Continuous and Intermittent Exposure to the Toxigenic Cyanobacterium Microcystis aeruginosa Differentially Affects the Survival and Reproduction of Daphnia curvirostris.

ABSTRACT

Anthropic eutrophication leads to water quality degradation because it may cause the development of harmful cyanobacterial blooms, affecting aquatic biota and threatening human health. Because in the natural environment zooplankters are exposed continuously or intermittently to cyanotoxins in the water or through cyanobacterial consumption, this study aimed to assess the effects of the toxigenic Microcystis aeruginosa VU-5 by different ways of exposure in Daphnia curvirostris. The acute toxicity produced by the cells, the aqueous crude extract of cells (ACE), and the cell-free culture medium (CFM) were determined. The effect on the survival and reproduction of D. curvirostris under continuous and intermittent exposure was determined during 26 d. The LC50 was 407,000 cells mL-1; exposure to the ACE and CFM produced mortality lower than 20%. Daphnia survivorship and reproduction were significantly reduced. Continuous exposure to Microcystis cells caused 100% mortality on the fourth day. Exposure during 4 and 24 h in 48 h cycles produced adult mortality, and reproduction decreased as the exposure time and the Microcystis concentrations increased. The higher toxicity of cells than the ACE could mean that the toxin's absorption is higher in the digestive tract. The temporary exposure to Microcystis cells produced irreversible damage despite the recovery periods with microalgae as food. The form and the continuity in exposure to Microcystis produced adverse effects, warning about threats to the zooplankton during HCBs.