Heterotrophic flagellates increase microalgal biomass yield

摘要

In time of scarcity of fossil energies, microalgae are attracting interest as a potential source of renewable energy due to their high growth rates and potential high lipid contents. Additionally, cultivation may be an abatement measure to remove surplus nutrients from eutrophicated ecosystems. At present, microalgal cultivations for biomass production are run mainly in monocultures, which are easily contaminated with competing microalgae or grazers. Furthermore, heterotrophic bacteria are highly abundant and may strongly reduce the yield in the target microalgae through competition for nutrients. In three laboratory experiments, we tested whether heterotrophic flagellates (Oxyrrhis marina and Cafeteria roenbergensis) can make nutrients bound in bacteria available for marine diatoms (Coscinodiscus granii and Odontella sinensis) and can shift the competition for inorganic nutrients towards the microalgae. Cultures were run with and without flagellates, under different conditions: without an external carbon source, in presence of organic matter (barley grains) or biogas wastewater. The presence of flagellates had a positive effect on microalgal growth, but this was context and species specific. The presence of the flagellates affected the maximum algal growth rates (r) especially in Coscinodiscus granii. A maximal biomass increase (29.93 ± 2.98 %) (mean ± standard deviation, n = 3) was observed for Coscinodiscus granii in F/2 + Si medium. Furthermore, although the flagellates were attributed to the detrital fraction, their presence resulted in a significant reduction of detritus. In conclusion, heterotrophic flagellates have the potential to increase nutrient use efficiency especially in algae bioreactors with slow-growing large phytoplankton taxa. This effect may be particularly relevant in organic polluted water.