Nutrient enrichment decreases dissolved organic carbon sequestration potential of tropical seagrass meadows by mediating bacterial activity

Abstract

Dissolved organic carbon (DOC) pool in seawater plays an important role in long-term carbon sequestration in seagrass meadows. Microbial activities (microbial communities and their extracellular enzymes) are the key determining factors of DOC decomposition and sequestration potential, and are affected by nutrient enrichment. However, there is little information on the response of microbial communities and carbon-degrading extracellular enzymes to nutrient loading within seagrass meadows, limiting our understanding of the driving mechanism of DOC decomposition under nutrient enrichment. Here, microbial communities (including bacteria and fungi) and representative extracellular enzyme activity (EEA) in three seagrass meadows with different nutrients levels were investigated across four seasons. Water temperature was the driving factor influencing the seasonal dynamics of EEAs. In addition, the hydrolysis rates of chitinase, β-glucosidase, and α-glucosidase were significantly higher at a high nutrient loading seagrass meadow than at a low nutrient loading meadow. Furthermore, higher relative abundance of bacterial groups, such as Actinobacteria, Bacteroidetes, Cellvibrionale, and Verrucomicrobia were in according with enhanced EEAs, suggesting that these K-strategists were likely involved in enzyme production and the subsequent remineralization of organic matter in seagrass meadows. In contrast with the bacterial community, fungal communities were not sensitive to nutrient concentrations, and there was no strong association between the given fungal groups and EEA. This may be attributed to the low taxonomic resolution of marine planktonic fungi or the minor role of fungi in EEA production. Overall, these results suggested that nutrient loading enhanced EEA levels, modified bacterial rather than fungal communities, and consequently accelerated DOC remineralization, thereby reducing DOC contribution potential of seagrass ecosystems to long term carbon sequestration.