| 内容提要: |
Plankton forms the base of the food chain in the marine ecosystem and have a profound effect on the global biogeochemical cycles. However, one member of this community, the planktonic fungi, have received less attention and understudied with respect to their diversity and ecology. The South China Sea (SCS) which is a semi-enclosed tropical marginal sea in Southeast Asia is a unique ecosystem. Although the Northern SCS is oligotrophic especially towards its centre, the unique combinations of environmental conditions such as monsoon-enhanced wind-induced mixing, ocean currents and regions of upwelling bring in intermittent nutrient-rich conditions. We undertook a large-scale study to explore the abundance and diversity of planktonic fungi in the SCS during a cruise to the region in May-June 2016. During the entire study period the range of wind speed, sea-surface water temperature, and salinity were 3.9-14.7m/s, 26-30 °C, and 31.7-34.5‰, respectively. Using quantitative PCR (qPCR) and Illumina Sequencing, we determined the fungal abundance and diversity, respectively, across 17 stations on 7 transects (A, D-F, K, Q, N) at different depths. The qPCR results revealed the ubiquitous presence of fungi across all stations and depths. The Fungi-specific 18S rRNA gene abundance varied from 6.0x104 to 7.0x107 copies/L seawater at the surface. The abundances between transects (except D) did not differ significantly (P>0.05, ANOVA), which might be attributed to the various circulatory mechanisms and ocean currents active in the SCS. The abundance exhibited an increasing trend from coastal Hainan (Q7 station) to the offshore station SEATS with its maximum at the offshore station D4 located South of Taiwan. One possible explanation for the high abundance in these offshore stations could be their location in the path of the major water currents (Ekman drift, South China Sea Warm Current, Kuroshio Current, and cyclonic eddy southwest of Dongsha Island) flowing towards Taiwan and the Luzon strait which may bring in nutrient-rich conditions. Besides, low temperature (28 °C) and high salinity (34‰) of the surface water at station D4 indicate region of upwelling resulting in increased abundance. Further, a decreasing trend in fungal abundance was observed with increasing depth (surface average abundance = 5x106 18S copies/L, 3000 m depth abundance = 5 x103 18S copies/L). Sequence analysis of D4, K7 and SEATS revealed a total number of 875 (0.97 cut-off) OTUs that were classified into five phyla namely Ascomycota, Basidiomycota, Glomeromycota, Rozellomycota and Mortierellomycota. A majority of sequences (> 95% of total sequences) retrieved from D4, SEATS could not be assigned to any known fungal phyla, possibly indicating the incompleteness of the fungal sequence database and/or the presence of novel and undescribed lineages in these offshore stations. Based on phylogenetic analysis, many of these taxonomically unassigned sequences were closely related to phyla Ascomycota, Basidiomycota, Glomeromycota and Rozellomycota. On the other hand, K7 (nearshore station) exhibited the highest diversity with low percentage of unassigned and unidentified fungal sequences (21.7% of total sequences). These results suggest that the fungal diversity in the offshore seawaters are mostly undescribed and are subject of further exploration. Further, FUNGuild analysis clustered the identified phyla into saprotrophs, symbiotrophs and pathotrophs. In conclusion, our study expands the current knowledge on planktonic fungi, their distribution pattern and highlights the possible mechanism of their abundance patterns in the SCS. |