Effective removal of the rotifer Brachionus calyciflorus from a Chlorella vulgaris microalgal culture by homogeneous solar photo-Fenton at neutral pH

In this study, a citrate-modified photo-Fenton process was successfully applied to decontaminate a Chlorella vulgaris microalgae culture spiked with the rotifer Brachionus calyciflorus (5 individuals mL-1). The applied treatment (1 mg L-1 Fe2}, 20 mg L-1 H2O2, 17.5 mg L-1 citric acid) had only moderate effects on viability and regrowth of the microalgae since, after a short post-treatment delay of a few days, they reached final cell den-sities similar to that obtained for microalgae cultures that were not spiked. The decontamination was effective as no regrowth of rotifers was observed in the microalgae cultures after treatment. The efficacy of the citrate -modified photo-Fenton treatment was also studied with a higher starting concentration of 20 rotifers mL-1 and was compared with a solar light/H2O2 treatment. Results show that both treatments had similar efficacies on the rotifer elimination, but that the citrate-modified photo-Fenton treatment had a lower negative impact on the regrowth of microalgae than the solar light/H2O2 treatment. However, when microalgae cultures were spiked with 20 rotifers mL-1, rotifers were only partially inactivated and post-treatment regrowth occurred, which highlights the importance to apply the photo-Fenton process at an early stage of a contamination to achieve full rotifer elimination. In any case, a contamination with 5 rotifers mL-1 is already a significant threat as numbers above 1000 rotifers mL-1 were reached after 14 days and caused the microalgae culture to fail. Overall, our treatment suggests that the citrate-modified solar photo-Fenton process is an environmentally friendly solution to support the maintenance of contaminant-free microalgal cultures.

Automated summary

In this study, a citrate-modified photo-Fenton process was successfully used to decontaminate a Chlorella vulgaris microalgae culture contaminated with the rotifer Brachionus calyciflorus. The treatment had moderate effects on microalgae viability and regrowth, and effectively eliminated the rotifers. Comparison with a solar light/H2O2 treatment showed that the citrate-modified photo-Fenton treatment had a lower negative impact on microalgae regrowth. However, full elimination of rotifers requires early-stage application of the photo-Fenton process.