The dissolved oxygen variation in seagrasses is influenced by DOC excretion and its associated microbes

Abstract

Tropical seagrass beds usually exhibit pronounced diel variations in dissolved oxygen (DO) and dissolved organic carbon (DOC) mediated by primary production, respiration, and microbial activities. With near-future climate change scenarios predicting elevated ocean temperatures and more frequent extreme conditions, understanding the mechanisms that influence the DO and DOC dynamics in these ecosystems is crucial. This study examined two seagrass sites with distinct community compositions on Dongsha Island (South China Sea): a semi-closed lagoon dominated by Halodule uninervis, and an open coast dominated by Thalassia hemprichii. Metabolic rates and DOC fluxes were quantified using in situ benthic chambers and active microbial communities were characterized via amplicon sequencing of 16S rRNA transcripts. Seagrass beds predominantly exhibited net autotrophy in summer, shifting toward a heterotrophic state in winter, whereas unvegetated sediments remained net heterotrophic. DOC fluxes varied, with the semi-closed lagoon acting as a net DOC source and the open coast predominantly functioning as a DOC sink, correlating with microbial activity rather than with primary production. Diverse microbial taxa emerged, varying across locations and seasons, and correlated with factors such as organic carbon, temperature, and DO. A surge in microbial activity over diel cycles likely drove the observed night time decline in DO. The contrasting trends in community productivity and DOC production between the two sites were influenced by different seagrass communities and relevant environmental factors. As ocean temperatures continue to rise, the net heterotrophy and sulfidic conditions observed in the lagoon may represent the future for tropical seagrass ecosystems, underscoring the need to maintain coastal water quality and understand microbial community resilience.