Interspecific competition between the bloom-causing dinoflagellates Hetrocapsa bohaiensis and the local species Chlorella pyrenoidosa

Transcriptome Analysis of the Harmful Dinoflagellate Heterocapsa bohaiensis Under Varied Nutrient Stress Conditions

The increasing prevalence of harmful algal blooms (HABs) driven by eutrophication, particularly in China's nearshore waters, is a growing concern. Dinoflagellate Heterocapsa bohaiensis blooms have caused significant ecological and economic damage, as well as mass mortality, in cultivated species. Nutrients are one of the primary inducers of H. bohaiensis blooms. However, the transcriptomic studies of H. bohaiensis remain sparse, and its metabolic pathways are unknown. This study analyzed the transcriptome of H. bohaiensis under varying nutrient conditions (nitrogen at 128, 512, and 880 μM; phosphate at 8, 6, and 32 μM), focusing on differential gene expression. The results indicated that deviations in nutrient conditions (higher or lower N:P ratios) led to a higher number of differentially expressed genes compared to the control (N:P ratios = 27.5), thereby underscoring their pivotal role in growth. Gene Ontology (GO) enrichment analyses showed that nutrient limitation upregulated the biosynthesis and catabolism processes while downregulating the cell cycle and division functions. The Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis revealed that, under nitrogen limitation, the proteasome pathways were upregulated, while photosynthesis and carbon fixation were downregulated; under phosphorus limitation, the proteasome pathways were upregulated and nitrogen metabolism was downregulated. These findings suggest that H. bohaiensis adapts to nutrient stress by adjusting its metabolic processes.

Control ultrafiltration membrane fouling in Chlorella-laden water treatment by integrated heat-activated peroxydisulfate pre-oxidation and coagulation treatment

The membrane fouling induced by algal extracellular organic matter (EOM) remain a bottleneck in restricting ultrafiltration (UF) application during harmful algal-water treatment. In current study, the application of heat-activated peroxydisulfate (PMS) and coagulation (Aluminum chlorohydrate, PACI) on membrane fouling behavior during Chlorella-laden water treatment was investigated. The membrane fouling mechanism was analyzed using the extended Derjaguin-Landau-Verwey-Over-beek (XDLVO) theory. The results revealed that separated heat-activated PMS could enhance the filtration flux of EOM at high PMS does >0.2 mM, whereas the membrane fouling was further alleviated by combined heat-activated PMS (0.2-1.0 mM) and PACI (20 mg/L) treatment, especially at low PMS dose. Combined heat-activated PMS and PACI pretreatment could effectively increase the adhesive repulsion between membrane and foulants and reduce the cohesion free energies between organic foulants than those by separated heat-activated PMS treatment, making the initial filtration flux reduced and the cake layer looser. Moreover, the organic foulants of proteins, polysaccharides, and humic-like organics were removed. Cake formation was the major fouling mechanism when EOM was treated with/without separated heat-activated PMS treatment, whereas the membrane fouling mechanism was changed from cake layer formation to pore blocking after combined heat-activated PMS and PACI treatment. Overall, this research provided a feasible method in membrane fouling control during Chlorella -laden water treatment.

Effects of nutrient ratios on a newly harmful dinoflagellate Heterocapsa bohaiensis: Evidence from growth, toxicity and transcriptome analyses

Heterocapsa bohaiensis is a newly identified dinoflagellate species that causes harmful blooms in coastal areas in China, Malaysian, and New Caledonian. These blooms have led to substantial economic losses for local aquaculture. Previous studies have mainly focused on understanding the toxicity of H. bohaiensis. However, the causes of H. bohaiensis blooms remain unknown. In this study, we aimed to ascertain nitrogen (N) and phosphorus (P) requirements for the growth and reproduction of H. bohaiensis. Additionally, we sought to understand the functional mechanisms by comparing the transcriptomes of H. bohaiensis under nutrient-limited conditions and control conditions. The results revealed a wide range of acceptable N:P ratios for H. bohainensis, attributed to a mechanism involving nutrient storage, which allowed H. bohainensis to sustain its growth even when either nitrate or phosphate was depleted. Higher N:P ratios (>27.5) were more conducive to the growth of H. bohainensis than f/2 medium or low ratios, which is related to the N:P ratios absorbed by H. bohainensis. The toxicity of H. bohainensis was significantly enhanced in N-limited or P-limited states. These findings underscore the significance of the physiological metabolism of H. bohainensis in adapting to environmental stresses induced by human activities and establishing the dominance of blooms.

Meta-analysis to identify inhibition mechanisms for the effects of submerged plants on algae

Submerged plants inhibit algae through shading effects, nutrient competition, allelopathy, and combinations of these mechanisms. However, it is unclear which mechanism is dominant, and how the inhibition intensity results from the traits of the plant and algae. In this study, we performed meta-analysis to quantitatively identify the dominant mechanisms, evaluate the relationship between inhibition intensity and the species and functional traits of the submerged plants or algae, and reveal the influences of external environmental factors. We found that allelopathy caused stronger inhibition than the shading effect and nutrient competition and dominated the combined mechanisms. Although the leaf shapes of the submerged plants influenced light availability, this did not change the degree of algae suppression. Algal species, properties (toxic or nontoxic) and external environmental factors (e.g., lab/mesocosm experiments, co-/filtrate/extract culture, presence or absence of interspecific competition) potentially influenced inhibition strength. Cyanobacteria and Bacillariophyta were more strongly inhibited than Chlorophyta, and toxic Cyanobacteria more than non-toxic Cyanobacteria. Algae inhibition by submerged plants was species-dependent. Ceratophyllum, Vallisneria, and Potamogeton strongly inhibited Microcystis, and can potentially prevent or mitigate harmful algal blooms of this species. However, the most common submerged plant species inhibited mixed algae communities to some extent. The results from lab experiments and mesocosm experiments both confirmed the inhibition of algae by submerged plants, but more evidence from mesocosm experiments is needed to elucidate the inhibition mechanism in complex ecosystems. Submerged plants in co-cultures inhibited algae more strongly than in extract and filtrate cultures. Complex interspecific competition may strengthen or weaken algae inhibition, but the response of this inhibition to complex biological mechanisms needs to be further explored. Our meta-analysis provides insights into which mechanisms contributed most to the inhibition effect and a scientific basis for selecting suitable submerged plant species and controlling external conditions to prevent algal blooms in future ecological restoration of lakes.