The Research On The Interspecific Competition Between Heterocapsa Bohaiensis Sp.nov And Co-existing Microalgae

Heterocapsa bohaiensis sp.nov is one of novel algae species causing harmful red tide in the coastal aquaculture area of Liaodong Bay.In addition to environmental factors,the competitive exclusion of dominant species in phytoplankton communities for coexisting species is a key process in the occurrence of red tides.In this study,Skeletonema costatum from the common red tide algae in the Bohai Sea,and Chlorella pyrenoidosa,the dominant algae in industrial aquaculture ponds were selected as co-existing species with H.bohaiensis.The interspecific competition relationship between H.bohaiensis and other species were studied,especially the role of allelopathic effects in the competition among several algae.The mixed culture process and the cultivation of cell-free filtrate were conducted in this paper.The results will contribute to explore the ecological mechanism of the occurrence of red tides in H.bohaiensis in the Bohai Sea.This experiment explored the competitive relationship between two species of red tide algae:S.costatum and H.bohaiensis.Firstly,the interspecific competition relationship between the two algae was studied by mixed culture,and different initial algae density was set experimentally.The proportions of H.bohaiensis and S.costatum were 1:1,1:2 and 1:3,respectively.The experimental results showed that the H.bohaiensis was dominant in the culture when the ratio of the initial algae density of H.bohaiensis and S.costatum was 1:1and 1:2 in the co-culture system,and the S.costatum was inhibited from three days or four until disappeared entirely.When the ratio of the initial algae density was 1:3,both algae can grow,and the growth of S.costatum would be inhibited to a certain extent,but the degree of inhibition was relative to the other two densities.Secondly,the filtrate culture experiments were carried out to explore the effect of allelopathy on the competition between the two species of algae.The filtrate of H.bohaiensis strongly inhibited the growth of S.costatum,whereas the filtrate of S.costatum had little effect on the growth of H.bohaiensis.The above results indicated that the growth of H.bohaiensis was inhibited by extracellular chemicals,so allelopathy was one of the interspecific mechanisms between them besides contact competition.The original dominant algae in the aquaculture area is C.pyrenoidosa,and it is an issue remaining to be solved that how H.bohaiensis competes with C.pyrenoidosa,and which algae becomes a dominant species of phytoplankton community and proliferates in large quantities to initiate red tide.Experiments were conducted to set up the initial algal density ratios of H.bohaiensis and C.pyrenoidosa at 1:1,1:3,1:5,1:15,1:100,and explore inter-competition results in two algae species.The experimental results showed it depended on the ratios of the initial algae ratios of the two algae and the initial density of H.bohaiensis that which algae dominated the population in the mixed culture.When the initial density of H.bohaiensis was up to 1×10~4 cells/mL,the H.bohaiensis has always dominated the community regardless of the initial algal density of C.pyrenoidosa.In the other hands,C.pyrenoidosa can significantly inhibit the growth of H.bohaiensis in the cell-free filtrate culture.When the concentration of C.pyrenoidosa filtrate was 60%,it can significantly inhibit the growth of H.bohaiensis until completely eliminated.The filtrate of H.bohaiensis showed no toxic effect on C.pyrenoidosa.The above results indicated that under the condition of sufficient light and nutrient,the interspecific competition mechanism between H.bohaiensis and C.pyrenoidosa was the competition of cell contact and utilization,and the competing results were related to population density.The allelopathy was not the reason for the competition.In summary,this paper studied the interspecific competition mechanism between H.bohaiensis and the co-existing species.The experiments found that H.bohaiensis adopt different competitive strategies to obtain growth advantages as one of harmful algae stains in different phytoplankton populations.From the point of view of red tides biological control,C.pyrenoidosa filtrate can be used to control red tides in the aquaculture area.