Psychrophilic properties of sulfate-reducing bacteria in Arctic marine sediments

Microorganisms in the seabed of most of the global oceans live at temperatures close to zero degrees, and in the polar regions even below. Respiration rates of endogenous sulfate-reducing microorganisms in the polar seabed, however, are highest in the range of 18-30 degrees C and much reduced at colder temperature. In the past, this was seen as indicative of poor adaptation to their cold habitat. We challenge this interpretation, and tested which temperature range allowed sulfate reducers to grow, by adding moderate amounts of volatile fatty acids to high-arctic sediment. Initially, sulfate reduction was highest at 27 degrees C in both Svalbard and NE Greenland. But sulfate reduction was not sustainable at this temperature and decreased rapidly over time. Below 26 degrees C, however, sulfate reduction rates increased exponentially over time, indicating growth of sulfate-reducing microorganisms. We used the increase in the sulfate reduction rates over 4 d to calculate potential growth rates of the endogenous sulfate reducers as function of temperature. From growth rates and respiration rates, we could further calculate the growth yield, also as function of temperature. Highest growth rates were observed at 18 degrees C and growth yields peaked at even lower temperatures between 0 degrees C and 10 degrees C. The maximum growth yield at low temperature revealed a strong psychrophilic adaptation of the sulfate reducers in these Arctic sediments. The fact that growth yield was maximized at in situ temperature but maximum potential growth rate was not, is an indication that yield is the more important parameter for microbial competition in marine sediments.