The Phylogenetic Position Of A Bloom-forming Achnanthoid Species And The Study Of Its Growth Characteristics

Achnanthidium catenatum is a fresh water bloom-forming achnanthoid species.For detecting the evolution status of the species,we separated and purified it and other six achnanthoid species from lakes and reservoirs in China,and their morphology and phylogeny of 18 S rDNA and rbcL gene were analyzed.The results of phylogenetic analyses divided these monoraphidinales species into three main clades.One of them concluded some species from genera Achnanthidium,Rossithidium,Psammothidium,Pauliella,Planothidium,Lemnicola,Eucocconeis and Cocconeis,and showed closely genetic relationship with asymmetrical biraphid species.The second one contained only one genus of Schizostauron,which showed closely genetic relationship with symmetrical biraphid species.The third clade contained the genus of Achnanthes,which showed closely genetic relationship with Nitzschia and Eunotia.Results showed that this was a polyphyletic group.Species from genus Achnanthidium mostly are periphytic except Achnanthidium catenatum,which is the only planktic species showing chain-forming.In this experiment,we explored the growth characteristics of the Achnanthidium catenatum strain which were collected from Ningbo Dongqian Lake,Zhejiang province.Firstly,the effect of phosphorus on growth and chain colony-forming of Achnanthidium catenatum were studied.Results showed that: there were no significant differences for the total cell density in the preliminary phase(p > 0.05)and the Achnanthidium catenatum were planktonic under different concentration of phosphorus.The total density had been a quick decline in high phosphorus concentration group(0.5,1.0,5.0mg/L)and the multicellular population decresed along the culture time after stationary phase and grew mainly in unicellular.Meanwhile,the total density in low phosphorus concentration group(0.01,0.05mg/L)showed a slow decline,but the multicellular colony also transferred into unicellular.The APA activity was closely negative related to the phosphorus concentration in the culture,in other words,the APA activity in low phosphorus concentration group was significantly higher than that of the high phosphorus concentration group.In the wake of consumption of nutrition,the APA activity increased for the insurance of the growth of Achnanthidium catenatum.There were no significantly differences in total EPS content between different groups(p > 0.05).However,the average EPS content of every cell when dominated by multicellular colony was significantly higher than that of dominated by unicellular(p < 0.01).It means that the colony-forming is relying on the EPS content.Secondly,experiments were arranged on the response strategy of Achnanthidium catenatum to the signal of zooplankton grazing.The results showed that the effect between different species(big and small size),different concentration of zooplankton culture were different.Within a certain range,the growth ratio of Achnanthidium catenatum were inhibited by the culture of zooplankton.And the inhibiting effect was weakened after boiling,which still facilitated the species to form more multicellular colony.As the extension of process,the Achnanthidium catenatum gradually transformed into a growth of unicellular dominated.At last,experiments were carried out on the response strategy of Achnanthidium catenatum to the signal of macro filamentous green algae(Cladophora sp.)competition.Compared with the zooplankton,the effect of boiled Cladophora sp.culture on Achnanthidium catenatum also can facilitate the species to form more multicellular colony.Different concentration of Filamentous green algae culture had certain effect on the growth of Achnanthidium catenatum.But it showed some facilitation to the diatom under low concentration and inhibition in high concentration.Achnanthidium catenatum tended to attach to the surface of f Cladophora sp.cell when they were cultured together,and the cell density in planktonic were very low.These total results suggested that to control a bloom of Achnanthidium catenatum using macro filamentous green algae is possible.