Growth and morphological responses of Chlorella vulgaris at different initial algal densities to treatment with Ceratophyllum demersum methanol extracts

The allelopathically inhibitory effects of submerged macrophytes on phytoplankton have been extensively studied and are utilised as an effective strategy for water restoration. However, this technique has been minimally implemented in natural in situ water bodies because of the unclear and complex interactions involved. Our study considered the uneven density distribution of algae in natural aquatic ecosystems and the biomass-dependent effects of submerged macrophytes on target algae. Ceratophyllum demersum methanol extracts of gradient concentration on Chlorella vulgaris with different initial algal density were conducted in the present laboratory study. Results indicated that methanol extracts of C. demersum could not only inhibit the growth of but also promote colony formation of C. vulgaris. C. vulgaris of a low density exerted increased inhibition and colony proportion responses to C. demersum extracts. By 0.42 g/mL C. demersum treatment, the inhibition rate on C. vulgaris under 0.02 IAD (Initial Alagal Density) and 0.05 IAD (p < 0.05) was 88.7%, 70.9%, respectively. We also suggested that the effects of C. demersum were biomass dependent, such that extracts with high concentration could produce increased inhibitory effects on C. vulgaris. According to GC-MS analysis, the study revealed five potential compounds (i.e. hexanoic, acetoacetic, azelaic, palmitic and stearic acid) in the C. demersum methanol extracts. However, the individual or combined effects of those compounds require further exploration. This study proposed certain theoretical basis for future water restoration by submerged macrophytes, that the biomass of the macrophytes and the density of the algae should both be taken into account.