Succession of Genetic Diversity of Botryococcus braunii (Trebouxiophyceae) in Two Japanese Reservoirs

Abstract

Abstract Botryococcus braunii is a green colonial alga that produces large volumes of liquid hydrocarbon. Therefore, B. braunii is expected to be useful as an alternative fuel resource. Natural blooms of B. braunii have been recorded in several lakes and reservoirs. Elucidation of natural B. braunii blooming would provide important information for the development of an open-pond cultivation system. In this study, we periodically assessed the genetic diversity and colony density of B. braunii populations, along with several environmental parameters, in two Japanese reservoirs (provisionally called “N” and “S”) from December 2008 to December 2009. Reservoir N had low numbers of B. braunii colonies whereas Reservoir S was characterized by periodic density increases that occurred in December 2008, and in March, September, and December 2009. Population genetics analysis using specific environmental sequences (PGA-SES method) was conducted for B. braunii populations for the first time. Among the B. braunii -dominated samples of Reservoir S, high levels of genetic diversity were observed in December 2008 and March 2009, whereas the diversity levels in September and December 2009 were low. The results suggest that B. braunii periodicity can be categorized into a high genetic diversity type and a low genetic diversity type. The high genetic diversity type may be caused by simultaneous growth of many genotypes, whereas the low genetic diversity type seems to be explained by increases in the cell density of only a few adapted genotypes.