Abstract
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.
Highlights
Oligotrophic oceans harbor one of the largest microbiomes on earth [1]
These results indicate that influential members of the ciliate community (i.e., As an approximation of the effect size of the Kuroshio Current intrusion (KCI) on microbial communities, we calculated members co-occurring with many others and members being abundant in the community) these sensitive operational taxonomic unit (OTU) to account for 4.9% of the total community sequences
The ciliate community displayed a prominent provincial distribution across the northern South China Sea (NSCS), i.e., the northern and southern groups, strongly associated with the Kuroshio water fraction. Such a distribution pattern had been observed in the microphytoplankton community in this region [26]. This distribution pattern of ciliates along the Kuroshio fraction gradient suggests that the strong biogeographic provincialism of ciliates in the NSCS was shaped by the Kuroshio intrusion
Summary
Oligotrophic oceans harbor one of the largest microbiomes on earth [1]. Understanding the mechanisms that control microbial communities in oligotrophic oceans is important [2]. Previous studies have shown that the KCI influenced the nutrient biogeochemistry of the euphotic zone [20], induced enrichment of DOC [21], enhanced ammonia oxidation [22], influenced microbial activity (i.e., primary production, heterotrophic bacterial abundance, and bacterial production) [23], and changed the community structure of bacteria [24], picoplankton [25], and microphytoplankton [26] in the northern South China Sea. Microbes, e.g., bacteria and phytoplankton, can directly use resources from the KCI to support their growth and biogeochemical activities. By employing high-throughput sequencing of rRNA gene transcripts, this study is the first to provide information on the spatial distribution of ciliate communities and the mechanism underlying lateral advection of ocean currents to modulate ciliate communistudy is the first to provide information on the spatial distribution of ciliate communities ties in the boundary zone of the NSCS.
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