[Show abstract][Hide abstract] ABSTRACT: Some free fatty acids are toxic to phytoplankton, and the toxic effects are multiple. However, precisely how they kill phytoplankton is debatable. Here we show that fatty acids result in damage to plasma membranes, which might account for their lethal effects on phytoplankton. In this study, we used two chlorophytes (Chlorella vulgaris Beij. and Monoraphidium contortum (Thur.) Kom.-Legn.) and a cyanobacterium (Anabaena P-9) as test organisms. When these organisms were treated with deleterious concentrations of fatty acids, a remarkable elevation of extracellular potassium (K+) was detected in the culture medium; this indicates that leakage of intracellular K+ occurred as a result of damage to the plasma membranes. Exposure to unsaturated fatty acids resulted in higher levels of leaked K+ than did exposure to saturated ones, and levels of leakage displayed a positive correlation with the susceptibility of the growth of organisms to fatty acids. Stressed phytoplankton cells also exhibited cell lysis followed by free release of phycobilins. The sequence of cytotoxic effects elucidated here suggests that fatty acids primarily affect the plasma membranes, leading to a change in membrane permeability and dissociation of phycobilins from the thylakoids. Severe damage to the plasma membranes would give rise to a disruption of the stressed cells.
[Show abstract][Hide abstract] ABSTRACT: The alga Botryococcus braunii Kützing (Chlorophyceae) present in Liyu Lake (Huanlien County, Taiwan) has toxic effects on a variety of aquatic organisms. Blooms of this alga, which typically occur in autumn, are associated with fish deaths in this lake. Experiments using 15 phytoplankton and 5 zooplankton isolated from Liyu Lake indicate that these plankton exhibit various susceptibilities to B. braunii. A close correlation between the degree of susceptibility tested in the laboratory and the absence of certain phytoplankton during B. braunii blooms in the lake was observed, suggesting allelopathic effects. Isolation, identification, and verification with authentic compounds indicated that allelochemicals were a mixture of free fatty acids, including -linolenic, oleic, linolic, and palmitic acids. Compared with other phytoplankton isolates, B. braunii produced significantly higher amounts of free fatty acids, particularly of oleic and -linolenic acids. The role of these fatty acids in favoring dominance of B. braunii in the natural environment was elucidated.