Impact of environmental gradients on the abundance and diversity of planktonic fungi across coastal habitats of contrasting trophic status

Fungal communities in the coastal waters have long been known to be dynamic with a significant role in organic matter cycling. However, the effects of environmental gradients on their community structures are poorly described. Here we studied three coastal sites off the South China Sea, namely Pearl River Estuary (PE), Shenzhen Bay (SB), and Daya Bay (DB) with contrasting trophic status and heterogenous local influences. Environmental analysis of these sites suggested higher nutrient and low salinity levels at PE and SB with wide variability compared to DB. Average molecular abundances (18S rRNA gene copy numbers) at sites PE (1.05 +/- 0.27 x 10(7) copies L-1) and SB (1.2 +/- 0.69 x 10(7) copies L-1) were similar and significantly higher (P < 0.05) than that at site DB (5.5 +/- 9.5 x 10(5) copies L-1). Although planktonic fungi were molecularly abundant at the three sites, live fungal bio-mass based on ergosterol assay was detected only at some stations of PE and SB. Both molecular abundance and live biomass were significantly correlated with chemical oxygen demand, nutrients, and phytoplankton biomass, supporting their role in detritus turnover. The fungal communities were unprecedently diverse with the ubiquitous dominance of Dikarya and the occasional predominance of Glomeromycota, Mucoromycota, Mortierellomycota, and Chytridiomycota. A total of 24 classes, 46 orders, 71 families, 59 genera, and eight species were classified within the eight detected phyla, including the new finding of ascomycetous class Geoglossomycetes in coastal waters. Salinity and nitrate were the significant (r(2) = 0.70, P < 0.05) factors that determined the beta-diversity of fungal communities. Overall, this study suggests that although planktonic fungi are ubiquitous in coastal habitats, their molecular abundances and diversities (both alpha and beta) are significantly determined by environmental gradients, particularly the salinity, COD and nitrate levels of coastal waters. (C) 2019 Elsevier B.V. All rights reserved.