海洋生态与环境科学重点实验室知识库

  有害藻华（Harmful Algal Blooms，HABs）频发严重威胁着海洋渔业、水产养殖业、旅游业及人类健康。改性粘土技术通过对天然粘土矿物进行表面改性，实现了对多种藻华生物的高效去除，目前已成为国内外藻华应急处置的首选方法。以往针对改性粘土的研究多以微藻营养细胞为实验对象。而某些藻华生物生活史中存在一些特殊阶段，这些处于特定阶段的藻细胞在藻华暴发中发挥着重要作用。改性粘土对于这些处于特殊阶段的典型藻华生物影响如何尚缺乏研究。浒苔（Ulva prolifera）微观繁殖体一直被认为是我国绿潮暴发的重要种源生物之一，本研究以浒苔微观繁殖体为研究对象，考察了不同体系改性粘土方法对其去除效果及萌发影响。孢囊的形成与萌发是某些甲藻藻华消亡及暴发的关键过程，本研究以典型有毒甲藻—太平洋亚历山大藻（Alexandrium pacificum）为研究对象，综合考察改性粘土絮凝沉降过程对甲藻孢囊形成及萌发的影响，同时关注改性粘土在絮凝沉降太平洋亚历山大藻后，对水体内及沉积絮体内麻痹性贝类毒素含量及组分的影响。为进一步考察改性粘土在敏感水域应用的安全性，以大菱鲆（Scophthalmus maximus）胚胎为模式生物，探究改性粘土对处于敏感期的鱼类胚胎-仔鱼阶段存活及生长发育的影响。针对三个不同的研究对象，所取得的几个阶段性成果如下：

  浒苔微观繁殖体：利用不同配比的硫酸铝改性粘土（AS-MC）及聚合氯化铝改性粘土（PAC-MC）对微观繁殖体进行去除，结果发现，同配比同浓度下，AS-MC去除效果优于PAC-MC，去除率随改性粘土及改性剂浓度增加而升高。对去除后水体内残余微观繁殖体及包含絮体在内的总体系微观繁殖体进行萌发培养，结果发现改性粘土在一定配比及浓度下能有效抑制其萌发，最高抑制率可达100%。

  太平洋亚历山大藻孢囊：通过室内培养太平洋亚历山大藻模拟其藻华的暴发过程，向藻液中分别添加0.2、0.4、0.6、0.8、1.0 g/L的改性粘土进行去除，去除结果显示，改性粘土对藻细胞的去除率随浓度升高而提升，适量改性粘土可将藻细胞密度在90 d内控制在较低水平。休眠孢囊密度在0.2、0.4 g/L改性粘土组中稍高于对照，在高于0.4 g/L的改性粘土组中低于对照。计算发现，总体而言，添加改性粘土并未显著提高休眠孢囊形成率。将形成的休眠孢囊在适宜条件下进行萌发培养，结果显示，添加改性粘土后孢囊形成率由对照组的68%降低到12.4%，即一定浓度的改性粘土不利于休眠孢囊的萌发。改性粘土可在24 h内迅速降低环境体系内的麻痹性贝类毒素含量，特别是水体中毒素含量，0.8 g/L改性粘土可将水体毒素含量迅速降至检测限以下。

  大菱鲆胚胎：选择典型经济硬骨鱼类—大菱鲆为模式生物，在其对环境变化最为敏感的胚胎阶段开展毒理学实验，探究改性粘土对其胚胎-仔鱼阶段存活及发育的影响。研究结果显示，不高于1.0 g/L的改性粘土不会造成胚胎存活率及孵化率的显著降低。改性粘土对大菱鲆胚胎24 h及48 h的半致死浓度（LC₅₀）分别为1.70、1.65 g/L，依此计算得到安全浓度为0.5 g/L，高于改性粘土现场使用浓度。持续跟踪初孵仔鱼的生长发育指标发现，不高于0.5 g/L的改性粘土未对初孵仔鱼的生长发育指标造成不良影响。

