Variations in Marine Bacterial and Archaeal Communities during an Ulva prolifera Green Tide in Coastal Qingdao Areas

Spatial and diel variations of bacterioplankton and pico-nanoeukaryote communities and potential biotic interactions during macroalgal blooms

We investigated spatial heterogeneity and diel variations in bacterioplankton and pico-nanoeukaryote communities, and potential biotic interactions at the extinction stage of the Ulva prolifera bloom in the Jiaozhou Bay, Yellow Sea. It was found that the presence of Ulva canopies significantly promoted the cell abundance of heterotrophic bacteria, raised evenness, and altered the community structure of bacterioplankton. A diel pattern was solely significant for pico-nanoeukaryote community structure. >50 % of variation in the heterotrophic bacterial abundance was accounted for by the ratio of Bacteroidota to Firmicutes, and dissolved organic nitrogen effectively explained the variations in cell abundances of phytoplankton populations. The factors representing biotic interactions frequently contributed substantially more than environmental factors in explaining the variations in diversity and community structure of both bacterioplankton and pico-nanoeukaryotes. There were higher proportions of eukaryotic pathogens compared to other marine systems, suggesting a higher ecological risk associated with the Ulva blooms.

Metatranscriptomic insights into the microbial metabolic activities during an Ulva prolifera green tide in coastal Qingdao areas

Green tide, a typical marine environmental disaster that profoundly influenced the coastal areas, has been occurred consecutively in the South Yellow Sea of China since 2007. Herein, the active microbial community structure and metabolic pathways in Qingdao offshore during an Ulva prolifera green tide were investigated by using metatranscriptomic approach. The dominant active microbial taxa at the outbreak phase were primarily a functional group that can utilize organic matters derived from U. prolifera, such as Lentibacter, Polaribacter and Planktomarina. While the taxa involved in biogeochemical cycles, including Phaeobacter, Pseudomonas and Marinobacterium, dominated the active microbial communities at the decline phase. The expression level of enzymes involved in U. prolifera polysaccharides degradation was significantly higher at the outbreak phase compared to the decline phase. At the same time, the main players Glaciecola and Polarbacter showed similar trends, suggesting that the low competitiveness for nutrients of related microorganisms at this phase made them degrade more U. prolifera polysaccharides to meet their own nutrient needs, thereby accelerating the degradation of U. prolifera. According to KEGG annotation, the biogeochemical pathways including nitrogen cycle, sulfur cycle and methane oxidation altered during the green tide, with thiosulfate oxidation and methane oxidation probably being the crucial pathways at the outbreak and the decline phase respectively. The coupling of sulfide oxidation and denitrification was also observed in this study. Furthermore, the green tide in Qingdao offshore might impact the greenhouse effects induced by CH4 and N2O through influencing the related microbial processes.

Virome reveals effect of Ulva prolifera green tide on the structural and functional profiles of virus communities in coastal environments

Viruses are widely distributed in marine environments, where they influence the transformation of matter and energy by modulating host metabolism. Driven by eutrophication, green tides are a rising concern in Chinese coastal areas, and are a serious ecological disaster that negatively affects coastal ecosystems and disrupts biogeochemical cycles. Although the composition of bacterial communities in green algae has been investigated, the diversity and roles of viruses in green algal blooms are largely unexplored. Therefore, the diversity, abundance, lifestyle, and metabolic potential of viruses in a natural bloom in Qingdao coastal area were investigated at three different stages (pre-bloom, during-bloom, and post-bloom) by metagenomics analysis. The dsDNA viruses, Siphoviridae, Myoviridae, Podoviridae, and Phycodnaviridae, were found to dominate the viral community. The viral dynamics exhibited distinct temporal patterns across different stages. The composition of the viral community varied during the bloom, especially in populations with low abundance. The lytic cycle was most predominant, and the abundance of lytic viruses increased slightly in the post-bloom stage. The diversity and richness of the viral communities varied distinctly during the green tide, and the post-bloom stage favored viral diversity and richness. The total organic carbon, dissolved oxygen, NO^3-, NO^2-, PO₄^3-, chlorophyll-a contents, and temperature variably co-influenced the viral communities. The primary hosts included bacteria, algae, and other microplankton. Network analysis revealed the closer links between the viral communities as the bloom progressed. Functional prediction revealed that the viruses possibly influenced the biodegradation of microbial hydrocarbons and carbon by metabolic augmentation via auxiliary metabolic genes. The composition, structure, metabolic potential, and interaction taxonomy of the viromes differed significantly across the different stages of the green tide. The study demonstrated that the ecological event shaped the viral communities during algal bloom, and the viral communities played a significant role in phycospheric microecology.