Long-Term Warming of Baltic Sea Coastal Waters Affects Bacterial Communities in Bottom Water and Sediments Differently

Abstract

Coastal marine ecosystems are some of the most diverse natural habitats while being highly vulnerable in the face of climate change. The combination of anthropogenic influence from land and ongoing climate change will likely have severe effects on the environment, but the precise response remains uncertain. This study compared an unaffected “control” Baltic Sea bay to a “heated” bay that has undergone artificial warming from cooling water release from a nuclear power plant for ~50 years. This heated the water in a similar degree to IPCC SSP5-8.5 predictions by 2100 as natural systems to study temperature-related climate change effects. Bottom water and surface sediment bacterial communities and their biogeochemical processes were investigated to test how future coastal water warming alters microbial communities; shifts seasonal patterns, such as increased algae blooming; and influences nutrient and energy cycling, including elevated respiration rates. 16S rRNA gene amplicon sequencing and geochemical parameters demonstrated that heated bay bottom water bacterial communities were influenced by increased average temperatures across changing seasons, resulting in an overall Shannon's H diversity loss and shifts in relative abundances. In contrast, Shannon's diversity increased in the heated surface sediments. The results also suggested a trend toward smaller-sized microorganisms within the heated bay bottom waters, with a 30% increased relative abundance of small size picocyanobacteria in the summer (June). Furthermore, bacterial communities in the heated bay surface sediment displayed little seasonal variability but did show potential changes of long-term increased average temperature in the interplay with related effects on bottom waters. Finally, heated bay metabolic gene predictions from the 16S rRNA gene sequences suggested raised anaerobic processes closer to the sediment-water interface. In conclusion, climate change will likely alter microbial seasonality and diversity, leading to prolonged and increased algae blooming and elevated respiration rates within coastal waters.

Versatile methanotrophs form an active methane biofilter in the oxycline of a seasonally stratified coastal basin

“…Currently, coastal ecosystems are facing warming, enhanced eutrophication, increased stratification and expansion of hypoxic or anoxic zones ( (Breitburg et al, 2018;Seidel et al, 2022). This will likely alter methane cycling and possibly result in larger methane emissions to the atmosphere.…”

“…Coastal and estuarine-shelf ecosystems contribute up to 75 % of marine methane fluxes to the atmosphere (Upstill-Goddard and Barnes, 2016; Dean et al ., 2018; Rosentreter et al ., 2021). Currently, coastal ecosystems are facing warming, enhanced eutrophication, increased stratification and expansion of hypoxic or anoxic zones ((Breitburg et al ., 2018; Seidel et al ., 2022). This will likely alter methane cycling and possibly result in larger methane emissions to the atmosphere.…”

Long-term warming modulates diversity, vertical structuring of microbial communities, and sulfate reduction in coastal Baltic Sea sediments

“…The other was an unaffected nearby bay with similar surrounding conditions. Previous studies in this experimental system show the effects of long-term warming on bottom water and surface sediment microbial communities, demonstrating an increased relative abundance of Picocyanobacteria in the heated bay in summer months and decreased overall bacterial alpha diversity ( Seidel et al, 2022a ). In addition, there was an increase in alpha diversity in the heated bay surface sediments and a shift of the microbial community toward the seafloor resulting in the thinning of the geochemical zones ( Seidel et al, 2022b ).…”

“…The control bay was chosen as it was as close as possible to the heated bay without being influenced by it. The two bays have been used in previous studies with further information therein ( Seidel et al, 2022a , b ).…”

“…For example, Hicks et al (2018) show that microbial communities within coastal sediments change their composition and activity with increasing temperature that in turn, also affects nutrient availability and will drive future changes in marine biogeochemical cycling. Within coastal benthic waters, higher temperatures will likely lead to a decrease in bacterial diversity ( Seidel et al, 2022a ). In contrast, higher oxygen consumption due to various factors such as temperature and eutrophication compresses the redox layers within sediments ( Mahmoudi et al, 2014 ) resulting in increased bacterial diversity ( Seidel et al, 2022b ).…”