Co-cultures of Microalgae with Other Organisms: Application in Wastewater Treatment

Microalgae represent a diverse group of photosynthetic microorganisms that play a fundamental role in the global carbon cycle as well as biomass and oxygen production, being at the base of food webs. Due to their high metabolic flexibility, microalgae can exploit different trophic regimes (photoautotrophy, heterotrophy, mixotrophy) and can adapt to various environmental conditions. In their habitats, microalgae have to co-exist with other microorganisms. This thesis summarises current knowledge about the co-cultures of microalgae with other groups of microorganisms, their cultivation and biotechnological applications. The central theme of the work is the interactions between microalgae and other microorganisms - primarily bacteria - which can be mutualistic, commensalistic, or parasitic in nature. This co existence can be harmful, but on the other hand, it may significantly improve the growth, productivity, and metabolic activity of microalgae, or can have a protective function. The key milieu for these interactions is the phycosphere - a space around the microalgal cell where significant exchange of nutrients, organic substances, and signalling molecules takes place, generating the so-called quorum-sensing signals that can regulate metabolism. Microalgae can be cultured in man-made systems. Various types of cultivation systems - open or closed - have been used for microalgae culturing. The trials in this thesis describe the influence of photobioreactor design on community composition, the effects of bacterial quorum-sensing molecules on carotenoid production, and the regulation of secondary metabolites in cyanobacteria. Data showed that co-cultivation of microalgae with bacteria or other microorganisms can increase the robustness of cultures, the efficiency of nutrient removal, and the ability to produce bioactive substances. Microalgae can also be used in systems remediating nutrients from wastewaters, and biomass is applied for agricultural purposes. The conclusion is that the interaction of microalgae with other microorganisms is not merely a threat of contamination, but represents an important, not fully understood tool for increasing the productivity of certain compounds. This thesis also emphasises the importance of linking the targeted use of microbial consortia in the production of some bioactive substances - pigments, biostimulants, biopesticides, and wastewater treatment. At the same time, it identifies the main challenges, in particular the variability of microbial communities in real conditions, the necessity to find suitable cultivation regimes and a detailed understanding of metabolic communication among organisms.