PHOTOSYNTHETIC ACTIVITY OF Microcystis IN FISH GUTS AND ITS IMPLICATION FOR FEASIBILITY OF BLOOM CONTROL BY FILTER-FEEDING FISHES

摘要

In a background of increasingly frequent and intensive Microcystis blooms occurred in eutrophic lakes, many countries, particularly China, have used filter-feeding fish to control algal blooms according to non-traditional biomanipulation theory. However, there is little information concerning the potential photosynthetic activity of cyanobacteria in the guts of planktivorous fishes. To make a scientific assessment of this algal control technology, we evaluated the effects of digestion by two fish species, silver carp and bighead carp, on the growth potential of Microcystis blooms in terms of photosynthetic activity, metabolic activity, up-floating velocity, morphological size, and toxin production. The results showed that: 1) the potential photosynthetic activity of Microcystis dropped significantly in fish foreguts after ingestion, but gradually increased and recovered in the midgut and hindgut; 2) digestion by planktivorous fish could significantly decrease the colonial size and up-floating velocity of Microcystis; 3) selective digestion by bighead carp caused a sharp rise in cellular microcystin levels; and 4) Microcystis retained a high growth potential after digestion by filter-feeding fish. These results indicate that planktivorous fish could be used for controlling blooms due to the degree of digestion and long retention of Microcystis in fish guts, but the controlling effect was limited because the potential photosynthesis activity of algae gradually recovered along the gut sections and the digested algae could grow normally after subsequent release into lakes. In addition to being less effective at controlling algal blooms than silver carp, the digestion process in bighead carp might confer an advantage to toxic Microcystis species.