The significance of heterotrophic bacteria in the microbial food web and carbon cycle in the northern East China Sea

Abstract

The quantification of heterotrophic bacteria parameters and related physical-chemical-biological factors is essential for understanding the relative importance of the microbial food web and the carbon cycle in marine ecosystems. In this study, we explored changes in the role of the microbial loop in the northern East China Sea (ECS) from both fishery and carbon cycling perspectives. Bacterial abundance (BA), production (BP), and respiration rates (BR) were measured seasonally from 2020 to 2022 in February (winter), May (spring), August (summer), and November (autumn). The highest BP (171±131 mg C m-2 d-1) was recorded in the summer. Moreover, elevated chlorophyll-a and primary production (PP) were observed, particularly with the highest dissolved organic carbon (DOC) concentrations among all seasons. In the northern ECS, an influx of Changjiang diluted water (CDW) during the summer has been observed annually. This indicates that nutrient input through CDW stimulates phytoplankton biomass and PP, supporting BP. The supply of allochthonous DOC is also increased by CDW, further supporting summer BP. Consequently, the northern ECS, unlike adjacent mid-latitude areas (Yellow Sea, East Sea), exhibits high BP exclusively during the summer season. The increasing importance of BP led to a higher BP:PP ratio (0.33±0.27) from 2020 to 2022 compared to the global BP:PP ratio (<0.20), with BP contributing up to 40% of the total basal production (BP+PP) in summer. Increased carbon flow through the microbial food web can decrease overall food web efficiency, likely contributing to the fluctuations observed in fishery resources in the northern ECS in recent decades. Because the bioavailability of DOC in the northern ECS varies with CDW inflow intensity and the input of recalcitrant terrestrial DOC stimulates BR, higher bacterial carbon demand (BCD) and lower bacterial growth efficiency (BGE) occur. Thus, the northern ECS appears to be a heterotrophic system (BCD:PP=3.6±1.6), where heterotrophic bacteria recycle a significant amount of allochthonous DOC to CO2 through respiration. Based on these findings, enhancement of the microbial food web in marine ecosystems can impact qualitative and quantitative variation in fisheries resources and carbon sequestration capacity.