AbstractmlrA is the only microcystin-degrading gene detected in Sphingomonas sp. MJ-PV. The gene has an extremely rare nucleotide sequence and homologous genes have not yet been discovered in the DNA database. We discovered the existence of a gene homologous to mlrA in new microcystin-degrading bacteria, MD-1 and Y2. These strains possessed mlrA homologues, and the identities of the genes of MD-1 and Y2 with the corresponding MJ-PV exceeded 98% and 84%, respectively. On the other hand, the mlrA gene was not detected in laboratory strains of the closely related Sphingomonas spp. strains employing hemi-nested polymerase chain reaction detection using two primer sets. Although the microcystin-degrading bacteria were closely related strains, they did not cluster together as the same species. We can conclude that the mlrA gene is conserved in three different bacterial species, and it is unique to microcystin degraders but not to the genus Sphingomonas.

We compiled and analyzed long-term data, including chemical, physical and phytoplankton community data, for the Lake Biwa ecosystem from 1962 to 2003. Analyses on environmental data indicate that Lake Biwa had experienced intensified eutrophication (according to total phosphorus concentration) in the late 1960s and returned to a less eutrophic status around 1985, and then exhibited rapid warming and thus increased water column stability since 1990. Total phytoplankton cell volume largely followed the trend of total phosphorus concentration, albeit short-term fluctuations existed. However, phytoplankton community shifted dramatically in response to those changes of environmental states. These shifts were cause by changes in trophic status driven by phosphorus loadings and physical properties in the water column driven by warming. Moreover, most phytoplankton species did not show a strong linear correlation with environmental variables, suggesting nonlinear transitions among different states.