Inactivation Of Cysts Of Scrippsiella Trochoidea By Different Physical And Chemical Treatments

Dinoflagellates are an important part of marine phytoplankton and contributor of primary productivity.In fact a large number of harmful algae blooms（HABs）are caused by dinoflagellates.Some dinoflagellates can be transported to other sea areas as the form of cysts via ships’ ballast water,and are introduced as the exotic species,and thus expand their geographic distribution.Ecological invasion caused by ballast water is now one of the greatest threats to biodiversity.Dinoflagellate cyst（dinocyst）walls are made of refractory organic matter,and thus are highly resistant to adverse stimulations.Therefore,the efficiency of ballast water treatments is based on the inactivation of dinocysts.Cysts of Scrippsiella trochoidea were used in this study to investigate effects of different physical and chemical treatments on the inactivation of cysts.Effects of heating,ultraviolet（UV）,ultrasonic（US）,freshwater and hydrogen peroxide（H2O2）and the combinations of two and three treatments on the germination of cysts of S.trochoidea were studied in order to understand different treatments on the inactivation of dinocysts.The treatments of this study included those on pure cysts and cysts mixed with sediments.The purpose of this study is to find an effective and feasible way to prevent the introduction of harmful algal bloom species via ship ballast water,and to provide some basic information for ballast water treatments.The results of this study will play an important role in protecting marine environments and ecosystems.Following results are drown from this study:1.Cysts of S.trochoidea have high germination efficiency,and the accumulative germination rates were over 90% in the control groups without any treatments.The initial excystment occurred at day 1-3 after incubation,and excystment lasted about 50 days.2.Heating inhibited cyst germination significantly,and the accumulative germination rates decreased gradually with increasing temperature.Meanwhile,heating also had a significant effect on the initial excyst time,and higher temperature prolonged the initial excyst time.Germination rates were significantly reduced at 35°C for 5h and 38°C for 1 h when treated on pure cysts.Furthermore,cysts were completely inactivated after treated at 38°C and 40°Cfor 5 h,42°C for 3 h,and 45°C for 2 h for pure cysts.Effects of heating on cyst germination were mitigated when mixed with sediments.The germination rates were 10%-23% higher in treatments with sediments.Meanwhile cysts in sediments were completely inactivated after treated at 42°C for 5 h.3.Ultrasonic（US,40 KHz,40-60 min）,ultraviolet（UV,253.7 nm,30-60 min）and freshwater（60-180 d）could efficiently inhibit cyst germination,with the accumulative germination rate of 24.60%,39.68%,36.67%,respectively based on pure cyst treatments.Furthermore,US and UV treatments prolonged the initial excyst time.When mixed with sediments,the effects of inactivation of cyst decreased about 15%-50% compared with the same treatments on pure cysts.However,none of these three treatments resulted in completely inactivation of cysts.4.H₂O₂ treatments significantly inhibited cyst germination as well.H2O2（>40 mg/L,>3 d）significantly inhibited cyst germination based on pure cyst treatments.Germination rates were only 2%¹7% higher on cysts in sediments compared to pure cysts.Cysts were completely inactivated after treated at 100 mg/L for 6 d on both pure cysts and cysts in sediments.5.Combinations of two treatments were more effective in the inactivation of cysts than those of single treatments.The inhibition rates of combined treatments were 20%-60% higher compared with those of single treatments based on pure cysts.Meanwhile,the inhibition rates of heating combined with US or UV were 5%³0% higher than those of combinations of US and UV treatments.Cysts were completely inactivated after treated at 38°C for 3 h combined with UV（253.7 nm,60 min）and US（40 KHz,60 min）.For cysts in sediments,the inhibitory efficiencies of combined treatments were about 15%-30% higher than those of single treatments.However,inactivation effects decreased about 7%-40% compared with those on pure cysts.Cysts were completely inactivated after treated at 40°C for 5 h combined with UV（253.7 nm,60 min）and US（40 KHz,60 min）.6.Combinations of three treatments were most effective to inhibit cyst germination.Cysts were completely inactivated after treated at 38°C for 1 h combined with US（40 KHz,60 min）and UV（253.7 nm,60 min）based on pure cysts.As to cysts in sediments,the inhibition rates decreased about 7%-17% compared with pure cysts after treated at 38°C for 1-3 h combinedwith US and UV treatments.Cysts in sediments were completely inactivated after 40°C for 3 h combined with US and UV treatments.7.The results from this study suggested that heating might be a feasible way for ballast water treatments by using the waste heat from the cooling system of the ship engine.US,UV and freshwater treatments inhibited cyst germination to some extent.Combinations of two or three treatments efficiently inactivated cyst germination,and thus could be feasible to prevent the introduction and worldwide expansion of harmful bloom species via ballast water.