  综上，在以往的基础上，本研究进一步探究改性粘土对于典型绿潮藻微观繁殖体、甲藻孢囊、毒素及非藻华水生生物敏感阶段的影响。研究发现在特定改性剂及配比下，适量改性粘土能够有效去除浒苔微观繁殖体并抑制其萌发；一定浓度改性粘土不会为底层环境中埋下更多具有萌发能力的孢囊作为藻华再次暴发的“种源”，且在有效去除有毒藻细胞24 h内，使得体系内藻毒素含量迅速降低；现场应用浓度范围内的改性粘土未对受试鱼类胚胎存活、孵化及初孵仔鱼的生长发育造成不良影响。本研究为全面了解改性粘土在治理有害藻华过程中的生态环境效应提供了一定的参考依据。

  The frequent occurrence of harmful algal blooms (HABs) poses great threats to the fishery, aquaculture, tourist and public health. The removal efficiency of clay to various microalgae has been improved greatly after surface modification. As a result, the modified clay technology is now regarded as the most promising method for HABs' mitigation. Previous studies usually focus on the removal efficiency of modified clay to the vegetative cells of microalgae. However, for some harmful algae, cells in special stages of their life history is crucial for the bloom. The effects of modified clay on these special cells is lack of research. The microscopic propagules of Ulva prolifera are regarded as one of the factors that seed the green tides. The effects of different modified clay on the removal and germination of microscopic propagules were investigated to provide references for typical green tides' control. For the dinoflagellates, cyst plays an important role in the initiation and termination of blooms. The Alexandrium pacificum, a typical toxic cyst-producing dinoflagellate, was investigated to assess the effects of modified clay flocculation to the cysts and paralytic shellfish toxins (PSTs). In addition, to assess the safety of using modified clay in the spawning and nursery grounds of fishes, the survival and hatching rate of turbot Scophthalmus maximus embryo were investigated as well as the growth and development of newly hatched larvae. The periodic results are as follows.

  Different modified clays were used to remove microscopic propagules. The results indicated that the removal efficiency increased with the rising amount of modified clay and modifier. AS modified clay was more effective than PAC modified clay, under the same concentration and modifier/clay ratio. Moreover, the germination of microscopic propagules could be inhibited completely under appropriate modified clay concentration.

  To investigate the effects of modified clay on the cyst formation and germination during HABs' mitigation, bloom of A. pacificum was simulated in the lab, and the different concentrations (0.2, 0.4, 0.6, 0.8 and 1.0 g/L) of modified clay were added for the bloom control. The results showed that modified clay could remove A. pacificum effectively, with higher concentrations of modified clay exhibited higher removal efficiencies. The vegetative cell density was limited in a long period with an appropriate concentration of modified clay, and the residue cells did not show a persistent growth. Finally, the resting cyst density in groups with 0.2, 0.4 g/L modified clay was slightly higher than the control, while resting cyst density in groups with more than 0.4 g/L modified clay was less than control. The resting cyst formation rate was not increased after modified clay addition. To assess the differences in germination ability, thirty resting cysts in experimental and control groups were randomly pipetted out after 90 d of modified clay addition. The results showed that the germination rate of resting cysts decreased from 68% to 12.4% after adding modified clay, which means the addition of modified clay may be unfavorable for the resting cyst germination. The effects of modified clay on the paralytic shellfish toxins (PSTs) in the water column and bottom flocs were also studied. Modified clay was able to reduce the PSTs of the environment within 24 h, with 0.8 g/L led to the PSTs content in the water column lower than the detection limit.

  Turbot (Scophthalmus maximus) embryos were used as a model to assess the effect of modified clay on this sensitive stage of fishes. The 24 and 48 h LC₅₀ (median lethal concentrations) of the modified clay were 1.70 and 1.65 g/L, respectively, and the safe concentration was 0.5 g/L, which was much higher than the modified clay concentrations used in mitigating HABs. Modified clay did not significantly influence the hatchability of turbot embryos, but when the concentration exceeded 0.50 g/L, the deformity rate of newly hatched larvae increased significantly. The total length, specific growth rate (SGR) and yolk sac absorption rate of larvae reached their peaks at 0.50 g/L and then gradually decreased as the concentration of modified clay increased.

  Based on the previous researches, this study further investigated the effects of modified clay on the dinoflagellate cyst, toxins, and non-target organisms. The results showed that appropriate amount of modified clay would not leave more cysts in the sediment to act as "seed", and were able to control the PSTs in 24 h. Modified clay with the concentration used in fields would not cause any adverse effects on the survival and development of the tested fish embryos. This study is expected to provide references in fully understanding the environmental effects of modified clay in HABs' mitigation.