The Efficacy And Mechanisms Of Fungal Suppression Of Freshwater Algal Species And Microcystin

Cyanobacterial blooms have been observed in eutrophic fresh waters all over the world and are serious worldwide environmental problem, commonly causing ecological, economic or possible animal even human health impacts by deterioration water quality and secreting cyantoxins. There is already a substantial amount of research on the control of cyanobacterial blooms and the degradation on cyantoxins. Algicidal microorganisms, especially bacteria, have attracted extensive attention as possible agents for inhibiting water blooms and degrading cyantoxins. Such capabilities, however, are rarely reported for fungi. In previous studies, with the exception of antibiotic-producing fungi that can lyse cyanobacteria by secretion of antibiotics, little is known about the algicidal ability of fungal strains on cyanobacteria without producing antibiotics. Moreover, compared with fungi, although algicidal bacteria have also the ability to inhibit algal cell growth and directly or indirectly break algal cell structure, a large population of bacteria can decrease water transparency, be hard to control, and bring toxic metabolites into the water.Cyanobacterial blooms, occurred in Taihu lake in 2007, had also brought serious detrimental effects to the public life, economical and social development. To control the cyanobacterial blooms, in this paper, the white-rot fungal strain Trichaptum abietinum 1302BG and Lopharia spadicea were studied on their algicidal ability.1. We studied and evaluated algicidal ability of the fungal strain on different freshwater algal species:Cyanophyta, Chlorophyta, Bacillariophyta, Dinophyta, Cryptophyta, and Euglenophyta. The results show that cell density of these freshwater algal species except Euglenophyta decreased rapidly after 24 h inoculated with fungi and algal cells were almost completely disappeared after 48 h incubation. These suggest that the fungal strain has great potential for the degradation of phytoplankton species.2. The scanning electron microscope and transmission electron microscope observation revealed also that the fungal strain had preying ability on the algal cells. The mechanism may be three stages of the algal degradation. Stage one, the algal cells physically contacted with the mycelia of TA-1302 under a shaking condition. Stage two, the algal cells were surrounded with mucous membrane secreted by the mycelia of TA-1302. The mycelial pellicle emerged some green color. Stage three, the algal cells were degraded. The color of the mycelial pellicle was restored from green to white at approximately 48 h. This result indirectly showed that the algal cells that came in contact with the mycelial networks were degraded last.3. This study also examined the ability of Trichaptum abietinum 1302BG, a white rot fungus, to degrade microcystin-LR in the harmful algal culture. Results showed that microcystin-LR could not be detected by HPLC after 12 h and that biomass removal rate of algal culture was 93.23% after 48 h incubation with the fungus. The new substance was also found in the degraded algal culture. There were high activities of catalase and peroxidase in algal culture incubated with the fungus. Meanwhile, the micronucleus test in the toxicity studies revealed that the degraded algal culture had low toxicity. Naturally occurring terrestrial fungus with the potential capability of inhibiting harmful phytoplankton species may pave a new way for controlling algal blooms.4. Moreover, mycelial plaque on solid-plate and dry weight of mycelial pellicles in liquid-plate revealed that the white rot fungi, Trichaptum abietinum 1302BG and Lopharia spadicea, could use the cyanobacterial blooms directly took from Taihu lake to grow and cyanobacterial blooms could be used as a glucose substitute in the medium of white rot fungi. The removal rate of cyanobacterial blooms directly took from Taihu lake was approximately 100% incubated with the fungi strain Trichaptum abietinum 1302BG and Lopharia spadicea. It may be a new way to dealing with the cyanobacterial blooms as a medium of fungi instead of directly discharged as organic fertilizers, lest the secondary pollution of environment.