Spatial structure of eukaryotic ultraplankton community in the northern South China Sea

Biodiversity exploration of Formosa Ridge cold seep in the South China Sea using an eDNA metabarcoding approach

The Formosa Ridge, also named Site F, is an active cold seep marine ecosystem site that has been studied since it was discovered on the continental slope of the northeast South China Sea (SCS). However, few studies have focused on the eukaryotic diversity at Site F. Environmental DNA (eDNA) technology is a non-invasive method applied in biodiversity surveys with a high species detection probability. In the present study, we identified multi-trophic biodiversity using eDNA metabarcoding combined with multiple ribosomal RNA gene (rDNA) markers. We detected 142 phytoplankton, 90 invertebrates, and 64 fish species by amplifying the 18S rRNA gene V4 region, the 18S rRNA gene V9 region, and the 12S rRNA gene. The results elucidated dissimilar trends of different assemblages with depth. The diversity of phytoplankton and invertebrate assemblages markedly decreased with depth, whereas little change was observed within the fish assemblage. We comprehensively assessed the relationship between the three assemblages and environmental factors (temperature, salinity, depth, dissolved oxygen, and chlorophyll a). These factors strongly impacted on phytoplankton and invertebrates, but only slightly on fish. We inferred the finding might be due to fish having a strong migration capacity and wide distribution. This study indicates that eDNA metabarcoding with multiple markers is a powerful tool for marine biodiversity research that is able to provide technical support and knowledge for resource management and biodiversity protection efforts.

Biodiversity exploration in autumn using environmental DNA in the South China sea

The South China Sea (SCS) is an important part of the Indo-Pacific convergence zone, with high biodiversity and abundant marine resources. Traditional methods are primarily used to monitor biodiversity. However, a few studies have used environmental DNA (eDNA) metabarcoding to research the assemblage structure of the SCS. This study used eDNA metabarcoding to survey the SCS assemblage and its relationship with environmental factors over a month-long time-series (August 30th to September 30th, 2020) of seawater samples from the central part of the SCS (9°-20°86' N, 113°-118°47' E). 32 stations were divided into six groups (A, B, C, D, E, F) according to longitude. We collected water samples, extracted eDNA, and amplified 18S rRNA gene V4 region (18S V4), 18S rRNA gene V9 region (18S V9), and 12S rRNA gene (12S). Krona diagrams were used to show species composition. We identified 192 phytoplankton, 104 invertebrate, and 61 fish species from 18S V4, 18S V9, and 12S, respectively. Generally, the three assemblage structures exhibited an increase in species diversity with increasing longitude. Group E had the highest fish diversity. Groups F and C had the highest phytoplankton and invertebrate diversity, respectively. Canonical correspondence analysis showed that four factors (chlorophyll a, depth, salinity, and temperature) were correlated with assemblage structure. Chlorophyll a was the main environmental factor that affected fish, phytoplankton, and invertebrate assemblage structures; salinity was strongly correlated with fish and invertebrate assemblage structures; temperature was a key factor that impacted fish and invertebrate assemblage structures; and depth was strongly correlated with invertebrate assemblage structure. Our results revealed that eDNA metabarcoding is a powerful tool for improving detection rate and using multiple markers is an effective approach for monitoring biodiversity. This study provided information that can be used to enhance biodiversity protection efforts in the SCS.

Biogeographical Distribution and Community Assembly of Active Protistan Assemblages Along an Estuary to a Basin Transect of the Northern South China Sea

Marine protists are essential for globally critical biological processes, including the biogeochemical cycles of matter and energy. However, compared with their prokaryotic counterpart, it remains largely unclear how environmental factors determine the diversity and distribution of the active protistan communities on the regional scale. In the present study, the biodiversity, community composition, and potential drivers of the total, abundant, and rare protistan groups were studied using high throughput sequencing on the V9 hyper-variable regions of the small subunit ribosomal RNA (SSU rRNA) along an estuary to basin transect in the northern South China Sea. Overall, Bacillariophyta and Cercozoa were abundant in the surface water; heterotrophic protists including Spirotrichea and marine stramenopiles 3 (MAST-3) were more abundant in the subsurface waters near the heavily urbanized Pearl River estuary; Chlorophyta and Pelagophyceae were abundant at the deep chlorophyll maximum depth, while Hacrobia, Radiolaria, and Excavata were the abundant groups in the deep water. Salinity, followed by water depth, temperature, and other biological factors, were the primary factors controlling the distinct vertical and horizontal distribution of the total and abundant protists. Rare taxa were driven by water depth, followed by temperature, salinity, and the concentrations of PO₄³⁻. The active protistan communities were mainly driven by dispersal limitation, followed by drift and other ecological processes.

Spatial variation of phytoplankton community structure in Daya Bay, China

Daya Bay is one of the largest and most important gulfs in the southern coast of China, in the northern part of the South China Sea. The phylogenetic diversity and spatial distribution of phytoplankton from the Daya Bay surface water and the relationship with the in situ water environment were investigated by the clone library of the large subunit of ribulose-1, 5-bisphosphate carboxylase (rbcL) gene. The dominant species of phytoplankton were diatoms and eustigmatophytes, which accounted for 81.9 % of all the clones of the rbcL genes. Prymnesiophytes were widely spread and wide varieties lived in Daya Bay, whereas the quantity was limited. The community structure of phytoplankton was shaped by pH and salinity and the concentration of silicate, phosphorus and nitrite. The phytoplankton biomass was significantly positively affected by phosphorus and nitrite but negatively by salinity and pH. Therefore, the phytoplankton distribution and biomass from Daya Bay were doubly affected by anthropic activities and natural factors.