Photoinhibition in outdoor Spirulina platensis cultures assessed by polyphasic chlorophyll fluorescence transients

Photoinhibition in outdoor cultures of Spirulina platensis was studied by m easuring the polyphasic rise of chlorophyll fluorescence transients, which provide information on the primary photochemistry of PSII. The maximum effi ciency of PSII photochemustry (F-v/F-m) declined in response to daily incre asing irradiance and recovered as daily irradiance decreased. The greatest inhibition (15%) in F-v/F-m was observed at 12:00 hr which responded to the highest irradiance. The absorption flux, the trapping flux, and the electr on transport flux per PSII reaction center increased in response to daily i ncreasing irradiance and decreased as irradiance decreased. The daily chang e in the concentration of PSII reaction centers followed the same pattern a s F-v/F-m. However, no significant changes in the probability of electron t ransport beyond Q(A) (Psi(o)) were observed during the day. The results sug gest that the decrease in F-v/F-m induced by photoinhibition in outdoor Spi rulina cultures was a result of the inactivation of PSII reaction centers. The results also suggest that the measurement of polyphasic fluorescence tr ansients is a powerful tool to study the mechanism of photoinhibition in ou tdoor Spirulina cultures and to screen strains for photoinhibition toleranc e.