Biodiversity, Distribution and Ecological Interactions of Algal-Bacterial Biofilms in Wastewater Treatment Systems

Nutrient contamination from wastewater threatens aquatic ecosystems by promoting harmful algal blooms and creating hypoxic dead zones. In nutrient-rich systems, algae uptake nitrogen and phosphorus while bacteria carry out nitrification and denitrification. Although algae and bacteria have been employed together to enhance nutrient removal processes in wastewater treatment systems, there is limited understanding of specific algal-bacterial interactions, roles of macroalgae and biodiversity of non-suspended microbes. In this study, next-generation sequencing of the V4 region of 16S rRNA and the V8-V9 region of 18S rRNA was used to examine the biodiversity of biofilm communities at wastewater treatment plants and the influence of water quality and algal-bacterial interactions on dominant microbes. Redundancy analysis revealed that the dominant algae genera strongly influenced by nitrogen and phosphorus content were Neochloris, Characiopodium, Ulva and Cladophora. Positive co-occurrence was primarily observed between algae and plant growth-promoting rhizobacteria (PGPR), namely Prototheca-Blastopirellula and Cladophora-Gordonia. Conversely, algae formed negative co-occurrence relationships with both PGPR (Characiopodium-Pseudomonas, Neochloris-Pseudomonas, Characiopodium-Acinetobacter) and gut microbes (Neochloris-Christensenellaceae R-7, Ulva-Christensenellaceae R-7, Cladophora-Christensenellaceae R-7, Ulva-Desulfovibrio). These results highlight the significance of macroalgae in biofilm communities and suggest potential algae-bacteria pairs for enhanced nutrient removal in on-site wastewater treatment.