Bacterial community dynamics on the seaweed Ulva ohnoi during a full cultivation cycle in a land-based aquaculture pond system

Seaweeds of the genus Ulva are valuable marine resources that serve as food for humans and animals, a biofilter, biomass for biofuels, and a source of bioactive metabolites. Ulva culture in land-based ponds systems is an convenient alternative to cultures in open waters due to better control of biotic and abiotic factors that affect biomass quality and productivity. Microbes can play a crucial role in the development and growth of Ulva by releasing beneficial metabolites. However, our current knowledge of the microbial composition in this aquaculture system is limited. This study explores the abundance, diversity, and taxonomic composition of bacterial communities on Ulva ohnoi throughout a 4-week pond cultivation cycle. Epiphytic bacterial communities were characterized and quantified using epifluorescence microscopy and digital image analysis. Taxonomic composition was determined through 16S rRNA gene amplicon sequencing and diversity indices. The results showed that the epiphytic bacteria consisted of three morphotypes: i) clusters of small spherical bacteria, ii) non-clustered spherical bacteria, and iii) oval-shaped bacteria in chains. These bacteria were mainly clustered in the apoplast. The bacterial abundance did not exhibit any discernible pattern throughout the cultivation cycle, with pH being the only physicochemical variable exerting some regulation on the abundance. The most abundant phyla were Proteobacteria, Actinobacteria, and Firmicutes. At the class level, Gammaproteobacteria, Actinobacteria, Bacteroidota, and Alphaproteobacteria dominated the microbiome. This study suggests that the Ulva biomass influenced the abundance and composition of the epiphytic bacteria probably due to the seaweed photosynthesis and respiration processes, which were reflected in the variations of pH in the seawater. This suggests that the final biomass may exert a feedback process between the host and the microbiome, which can serve as an indicator of the quality and productivity of the biomass during the culture cycle.