In this study, the impacts of nitrogen loading rates on microalgal communities and intracellular storage compounds in a chemostat enrichment system were investigated. The chemostat was operated at a constant dilution rate of 0.5 d-1 with different nitrogen loading rates of 10, 20, 32.5, 42 and 63 mgN.l-1.d-1. The cultures with the lowest nitrogen loading rates showed nitrogen fixation capacity. Diverse microalgal communities were observed for nitrogen loading rates of 10 and 20 mgN.l-1.d-1. Chlorella sorokiniana and Chlorella vulgaris were dominant species at nitrogen loading rates of 42 and 32.5 mgN.l-1.d-1, respectively. Different species of Arthrospira platensis, Chlorella sp., Rhopalodia coexisted under the light-limited condition at NLR of 63 mgN.l-1.d-1. Lipids accumulation overtook starch accumulation at all nitrogen loading rates. mgN.l-1.d-1. Lipids accumulation overtook starch accumulation at all nitrogen loading rates. Specifically, the concentration of lipids in the enriched species cells, Chlorella sorokiniana was 4.2 times higher than the amount of accumulated starch. The results demonstrate that the approximately balanced addition of nutrients and light in chemostat enrichment culture could be an important trait to enrich a potential lipid production organism.
Lactic acid producing bacteria are important in many fermentations, such as the production of biobased plastics. Insight in the competitive advantage of lactic acid bacteria over other fermentative bacteria in a mixed culture enables ecology-based process design and can aid the development of sustainable and energy-efficient bioprocesses. Here we demonstrate the enrichment of lactic acid bacteria in a controlled sequencing batch bioreactor environment using a glucose based medium supplemented with peptides and B vitamins. A mineral medium enrichment operated in parallel was dominated by Ethanoligenens species and fermented glucose to acetate, butyrate and hydrogen. The complex medium enrichment was populated by Lactococcus, Lactobacillus and Megasphaera species and showed a product spectrum of acetate, ethanol, propionate, butyrate and valerate. An intermediate peak of lactate was observed, showing the simultaneous production and consumption of lactate, which is of concern for lactic acid production purposes. This study underlines that the competitive advantage for lactic acid producing bacteria primarily lies in their ability to attain a high biomass specific uptake rate of glucose, which was two times higher for the complex medium enrichment when compared to the mineral medium enrichment. The competitive advantage of lactic acid production in rich media can be explained using a resource allocation theory for microbial growth processes.