The combination of microalgal culture and wastewater treatment is an emerging topic. This study investigated the use of different microalgae to treat different types of dairy farm wastewater. The results showed that the removal of ammonia nitrogen and total phosphorus by mixed microalgae was over 99% and 80%, respectively. The highest production of protein in biomass and extracellular polymeric substances was observed in high-concentration wastewater. In the phycosphere, the abundance of Proteobacteria and Cyanobacteria increased, while that of Bacteroidota decreased. Phycosphere bacteria were strongly correlated with microalgal growth and the composition of extracellular polymeric substances, especially with bound extracellular polymeric substances relative to soluble extracellular polymeric substances. Genes associated with photosynthesis and respiration in phycosphere bacteria were upregulated, contributing to the material exchange capacity in the microalgal-bacterial systems. The interaction between microalgae and phycosphere bacteria thus represents the core of the binary cultivation system-based wastewater treatment and requires further investigation.

Phycosphere bacteria can regulate the dynamics of different algal blooms that impact marine ecosystems. Phaeocystis globosa can alternate between solitary free-living cells and colonies and the latter morphotype is dominate during blooms. The mechanisms underlying the formation of these blooms have received much attention. High throughput sequencing results showed that the bacterial community composition differed significantly between colony and solitary strains in bacterial composition and function. It was found that the genera SM1A02 and Haliea were detected only among the colony strains and contribute to ammonium accumulation in colonies, and the genus Sulfitobacter was abundant among the colony strains that were excellent at producing DMS. In addition, the bacterial communities of the two colony strains exhibited stronger abilities for carbon and sulfur metabolism, energy metabolism, vitamin B synthesis, and signal transduction, providing inorganic and organic nutrients and facilitating tight communication with the host algae, thereby promoting growth and bloom development.

The phycosphere of phytoplankton harbors a diversity of microbes that can potentially interact with the host phytoplankton cells. In spite of increasing reports regarding the interactions between some model phytoplankton and bacteria, there remain many unknowns regarding interactions between the dinoflagellate Akashiwo sanguinea and its phycosphere bacteria. Here we used both cultivation and metagenomic sequencing methods to investigate the microbial community composition in A. sanguinea batch cultures under various growth conditions. The microbial community dynamics under different growth stages and nutrient (nitrogen and iron) depletion scenarios were determined by Illumina MiSeq sequencing of 16S rRNA gene amplicons. Rhodobacteraceae, Alteromonadaceae and Flavobacteriaceae were the main bacterial families and varied significantly under different states of the host A. sanguinea. Selective fluorescence in situ hybridization was also performed to label Rhodobacterales and Alteromonas clade bacteria to confirm their attachment to A. sanguinea cells under confocal laser scanning microscopy. Plate streaking protocol isolated 19 bacterial strains from A. sanguinea cultures, identified through 16S rRNA analysis. Most culturable strains (11 of 19) belonged to Rhodobacteraceae consistent with Illumina MiSeq sequencing data. The possible interaction pathway between these epibiotic bacteria and A. sanguinea in the phycosphere is also discussed.

This study demonstrates that ecologically engineered bacterial consortium could enhance microalgal biomass and lipid productivities through carbon exchange. Phycosphere bacterial diversity analysis in xenic Chlorella vulgaris (XCV) confirmed the presence of growth enhancing and inhibiting microorganisms. Co-cultivation of axenic C. vulgaris (ACV) with four different growth enhancing bacteria revealed a symbiotic relationship with each bacterium. An artificial microalgal-bacterial consortium (AMBC) constituting these four bacteria and ACV showed that the bacterial consortium exerted a statistically significant (P<0.05) growth enhancement on ACV. Moreover, AMBC had superior flocculation efficiency, lipid content and quality. Studies on carbon exchange revealed that bacteria in AMBC might utilize fixed organic carbon released by microalgae, and in return, supply inorganic and low molecular weight (LMW) organic carbon influencing algal growth and metabolism. Such exchanges, although species specific, have enormous significance in carbon cycle and can be exploitated by microalgal biotechnology industry.