链状亚历山大藻赤潮衰亡的生理调控

研究了链状亚历山大藻在对数生长期、衰亡期、高氮、低氮条件下,藻细胞中可溶性蛋白含量、超氧化物歧化酶(SOD)活性、丙二醛(MDA)、过氧化氢(H2O2)和还原型谷胱甘肽(GSH)含量、光合速率和呼吸速率、DNA降解、端粒酶活性的变化。结果表明:在衰亡期、高氮、低氮条件下链状亚历山大藻细胞中可溶性蛋白、GSH含量、光合速率和呼吸速率下降;SOD活性(低氮条件除外)、H2O2、MDA含量上升;端粒酶活性和DNA Ladder随着藻细胞生长而变化,并在衰亡时期,出现了明显的DNALadder。研究结果显示链状亚历山大藻衰亡过程的反应表现为:蛋白质合成受阻或降解,产生大量氧化中间产物(MDA,H2O2等),抗氧化系统被激活,GSH等非酶抗氧化物质被大量消耗,SOD等酶抗氧化物被激活;另外表现为光合速率和呼吸速率下降;同时活性氧自由基(Reactive Oxygen Species,ROS)的积累诱发了细胞凋亡,核酸内切酶被激活,选择性降解染色质DNA。推测低氮、高氮条件均可以加快藻细胞的衰亡的生理过程,链状亚历山大藻的赤潮衰亡是一种有序的死亡过程。

Physiological regulation related to the decline of Alexandrium catenella

Over the last several decades, harmful algal blooms (HABs) have emerged as a global environmental problem because of their more frequent occurrence and because of the threat they pose to the health of humans and other organisms. Great efforts have been made to elucidate the ecological and biological features of red tide events, using approaches ranging from molecular and cell biology to large-scale field surveys, numerical modeling, and remote sensing from space. However, studies on the molecular mechanisms of red tides, including those involved in their decline phase, are still limited. Researchers believe that the decline phase of red tides represents a process of programmed cell death (PCD). PCD, which is controlled by multiple factors, is an active, gene-regulated process that has evolved in most organisms. Alexandrium catenella is an important causative dinoflagellate associated with HABs and paralytic shellfish poisoning. In this study, we determined physiological and biochemical indices of A. catanella including soluble protein content, superoxide dismutase (SOD) activity, malondialdehyde (MDA) content, reduced glutathione (GSH) content, hydrogen peroxide (H₂O₂) content, photosynthetic rate, respiratory rate, DNA laddering, and telomerase activity. These biochemical analyses were conducted using cells of A. catenella collected after different periods of growth and under different growth conditions (i.e., with different concentrations of nitrogen in the medium). There were differences in several parameters between the decline phase and the logarithmic phase. There was an increase in peroxidation in A. catenella during the decline phase and under low-and high-nitrogen conditions. This was characterized by increased SOD activity (except under low-nitrogen growth conditions), MDA content, and H₂O₂ content, and decreases in soluble protein content, GSH content, photosynthetic rate, and respiratory rate. The results suggested that excessive reactive oxygen was the main reason for the decline of A. catenella. Based on these experimental results, we hypothesized that the physiological process during the decline phase of A. catenella was as follows: First, metabolism slowed, resulting in a decrease in intracellular protein levels. Secondly, cells produced large amounts of peroxide, which activated the cellular antioxidant system. SOD was activated in response to the massive accumulation of ROS, and other non-enzymatic antioxidants such as GSH were consumed. In spite of these changes, the oxidation and antioxidant system remained unbalanced, resulting in excess accumulation of H₂O₂ and other ROS. Ultimately, those ROS led to serious membrane lipid peroxidation and the release of MDA. The decrease in SOD activity under low-nitrogen conditions may have been due to a lack of nitrogen, which is necessary for protein synthesis. Finally, excessive accumulation of ROS induced apoptosis, and the endonuclease was activated to selectively degrade chromosomes, resulting in DNA laddering. The photosynthetic rate decreased as a result of decreases in chlorophyll content and RUBP carboxylase activity. The respiration rate decreased as a result of the decreased volume of mitochondria and smaller area of their internal cristae. Telomerase activity also changed during the cell growth and decline periods. Low-nitrogen and high-nitrogen conditions accelerated the physiological process of cell aging. These results are consistent with the hypothesis that the decline of A. catenella is a controlled process of cell death. Our findings reveal some of the physiological changes that occur during the decline phase of A. catenella. These data will help us to understand the decline mechanisms of red tide algae, and provide a foundation for studying the molecular mechanisms of red tide dynamics. Such information will be useful for developing methods to monitor and control red tides.