Microalgae to Biofuel

The growth of a locally isolated strain of green algae, Chlorella sp., selected for its promise as a biodiesel feedstock, was studied in wastewater effluent from the municipal wastewater treatment plant in Tallahassee FL. Nutrient concentration and microbial community composition within the effluent were profiled at monthly intervals. Adequate nutrients for algal cultivation were observed along with a dynamic microbial community of zooplankton, green algae, diatoms, and a bacterial community that included Proteobacteria, Bacteroidetes, Cyanobacteria, Firmicutes, and Actinobacteria. The importance of this microbial community in the cultivation of Chlorella and the impact on biofuel production was investigated utilizing mesocosm incubations. Size exclusion was employed to examine the biogeochemical interactions between the Chlorella culture and the resident wastewater microorganisms. The accumulation of algal cells and chl a biomass, the production of oxygen, and the consumption of inorganic carbon and nutrients, along with changes in the microbial community composition were monitored in treatments that included the total wastewater microbial community, the wastewater bacterial community, and the Chlorella culture with no effluent microorganisms. The treatment that excluded the wastewater microbial community and allowed the Chlorella culture to grow uninfluenced, consistently demonstrated the highest abundances of algal cells. A limited abundance of algal cells and chl a biomass were observed in the treatments that contained the total wastewater microbial community, while no limitation in oxygen production or nutrient consumption was observed in these treatments. Given the presence of zooplankton that are known to graze on algal cells, it appears that a top-down control inhibits the accumulation of algal biomass in raw wastewater effluent from a municipal treatment plant. In the treatments that contained the native bacterial community, competition between the Chlorella culture and phototrophic bacteria, i.e. cyanobacteria, was observed. These treatments demonstrated high chl a biomass, but limited accumulation of algal cells, as well as significant consumption of nutrients, indicating that cyanobacteria may out-compete Chlorella for limiting nutrients. However, the heterotrophic bacterial community did appear to have a significant impact on algal growth. Treatments in which the cyanobacterial community was inhibited did not demonstrate a draw-down of limiting nutrients or a limitation in the accumulation of algal cells. In addition, shifts in the composition of the bacterial community, including a reduction in the relative abundance of Betaproteobacteria with a simultaneous increase in the relative abundance of Alphaproteobacteria, Nostocophycideae, and Oscillatoriophycideae were observed, but these shifts occurred independently of the presence of the Chlorella culture, implying that there is not a strong relationship between these two groups. The limitation that occurred in the growth of Chlorella due to competition and top-down controls indicates that without significant manipulations of the microbial community, the cultivation of an algal monoculture in wastewater effluent from a municipal treatment plant may be unrealistic for efficient biofuel feedstock production.