Temperature control of acterial carbonmineralization processes in marine sediments

Abstract

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.