The Baltic Sea from the present to future : microbial carbon & nutrient cycling in a changing climate

Abstract: Climate Change is caused by the accelerated increase of anthropogenic greenhousegas emissions to the atmosphere and affects all ecosystems on our planet. A resultof higher CO2 uptake by the oceans as well as an increase of heat trapped in theatmosphere leads to, for example acidification, stratification, sea-level rise, oxygenloss, and temperature increase of the earth’s waterbodies. The IntergovernmentalPanel on Climate Change (IPCC) predicts the earth’s surface temperature to risebetween 1.0-5.7°C by the year 2100 and ocean temperatures are predicted to rise byup to 2.0°C.This thesis focuses on the effects of environmental changes on microbes and theirfunctions in coastal Baltic Sea sediments and overlying bottom-waters. The studiesexamine potential effects of ongoing climate change in combination with coastaleutrophication, as well as long-term warming due to e.g. climate change within anatural fluctuating system and a laboratory based incubation experiment.Investigation of coastal sediment and overlying bottom-waters showed thatpotential future changes on bacterial communities due to eutrophication incombination with climate change relies on the water depth and oxygen supply. Inaddition, the study of a natural seasonal fluctuating and long-term artificially heatedcoastal bay (compared to an unaffected control bay) gave insights into how theecosystem might react to future climate change. On one hand, bottom waters in theheated bay showed decreased bacterial diversity, suspended seasonal patterns pluselevated and prolonged cyanobacterial blooming. On the other hand, surfacesediment communities in the heated bay had an altered microbial community withdecreased seasonal variation and higher diversity likely due to a shallowing ofgeochemical layers. Furthermore, increased energy production occurred althoughhigher stress RNA transcripts suggested that the microbial community’stemperature optima were below that of the water. Nevertheless, incubationexperiments showed that exposure to short-term elevated temperatures shifted thecontrol bay microbial community closer to that of the heated bay with a similarresponse on RNA level at higher temperatures (28 °C).In summary, this thesis provides new insights into ongoing and likely future climatechange effects on coastal microbial communities, which are key players for nutrientandenergy cycling of the marine ecosystem.