Studies On Allelospoly Between Gracilaria Lemaneiformis (Bory) Weber Bosse And Microalgae

More and more contamination from land and aquaculture polluted the sea in the last several decades, as a result of which, eutrophication became a common ecological phenomenon in coastal area, neritic ecosystems were destroyed greatly, and harmful algal blooms (HABs) broke out in more localities at a much greater frequency. Utilizing macroalgae to remove nutrients from seawater is an alternative measure concerned by many scientists because of its combined merit. In this dissertation we researched the competition outcome between Gracilaria lemaneiformis and 5 kinds of red tide causative microalgae separately. These microalgae belong to dinoflagellate, raphidophyceae, and diatom, separately. Results showed that G. lemaneiformis could inhibit the growth of all of the target microalgae by allelospoly. Then we chose Scrippsiella trochoidea, a familiar red tide causative dinoflagellate, as target microalga to reaearch the allelospoly with G. lemaneiformis in detail. When initial biomass ratio (G. lemaneiformis to S. trochoidea) was relatively low, cell density in co-culture systems declined mainly because G. lemaneiformis coexisting competed on nitrate with S. trochoidea. When initial biomass ratio was relatively high, the decline of cell density in co-culture systems maybe attributed to some other reasons. Further research showed that it should be arisen by cell direct contact. Nitrate uptake kinetics of these two algae showed that S. trochoidea could absorb nutrients at a higher rate and could adapt itself in lower nitrate concentration than G. lemaneiformis. However, it was eliminated when co-cultured with G. lemaneiformis. That might be arisen by the fact that the latter could store nitrate intracellular at a higher efficiency and possess a much higher tolerance to oligotrophication. Because of the competition on nutrients from G. lemaneiformis, S. trochoidea could store fewer nutrients intracellular. That was an important reason for the competition outcome of G. lemaneiformis and S. trochoidea. Furthermore, we studied the effect of initial nutrients conditions on the competition outcome. When they were co-cultured under P-limited condition, G. lemaneiformis could stimulate the growth of S. trochoidea. When they were co-cultured under common and N-limited conditions, G. lemaneiformis could inhibit the growth of S. trochoidea. When the initial N:P ratio was 16:1 (Redfield value), the extent of inhibition was slightest. That might be attributed to the fact that S. trochoidea could grow best when it was mono-cultured in N:P ratio of 16:1, compared with other N:P ratios. Under semi-continuous culture, when initial nitrate concentration was 10μmol·L^-1 and G. lemaneiformis biomass was 0.2gFW·L^-1, G. lemaneiformis could stimulate the growth of S. trochoidea. In other conditions of initial nitrate concentrations and G. lemaneiformis biomass, G. lemaneiformis could inhibit the growth of S. trochoidea. The growth of S. trochoiea was inhibited much seriously when initial nitrate concentration was 50μmol·L^-1, maybe because conditions of relatively high nutrients concentrations were suitable for the growth of G. lemaneiformis.