Abstract
The deep chlorophyll maximum (DCM) harbours diverse microbial communities and regulates carbon fixation and sequestration in marine ecosystems. Although variations in the DCM microbial community diversity and composition across oceans are reported, we still have a limited knowledge about the ecological mechanism driving the biogeography of DCM microbial communities and their contributions to ecosystem functioning. Here, based on DNA-sequencing data (16S rRNA and 18S rRNA genes) with two size fractions collected from coastal to offshore regions of the South China sea (SCS), we found that both size-fractionated bacterial and protistan communities in the DCM exhibited a distance-decay pattern, which was strongly governed by deterministic processes (62.7%-72.2%) and mainly explained by environmental factors (34.1%-37.7%). Under warmer temperature and fresher salinity levels, the decreased abundance of protistan parasites induced the weakening parasitic interactions with other protists, whereas the increased abundance of protistan consumers enhanced the phagocytic interactions with bacteria. Such reconstruction of microbial communities induced by environmental variations was thought to constrain particulate organic carbon (POC) flux, as revealed by the partial least square regression model illustrating a strong correlation between the composition of functional protistan groups and POC content of DCM (R-2 = 0.48; r = 0.75; p < 10(-6)). This work highlights the importance of environmental filtering in shaping DCM microbial communities and extends our understanding of the declined carbon flux of the DCM in the SCS under future scenarios. Read the free Plain Language Summary for this article on the Journal blog.