Effects of temperature and salinity on microbial degradation of bacterial necromass in urban river sediments in outdoor mesocosm experiments

Microorganisms play a key role in the functioning of healthy river ecosystems because they consume carbon and nutrients from dead biomass derived from algae and other higher organisms, or "necromass", to feed it back into the food web. This so-called microbial loop is known to be substantial in aquatic systems, but it is so far unknown to which extent microorganisms perform self-recycling, i.e. recycling of necromass derived from microorganisms, and how this process is affected by multiple stressors such as increased temperature and salinity. In this study, we investigated microbial self-recycling in the sediment of the urban river Boye within a food-web context before, during, and after a period of increased temperature and salinity using the outdoor flow-through mesocosm system "ExStream". Rates of self-recycling were measured in additional microcosms with sediment and water from ExStream as an increase in 13CO2-concentration over time resulting from the microbial degradation of 13C-labelled microbial necromass, which was offered either in the form of intact but dead Escherichia coli cells, or lysed E. coli cells. The results showed that microbial self-recycling was highest in the first days of incubation suggesting that microbial necromass is an easily biodegradable carbon source. Increased salinity had no effect, but increased temperature or temperature and salinity strongly increased the rate of whole cell-necromass recycling compared to the unstressed control, while it had no effect on lysed cell-necromass recycling. After stressor removal, rates of self-recycling were not distinguishable from the unstressed control, showing the resilience of this community function. The compositions of the necromass-degrading communities were highly similar and little affected by stressor increase or release but changed during the course of the experiment. This indicated that both necromass types stimulated growth of the same organisms and that stressor levels were rather low for microorganisms and dominated by the effects of the seasonal variation in the Boye. The study suggests that microbial communities in urban rivers exposed to moderate levels of multiple anthropogenic stressors will be rather stressor-resistant and show fast recovery of community functioning.