# Warming Causes a Decline in Baltic Sea Coastal Sediment Microbial Abundance

## TL;DR

Long-term warming in the Baltic Sea has reduced sediment microbial cell abundance and altered microbial communities, which could impact the marine ecosystem.

## Contribution

This study provides empirical evidence that long-term warming decreases sediment microbial abundance and alters biogeochemical zones.

## Key findings

- Sediment cell abundances decreased in the heated bay compared to the control bay.
- Prolonged warming shallows sediment biogeochemical zones and related microbial communities.
- Live/dead cell ratios showed no significant differences between the heated and control bays.

## Abstract

Long‐term ocean warming impacts the marine environment, and these effects will be exacerbated by future climate change affecting, e.g., biogeochemical processes and microbial communities. However, how the sediment microbial cell abundance and live/dead ratio respond to warming is poorly understood. In this study, sediment core samples were collected from a Baltic Sea bay artificially heated on average 5°C for > 50 years above a nearby (control) bay unaffected by the heating. Contrary to the expected increased productivity in the heated bay, qPCR‐based sediment cell abundances showed decreased cell numbers along the sediment depth gradient in the heated bay compared to the control bay. This could reflect that a portion of the cells' metabolic energy was diverted to a heat related stress response rather than being used for replication. In addition, live/dead cell ratios showed no clear differences in either bay suggesting the majority of the cells were alive. Finally, sediment depth gradient 16S rRNA gene sequencing confirmed previous studies, showing that prolonged warming shallows sediment biogeochemical zones and related microbial communities. In conclusion, future climate change related warming will likely decrease microbial cell abundances that form part of the food web base, potentially impacting the entire ecosystem.

Long‐term warming in the Baltic Sea coastal area has led to a decrease in sediment microbial cell abundance, alongside a shallowing of sediment biogeochemical zones and associated microbial communities. These changes may become more pronounced in marine sediments under future climate change‐related warming.