Marine macroalgae and their associated bacterial communities affect larval settlement and survivorship of the coral Pocillopora damicornis.

Macroalgae play crucial roles as major habitat-forming organisms in marine ecosystems, having significant impacts on coral recruitment and reef recovery. However, the interactions between marine macroalgae and coral larvae remain poorly understood. Furthermore, little is known whether differences in bacterial assemblages associated with macroalgae may play roles in this process. Here, we comprehensively investigated the impacts of different macroalgae and their associated microbiomes on larval settlement and survival of coral Pocillopora damicornis. The results revealed significant variations in larval settlement and survival rates when exposed to different macroalgal species. The highest settlement rate, reaching 90%, was observed in the presence of the red alga Hypnea pannosa, followed by green algae Caulerpa serrulata, C. racemosa, and brown algae Turbinaria gracilis, Sargassum polycystum. Correspondingly, similarities in bacterial compositions were observed between H. pannosa and C. racemosa, as well as between T. gracilis and S. polycystum, implying associated bacterial may be related with the algal functions. Furthermore, macroalgae that facilitate larval settlement exhibited higher abundances of amplicon sequence variants (ASVs) associated with the metabolism of dimethylsulfoniopropionate or the antagonism of known coral pathogens. However, the brown alga Padina boryana failed to induce larval settlement with survival rate of zero after 120 h. The algal species harbored more abundances of ASVs related to Rhizobiaceae. These findings highlight the significant impact of macroalgae and their associated microbiomes on coral recruitment, as they influence both larval settlement and survival rates.

Microplastics inhibit the growth of endosymbiotic Symbiodinium tridacnidorum by altering photosynthesis and bacterial community.

Microplastics, ubiquitous anthropogenic marine pollutants, represent potential threats to coral-Symbiodiniaceae relationships in global reef ecosystems. However, the mechanism underlying the impacts of polystyrene microplastics (PS-MPs) on Symbiodiniaceae remains poorly understood. In this study, the cytological, physiological, and microbial responses of Symbiodinium tridacnidorum, a representative Symbiodiniaceae species, to varying concentrations of PS-MPs (0, 5, 50, 100, and 200 mg L-1) were investigated. The results revealed that microplastic exposure inhibited cell division, resulting in reduced cell density compared to control group. Furthermore, algal photosynthetic activity, as indicated by chlorophyll content, Fv/Fm, and net photosynthetic rate, declined with increasing microplastic concentration up to 50 mg L-1. Notably, elevated levels of microplastics (100 and 200 mg L-1) prompted a significant increase in cell size in S. tridacnidorum. Transmission electron microscopy and fluorescence microscopy indicated that hetero-aggregation was formed between high levels of PS-MPs and algal cells, ultimately causing damage to S. tridacnidorum. Moreover, the impact of PS-MPs exposure on the bacterial community associated with S. tridacnidorum was investigated. The results showed a reduction in alpha diversity of the bacterial community in groups exposed to 50, 100, and 200 mg L-1 of microplastics compared to those treated with 0 and 5 mg L-1. Additionally, the relative abundance of Marinobacter, Marivita, and Filomicrobium significantly increased, while Algiphilus and norank Nannocystaceae declined after microplastic exposure. These findings suggest that MPs can inhibit the growth of S. tridacnidorum and alter the associated bacterial community, posing a potential serious threat to coral symbiosis involving S. tridacnidorum.