Competition and succession between the oily alga Botryococcus braunii and two green algae Chlorella vulgaris and Chlamydomonas reinhardtii

Contamination by other algal species is a major problem in the culture of the oil-producing green alga Botryococcus braunii (Bb). In this study, the population interactions between Bb and two small green algae, Chlorella vulgaris (Cv) and Chlamydomonas reinhardtii (Cr), were separately investigated in two controlled experiments. Filtrates of Cv or Cr significantly stimulated the growth of Bb in the first 9-11 days when the filtrates were derived from a low- or medium-density algal culture. However, filtrates of Cv and Cr from the high-cell-density cultures significantly inhibited Bb growth from day 11 and 7, respectively. In Bb-Cv competition, the growth of Bb was significantly inhibited by Cv at the biomass ratio of Bb/Cv = 1:1. However, when the Cv component reduced to 0.5 (Bb/Cv = 1:0.5) or 0.25 (Bb/Cv = 1:0.25), the growth of Bb was faster than that in the control in the first 13 days. Similarly, in Bb-Cr competition, the biomass ratios of 1:0.5 and 1:0.25 (Bb/Cr) favoured the growth of B. braunii in the first 15 days, but the growth of Bb decreased in the Bb/Cr ratio of 1:1 from day 15. By contrast, the growth of Cv and Cr were not affected by the presence of Bb in all biomass ratios. These results suggest that Cv and Cr could impact the growth of Bb by metabolites in the culture medium, and when the biomass of Bb is two to four times that of other algae, this oily alga becomes competitive over other algal species. This study reveals the mechanism of population dynamics of this oily alga competing with small and fast-growing algae. The results are applicable in reducing algal competition and regulating species succession between slow and fast-growing algae species.