Alexandriumpacificum is a typical toxic bloom-forming dinoflagellate, causing serious damage to aquatic ecosystems and human health. Many bacteria have been isolated, having algicidal effects on harmful algal species, while few algicidal bacteria have been found to be able to lyse A. pacificum. Herein, an algicidal bacterium, Shewanella Y1, with algicidal activity to the toxic dinoflagellate A. pacificum, was isolated from Jiaozhou Bay, China, and the physiological responses to oxidative stress in A. pacificum were further investigated to elucidate the mechanism involved in Shewanella Y1. Y1 exhibited a significant algicidal effect (86.64 ± 5.04% at 24 h) and algicidal activity in an indirect manner. The significant declines of the maximal photosynthetic efficiency (F_v/F_m), initial slope of the light limited region (alpha), and maximum relative photosynthetic electron transfer rate (rETRmax) indicated that the Y1 filtrate inhibited photosynthetic activities of A. pacificum. Impaired photosynthesis induced the overproduction of reactive oxygen species (ROS) and caused strong oxidative damage in A. pacificum, ultimately inducing cell death. These findings provide a better understanding of the biological basis of complex algicidal bacterium-harmful algae interactions, providing a potential source of bacterial agent to control harmful algal blooms.

The Beibu Gulf, a vital region for marine biodiversity and aquaculture, is increasingly affected by nutrient‐driven ecological shifts in the phytoplankton community. This study combined morphology and eDNA metabarcoding (18S rDNA V4) to investigate phytoplankton diversity and environmental drivers during summer and winter in the Beibu Gulf. Metabarcoding detected 3.5 times more phytoplankton species, contributing to higher species diversity and richness than morphology. Metabarcoding identified 200 phytoplankton genera from eight phyla, while morphology only identified 49 genera from six phyla. Both methods revealed different dominant phytoplankton communities. Bacillariophyta and Haptophyta dominated the phytoplankton community based on morphology, in summer and winter, respectively; meanwhile, Dinophyta dominated in both seasons under metabarcoding due to their high 18S rDNA copy number. Altogether, 83 HAB and/or toxic species were identified, among which 10 were dominant, suggesting a high risk of HAB outbreaks in the Beibu Gulf. Phytoplankton abundance increased from south to north and west to east in both seasons, following the high input of dissolved inorganic nitrogen (DIN) and silicate. Excess ammonium input can promote the dominance of Scrippsiella trochoidea and Heterocapsa circularisquama, positioning them as emerging HAB species, while excess DIN caused extreme phosphorus limitation and favored the dominance of Phaeocystis globosa in the Beibu Gulf. This study provided a comprehensive description of the influence of environmental drivers on the phytoplankton community in the Beibu Gulf.

Immobilized microbial materials can effectively remove pollutants from surface water, and a biochar/clay composite particle (BCCP) material is prepared with immobilized Flavobacterium mizutaii sp. and Aquamicrobium sp. to remove ammonia nitrogen (NH₄⁺–N) and petroleum hydrocarbons (PHCs). The results indicated that the optimal ratios of biochar, Na₂SiO₃ and NaHCO₃ were 15%, 3%, and 3%, and the adsorption process was found to be better described with the pseudo-second-order kinetic equation. The individual immobilization of Flavobacterium mizutaii sp. and Aquamicrobium sp. with sodium alginate–polyvinyl alcohol (PVA + SA) achieved 80% and 90% removal efficiencies for NH₄⁺–N and PHCs at the 10th d. The composite immobilization of two efficient bacteria could degrade 82.48% NH₄⁺–N and 74.62% PHCs. In addition, immobilization relieved the effects of temperature and salinity. This study can provide guidance for the application of immobilized microbial composite materials in natural water environments.

Being a biologically diversed hotspot in the global marine ecosystem, the Beibu Gulf is inhabited by a high diversity of fish and serves as a vital fishing ground in China. Due to continuous overfishing, the fishery resource has drastically declined in the Beibu Gulf. However, information about the ichthyoplankton assemblages in this area is still lacking. In this present study, ichthyoplankton diversity, spatial and temporal distribution patterns, and assemblage structures were examined using the specimens collected in the late summer–autumn and winter of 2022 in the Beibu Gulf, and the relationship between ichthyoplankton assemblage and environmental variables was studied. A total of 117 ichthyoplankton taxa, belonging to 13 orders and 42 families, were recorded. The most abundant families were Gerreidae, Leiognathidae, and Sillaginidae in late summer–autumn, accounting for 38.74%, 27.95%, and 9.94%, respectively. Sparidae, Platycephalidae, and Sillaginidae were the most abundant families in winter, accounting for 34.03%, 17.15%, and 8.20%, respectively. Cluster analysis identified five assemblages in late summer-autumn and four assemblages in winter. The most characteristic species in each cluster were Terapon jarbua, Sillago sihama, Leiognathus brevirostris, Mene maculate, and Scomberoides tol in late summer-autumn and Scomberomorus commerson, Acanthopagrus latus, Sillago sp., and Evynnis cardinalis in winter. The results of the canonical correspondence analysis indicated that pH, chlorophyll-a (Chl-a), depth, dissolved oxygen (DO), sea surface salinity (SSS), total nitrogen (TN), and total phosphorus (TP) were the major environmental variables affecting the ichthyoplankton assemblage structure in the Beibu Gulf. The finding of this study will provide valuable information in conserving fish spawning grounds and developing fishery management practices to protect fishery resources in the Beibu Gulf.

In recent years, the widespread application of antibiotics has raised global concerns, posing a severe threat to ecological health. In this study, the occurrence, source, and ecological risks of 39 antibiotics belonging to 5 classes in mangrove sediments from Lianzhou Bay, China, were assessed. The total concentrations of the antibiotics (∑39 antibiotics) ranged from 65.45 to 202.24 ng/g dry weight (dw), with an average of 142.73 ± 36.76 ng/g dw. The concentrations of these five classes of antibiotics were as follows: Sulfonamides (SAs) > Tetracyclines (TCs) > Fluoroquinolones (QUs) > Penicillin (PCs) > Macrolides (MLs). The spatial distribution of antibiotics varied as high tidal zone > middle tidal zone > low tidal zone. The total organic carbon (TOC), pH, nitrate (NO₃⁻-N), and nitrite (NO₂⁻-N) of the sediment significantly influenced the distribution of antibiotics (p < 0.05). A source analysis identified untreated sewage from aquaculture as the primary source of antibiotics in the local mangrove. A risk assessment revealed that ciprofloxacin, norfloxacin, ofloxacin of QUs, and tetracycline of TCs exhibited medium risks to algae in certain sampling sites, while other antibiotics exhibited low or no risks to all organisms. Nevertheless, the total risk of all the detected antibiotics to algae was medium in 95% of the sites. The overall ecological risk level of antibiotics in the middle tidal zone was slightly lower than in the high tidal zone and the lowest in the low tidal zone. In summary, the experimental results provided insights into the fate and transport behaviors of antibiotics in mangrove sediments from Lianzhou Bay.