Influence of heterotrophic and mixotrophic cultivation on growth behaviour of terrestrial cyanobacteria

Abstract

Terrestrial cyanobacteria are a promising source for the production of biotechnological interesting metabolites. Some of these organisms of potential economic importance live in form of phototrophic biofilms and are therefore difficult to cultivate under laboratory conditions. However, there are some cultivation systems that mimic natural growth conditions, they are neither applicable for a scale-up, nor commercially available. Nevertheless, common cultivation systems like open ponds are designed for submerged conditions and an exchange against emerse photobioreactor systems (ePBRs) would be a cost-intensive procedure. Heterotrophic and mixotrophic cultivation could provide an alternative. In this work, cells of non-axenic mixed cultures of the terrestrial cyanobacteria Trichocoleus sociatus and Nostoc muscorum were cultivated for 14 days under phototrophic, heterotrophic and mixotrophic submerged conditions. The influence of different carbon sources, such as glucose, fructose, galactose, raffinose and potassium acetate, as well as the difference between batch and fed-batch cultivation on growth behaviour was analyzed. Mixotrophic cultivation led to higher cell concentrations as phototrophic and heterotrophic cultivation, regardless of which carbon source was used. A medium supplement of 2.53 g L−1 raffinose yielded a cell dry weight of 3.77 g L−1 for T. sociatus under mixotrophic fed-batch conditions, whereas for N. muscorum the mixotrophic fed-batch mode with glucose was the best option. It led to a dry weight of 2.46 g L−1 after 14 days of cultivation. It was observed that the two strains were able to use several carbohydrates as carbon sources and even much better than acetate. The composition of heterotrophic and mixotrophic non-axenic cultures was analyzed using qPCR during the whole cultivation period. Although there were differences in carbon utilization between the strains, the results indicate that in general terrestrial cyanobacteria are able to grow heterotrophically and that the increase in dry weight is not attributed to the growth of heterotrophic coexisting organisms.