Eutrophication influences diversity and community-level change points of mycoplankton in subtropical estuaries

Abstract

Mycoplankton are essential for biogeochemical cycles in natural water bodies. However, the distribution of the mycoplanktonic community and its community-level change points in subtropical estuaries remain unclear. In this study, we employed 18S rRNA high-throughput sequencing to explore the mycoplanktonic community structure and environmental thresholds in the Dafengjiang River Estuary. Agaricostilbomycetes and Saccharomycetes are the dominant classes in the Dafengjiang River Estuary. The alpha and beta diversities of the mycoplanktonic communities showed significant differences (p < 0.05) across the seasons. Distance-based redundancy analysis (db-RDA) suggested that the main driver of the total community was eutrophication level, and the key factors for oligotrophication, medium eutrophication, and high eutrophication were dissolved inorganic phosphorus (DIP), ammonium (NH₄ ⁺), and chlorophyll-a (Chl-a), respectively. Threshold Indicator Taxa Analysis (TITAN) exhibited the community-level change points of mycoplankton along the eutrophication gradients were DIP (6-15.5 μg/L), NH₄ ⁺ (61.5-62.5 μg/L) and Chl-a (2.55-9.3 μg/L), respectively. Random forest analysis revealed that Rhizophydium, Aspergillus and Vanrija were sensitive to eutrophication status and could serve as bioindicator genera for environmental changes. Overall, our study enhances our understanding of the diversity and community-level change points of mycoplankton in subtropical estuaries and lays the theoretical foundation for the environmental monitoring of subtropical estuaries.

Keywords: 18S rRNA gene; TITAN; community-level change points; mycoplankton; subtropical estuary.

Figure 1

Localization of the sixteen sampling locations in the Dafengjiang River Estuary.

Figure 2

(A) Mycoplanktonic community composition at different years (2018 and 2020) and seasons in Dafengjiang River Estuary were evaluated based on relative abundance of different taxa at the class level. (B) The alpha diversity (Shannon) at different years (2018 and 2020) and seasons. The alpha diversity shown by boxplot. SP, spring; SU, summer; FA, fall; WI, winter.

Figure 3

Distance-based redundancy analysis (db-RDA) ordination plots of the relationship between mycoplanktonic communities and environmental factors. (A) The total community; (B) Oligotrophication; (C) Medium eutrophication; (D) High eutrophication; (E) Relationship between regression coefficient R2 and environmental variables. SP, spring; SU, summer; FA, fall; WI, winter. Temp, temperature; pH, pH; DO, dissolved oxygen; DIN, dissolved inorganic nitrogen; DIP, dissolved inorganic phosphorus; TOC, total organic carbon; COD, chemical oxygen demand; TN, total nitrogen; TP, total phosphorus; EI, eutrophication index. *, p < 0.05; **, p < 0.01; ***, p < 0.001.

Figure 4

Threshold Indicator Taxa Analysis (TITAN) sums of negative (z-) and positive (z+) of responding species to all candidate change points of key environmental factors. The black solid line and red dotted line depict the cumulative frequency distributions of change points for z- and z + taxa groups, respectively. (A) Oligotrophication; (B) Medium eutrophication; (C) High eutrophication.

Figure 5

Random Forest classification of the top 20 important genera. Left: Gini index-based analysis of the top 20 taxa, indicating their importance in distinguishing distinct eutrophication levels. Middle: abundance levels of the top 20 genera. Right: Pearson correlations between the relative abundances of the top 20 genera and environmental parameters. Temp, temperature; pH, pH; DO, dissolved oxygen; DIN, dissolved inorganic nitrogen; DIP, dissolved inorganic phosphorus; TOC, total organic carbon; COD, chemical oxygen demand; TN, total nitrogen; TP, total phosphorus. *, p < 0.05; **, p < 0.01; ***, p < 0.001.