Allelopathic growth inhibition of Prorocentrum micans (Dinophyta) by Ulva pertusa and Ulva linza (Chlorophyta) in laboratory cultures

Allelopathic effects of several concentrations of fresh tissue and dry powder of two bloom-forming green macroalgae Ulva pertusa and Ulva linza on the red tide microalga Prorocentrum micans were investigated in coexistence culture systems. Preliminary studies on the algicidal effects of one aqueous and four organic solvent extracts from the macroalgae on the microalga were carried out to confirm the existence of allelochemicals in the tissue of these two macroalgae. The effects of filtrate of used macroalgal culture medium were tested on P. micans using a single initial or semi-continuous filtrate addition. Furthermore, the potential effects of the microalga on these two macroalgae were also evaluated. The results of the coexistence experiments demonstrated that the growth of P. micans was strongly inhibited by the addition of both the fresh tissue and the dry powder of both species of Ulva. Aqueous and methanol extracts of both macroalgae had strong growth inhibitory effects on P. micans, while the other three organic solvent extracts (acetone, ether and chloroform) had no apparent effect on its growth. This suggests that the allelochemicals from both macroalgae had relatively high polarities. The filtrate of used macroalgal culture medium exhibited no apparent growth inhibitory effect on the microalga under initial filtrate addition whereas the growth of P. micans was significantly inhibited under semi-continuous filtrate addition, which suggested that continuous release of small quantities of rapidly degradable allelochemicals from the fresh tissue of both macroalgae was effective in inhibiting the growth of P. micans. In contrast, the microalga had no apparent effect on the growth of either macroalgal species in coexistence experiments.