Effects Of Multiple Environmental Factors On The Growth Of Heterosigma Akashiwo

Under the dual factors of global warming and anthropogenic activities,the environment has been altered,such as increased atmospheric carbon dioxide and seawater eutrophication.As a result,both frequency and intensity of harmful algal blooms(HABs)in coastal waters are increasing worldwide,threatening marine ecosystem,fishery industry,and human health through food web accumulation.Heterosigma akashiwo,one of the HAB species,causes red tide outbreaks in the coast of China from time to time.Due to its toxicity,H.akashiwo blooms have killed a large amount of fish,and resulted in significant economic losses.In order to further understand outbreak mechanism of H.akashiwo as to monitor and control HABs,it is of great significance to study H.akashiwo under multiple environmental factors.Based on the observed nutrient concentration ranges in the East China Sea(ECS),four combinations of nitrate and phosphate were set up in this laboratory study:high nitrate and high phosphate(HNHP:NO₃^-30?M+PO₄^3-1?M),high nitrate and low phosphate(HNLP:NO₃^-30?M+PO₄^3-0.5?M),low nitrate and high phosphate(LNHP:NO₃^-15?M+PO₄^3-1?M)and low nitrate and high phosphate(LNLP:NO₃^-15?M+PO₄^3-0.5?M).Based on IPCC report,three combinations of CO₂ and temperature were employed:Control group(430 ppm+20?),High-CO₂ group(1000ppm+20?)and High-CO₂&T group(1000 ppm+25?).The effects of multiple environmental factor changes on growth of H.akashiwo were measured by monoculture and bi-algal culture experiments,respectively.Results showed that the maximum cell density of H.akashiwo of the low phosphate treatments was significantly lower(about 20-65%)than that of high phosphate treatments in both monoculture and bi-algal culture experiments.Also,the specific growth rate of H.akashiwo of the low phosphate treatments was significantly lower(about 10-18%)than that of the high phosphate treatments in monoculture.In addition,the cell density of H.akashiwo was not statistically different in different nitrate treatments.Compared with the Control group,increased CO₂ and temperature could enhance the cell density in the exponential growth phase(about 0.2-2.9 times higher than Control groups).With respect to species competition in all treatments,except for HNHP and LNLP,H.akashiwo outcompeted Skeletonema costatum during the later phases of experiments.It may indicate that H.akashiwo has a stronger competitive advantage than S.costatum.In summary,phosphate concentration was found to significantly influence the growth of H.akashiwo,and phosphate was a major limiting nutrient in the growth and development of this HAB species.H.akashiwo is able to become a dominant species in coastal waters and has the potential to induce large-scale blooms under increasing CO₂ and temperature,particularly with sufficient nutrients.Based on this study,it suggests that control of nutrient concentration,particularly phosphate,could be an effective measure to reduce the H.akashiwo outbreak.This study provides a new understanding of the growth of H.akashiwo under varying but realistic nutrient concentrations,which could serve as a reference for coastal water management and marine ecological protection.