Grazers play different roles in the microbial loop of oligotrophic and eutrophic subtropical marine ecosystems

Abstract

In planktonic oceanic communities, heterotrophic bacteria (HB), Synechococcus spp. (SYNE), Prochlorococcus (PRO), and phototrophic picoeukaryotes (PPE) (<2 μm in size) are the numerically dominant groups. Recent advancements in understanding the physiology and ecology of these small cells have shed light on the significant roles that microbes play in global biogeochemical cycles and food webs. In order to investigate the interaction between HB, picophytoplankton and nanoflagellate grazers, estimated the effects of grazers on HB and picophytoplankton using the size-fractionation experiments at two stations in the subtropical western Pacific: Matsu (St. M) and Penghu (St. P). At St. M, there is clear evidence for top-down control of SYNE and PRO due to the relatively high abundance recorded in <2 μm filtered treatment within the 24-h sampling period. In contrast, at St. P, where higher temperatures and lower nutrient levels were observed compared to St. M, we suggest that the bottom-up factor (resource supply) is the primary driver of picophytoplankton growth. Furthermore, while PPE may not be the most abundant group, it is a dominant contributor to picophytoplankton biomass at both stations. It highlights the complex ecology of microbial communities in different subtropical Pacific oceanic regions, providing new insights into the important role these organisms play in ocean carbon cycling.