Density and Composition of Cohabiting Bacteria in <i>Chlorella vulgaris</i> CCAP 211/21A Is Influenced by Changes in Nutrient Supply

Microalgae have considerable potential as a renewable feedstock for biochemical and bioethanol production that can be employed in processes associated with carbon capture. Large-scale microalgae cultivations are often non-axenic and are often cohabited by bacteria. A better understanding of the influence of cohabiting bacteria on microalgae productivity is required to develop sustainable synthetic co-culture processes at scale. Nutrient limitation is a frequently employed strategy in algal cultivations to accumulate energy reserves, such as lipids and carbohydrates. Here, a non-axenic culture of an estuarine green microalga, <i>Chlorella vulgaris</i> CCAP 211/21A, was studied under nutrient replete and deplete conditions to assess how changes in nutrient supply influenced the cohabiting bacterial population and its association with intracellular carbohydrate accumulations in the alga. Nutrient limitation resulted in a maximum carbohydrate yield of 47%, which was 74% higher than that in nutrient replete conditions. However, the latter condition elicited a 2-fold higher carbohydrate productivity. Three cohabiting bacterial isolates were cultivable from the three culture conditions tested. These isolates were identified using the 16S rRNA gene sequence to belong to <i>Halomonas</i> sp. and <i>Muricauda</i> sp. The composition of the bacterial population varied significantly between the growth conditions and time points. In all cases and at all time points, the dominant species was <i>Halomonas</i> isolates. Nutrient depletion resulted in an apparent loss of <i>Muricauda</i> sp. This finding demonstrates that nutrient supply can be used to control cohabiting bacterial populations in algal cultures, which will enable the development of synthetic co-culture strategies for improving algae productivity.