Antibacterial activity of green, photosynthetic microalgae and associated secondary metabolites produced by cultivation in extreme environments

Abstract

Green microalgae are vastly diverse unicellular, eukaryotic, and photosynthetic organisms that reside in freshwater, marine and terrestrial environments. Microalgae produce metabolites for growth and survival, including fatty acids, lipids, proteins, carbohydrates, phenolics and pigments. Some of these compounds possess antimicrobial bioactivities such as antibacterial, antifungal, antiviral, and antioxidant which are valuable for industrial application as pharmaceuticals and dietary supplements. The production of bioactive metabolites can be enhanced or modified by abiotic stress conditions such as pH, temperature, salinity, nutrients, and heavy metal concentrations. The influence of abiotic stress conditions on the production of metabolites and antibacterial bioactivity of wild microalgal strains from extraordinarily extreme environments has been investigated. Microalgae collected from various acidic, anthropogenically influenced waterbodies were cultivated at acidic and neutral pH to identify and compare the metabolic profile and antibacterial bioactivity of extremophilic microalgae. Microalgae from extreme environments exhibited promising antibacterial activity against predominantly Gram-positive bacteria, most prominently B. cereus, and in some cases against Gram-negative bacteria, such as E. coli. Cultivation at low pH influenced the metabolite profile of acid tolerant microalgae cultures, promoting lipid and fatty acid accumulation with significant negative correlation (r) linking decreasing medium pH with increasing fatty acid content. This work is valuable in identifying abiotic cultivation conditions for the enhanced production of desired, bioactive metabolites that could be applied in pharmaceuticals and nutraceuticals.