Sun P., Zhang S.L., Wang Y. and Huang B.Q.. 2021. Microorganisms, 9(5):1104.

Kuroshio Current intrusion (KCI) has significant impacts on the oceanographic conditions and ecological processes of the Pacific-Asian marginal seas. Little is known to which extent and how, specifically, the microzooplankton community can be influenced through the intrusion. Here, we focused on ciliates that often dominated the microzooplankton community and investigated their communities using high-throughput sequencing of 18S rRNA gene transcripts in the northern South China Sea (NSCS), where the Kuroshio Current (KC) intrudes frequently. We first applied an isopycnal mixing model to assess the fractional contribution of the KC to the NSCS. The ciliate community presented a provincial distribution pattern corresponding to more and less Kuroshio-influenced stations. Structural equation modeling revealed a significant impact of the KCI on the community, while environmental variables had a marginal impact. KCI-sensitive OTUs were taxonomically diverse but mainly belonged to classes Spirotrichea and Phyllopharyngea, suggesting the existence of core ciliates responding to the KCI. KCI-sensitive OTUs were grouped into two network modules that showed contrasting abundance behavior with the KC fraction gradient, reflecting differential niches (i.e., winner and loser) in the ciliate community during the Kuroshio intrusion scenarios. Our study showed that the Kuroshio intrusion, rather than environmental control, was particularly detrimental to the oligotrophic microzooplankton community.

Figure 1. (A) The ciliate co-occurrence network. OTUs are colored by the KCI sensitive-OTUs types. Red and blue represent the sensitive OTUs in more and less Kuroshio-influenced stations, respectively. Shaded areas represent the network modules that KCI-sensitive OTUs accumulated. (B,C) Qualitative taxonomic composition of the KCI-sensitive modules is reported as proportional OTUs numbers (B) and relative abundance (C); (D) Regression relationship between R_K and cumulative relative abundance of KCI-sensitive modules. M: more Kuroshio-influenced stations; L: less Kuroshio-influenced stations.