin) and low molecular weight substances (lactate and acetate) on the metabolic activity and community structure of bacteria involved in initial hydrolytic and fermentation steps and terminal oxidation was investigated during two separate experiments (Chapter 4). Substrate addition simulates the high input of organic matter into the sediment after a phytoplankton bloom, following upwelling events.doctoral thesis577 120