Transitional ecosystem states in coastal waters undergoing changing oxygen conditions revealed by coupled geochemical and microbial dynamics

Background Coastal oxygen depletion is a major driver of changes in sediment biogeochemistry and microbial functioning. However, oxygen states vary over space and time, which results in temporal dynamics that are so far under explored. This study examined three sites with differing oxygen regimes across a year to investigate how temporal oxygen variability shapes surface sediment microbial communities and activity. Methods Coastal Baltic Sea surface sediments were sampled over one year at three sites with differing long-term oxygen regimes—one site that was predominantly oxic, one with predominantly low-oxygen conditions, and one intermediate site with fluctuating oxygen availability. Sediment biogeochemistry, 16S rRNA gene amplicon sequencing, and metatranscriptomic analyses were combined to assess temporal changes in microbial community composition and transcriptional activity. Results The results showed that long-term oxygen history was the primary factor structuring both microbial community composition and transcriptional profiles. The shallow site was characterized by mostly oxygenated conditions and exhibited relatively stable communities and transcriptional activity over time. The deep site was predominantly characterized by long-term low-oxygen conditions and was dominated by anaerobic metabolic patterns. In contrast, the intermediate site, that was exposed to more pronounced fluctuating oxygen conditions, showed the highest temporal variability in both community composition and RNA transcripts. This site also displayed stronger shifts in transcriptional activity over time, indicating a higher sensitivity of microbial processes to environmental variability compared to the other sites. Conclusion These results suggested that transitional oxygen habitats represented dynamic ecological states where microbial communities and functions rapidly responded to changing environmental conditions. Overall, oxygen availability strongly structured both microbial composition and activity in coastal sediments, with transitional habitats exhibiting enhanced temporal variability. These findings further indicated that environments experiencing fluctuating oxygen conditions may be particularly responsive to ongoing environmental change in coastal systems.