Karenia mikimotoi is a well-known harmful algal bloom species. Blooms of this dinoflagellate have become a serious threat to marine life, including fish, shellfish, and zooplanktons and are usually associated with massive fish death. Despite the discovery of several toxins such as gymnocins and gymnodimines in K. mikimotoi, the mechanisms underlying the ichthyotoxicity of this species remain unclear, and molecular studies on this topic have never been reported. The present study investigates the fish-killing mechanisms of K. mikimotoi through comparative proteomic analysis. Marine medaka, a model fish organism, was exposed to K. mikimotoi for a three-part time period (LT25, LT50 and LT90). Proteins extracted from the whole fish were separated by using two-dimensional gel electrophoresis, and differentially expressed proteins were identified with reference to an untreated control. The change in fish proteomes over the time-course of exposure were analyzed. A total of 35 differential protein spots covering 19 different proteins were identified, of which most began to show significant change in expression levels at the earliest stage of intoxication. Among the 19 identified proteins, some are closely related to the oxidative stress responses, energy metabolism, and muscle contraction. We propose that oxidative stress-mediated muscle damage might explain the symptoms developed during the ichthyotoxicity test, such as gasping for breath, loss of balance, and body twitching. Our findings lay the foundations for more in-depth studies of the mechanisms of K. mikimotoi’s ichthyotoxicity.

Karenia mikimotoi is one of the most damaging ichthyotoxic dinoflagellate species commonly found in China. However, its growth and ichthyotoxicity responses to salinity changes are still largely unknown. In this study, the growth and ichthyotoxicity of three K. mikimotoi strains, Hong Kong strain KMHK, Japanese strain NIES2411 and New Zealand strain CAWD133, under different salinities (25 to 35 ppt), initial algal densities (5 to 40 thousand cells) and growth phases were investigated. Results indicated that the optimum salinity for all three strains was 30 ppt. The Japanese strain achieved the highest maximum cell densities (cells mL−1) and the New Zealand strain achieved the highest specific growth rate. The Hong Kong and New Zealand strains could not tolerate the low salinity at 25 ppt and the algal cells burst after 3 days of exposure. The average cell widths of all three algal strains in 35 ppt salinity were significantly larger than that in 30 ppt. The acute toxicity test performed on Oncorhynchus mykiss gill cell line RTgill-W1 revealed that the median lethal times for KMHK and NIES2411 were 66.9 and 31.3 min, respectively, and their ichthyotoxicity was significantly affected by algal cell density and growth phase. Nevertheless, CAWD133 did not pose any ichthyotoxicity. The gill cell viability levels at 30 min were reduced from 96 to 61% and 95 to 39% for KMHK and NIES2411, respectively, when the algal cell density increased from 5 × 103 to 4 × 104 algal cells mL−1. Both KMHK and NIES2411 at stationary phase also had higher toxicity than at log phase, with a 27% reduction of gill cell viability, and exerted higher toxicity to the gill cells under extremely low (28 ppt) or high (35 ppt) salinity. These findings demonstrated that the growth–ichthyotoxicity response of Karenia mikimotoi to salinity was not only strain-specific but also depended on its density and growth phase. Study on the effects of salinity on the growth and toxicity of K. mikimotoi is greatly limited. Results from the present study provide valuable insight on the growth and toxicity of different K. mikimotoi strains, which is important in understanding their occurrence of algal bloom and fish-killing action.

Biosorption is an effective method for removing heavy metal ions from wastewater. In the current study, the biosorption capacity of a microalgae Prorocentrum triestinum strain AD1 was investigated for cadmium removal. The efficient biomass concentration was found to be 5 g/L. Based on the Langmuir adsorption model, the maximum adsorption capacity (qmax) value of cadmium removal was found to be 0.0196 mmol/g. The investigation results of the AD1 biosorption kinetics showed that the effective contact time on biosorption was 3 h, and the adsorption kinetics fitted well with the pseudo-second-order model. The optimum pH of biosorption was found to be 5. On the other hand, HCl could act as an efficient desorbent for cadmium recovery from AD1, with an optimum concentration of 0.01 M. These results suggest that the biomass of P. triestinum has great potential for the removal of cadmium from wastewater as an efficient biosorbent.