Production of microalgal external organic matter in a Chlorella-dominated culture: influence of temperature and stress factors

Although microalgae are recognised to release external organic matter (EOM), little is known about this phenomenon in microalgae cultivation systems, especially at large scale. A study was carried out on the effect of microalgae-stressing factors such as temperature, nutrient limitation and ammonium oxidising bacteria (AOB) competition in EOM production by microalgae. The results showed non-statistically significant differences in EOM production at temperatures in the range of 25-35oC. However, when the temperature was suddenly raised from 25 to 35oC for 4h polysaccharide production increased significantly, indicating microalgae stress. Nutrient limitation also increased EOM production. No significant differences were found in EOM production under lab conditions when the microalgae competed with AOB for ammonium uptake. However, when EOM concentration was monitored during continuous outdoor operation of a membrane photobioreactor (MPBR) plant, nitrifying bacteria activity was likely to be responsible for the increase in EOM concentration in the culture. Other factors such as high temperatures, ammonium-depletion and low light intensities could also have induced cell deterioration and thus have influenced EOM production. An increase in EOM showed no significant effect in membrane performance under the operating conditions tested since fouling rate evolution did not follow the same trend than EOM concentration.