IN-SITU MONITORING OF CHLOROPHYLL FLUORESCENCE TO ASSESS THE SYNERGISTIC EFFECT OF LOW-TEMPERATURE AND HIGH IRRADIANCE STRESSES IN SPIRULINA CULTURES GROWN OUTDOORS IN PHOTOBIOREACTORS

A chlorophyll fluorescence technique was applied to an in situ study o n the effects of low temperature and high light stresses on Spirulina cultures grown outdoors in controlled tubular photobioreactors at high (1.1 g L(-1)) and low (0.44 g L(-1)) biomass concentrations. Diurnal changes in PSII photochemistry (F-v/F-m) after 15 min of darkness, or in the light (dF/F-m(')), and non-photochemical (qN) quenching were me asured using a portable, pulse-amplitude-modulated fluorometer. The de pression of the F-v/F-m ratio of Spirulina cultures grown outdoors at 25 degrees C (i.e. 10 degrees C below optimum for growth) and 0.44 g L (-1), reached 30% at the middle of the day. At the same time of the da y the dF/F-m(') ratio showed a reduction of up to 52%. The depression of both F-v/F-m and dF/F-m(') was lower in the cultures grown at 1.1 g L(-1). Photoinhibition reduced the daily productivity of the culture grown at 0.44 g L(-1) and 25 degrees C by 33% with respect to that gro wn at 35 degrees C. Changes in the growth yields of the cultures grown under different temperatures and growth rates correlate well with ana logous changes in photon yield (dF/F-m'). Simple measurements of photo chemical yield (F-v/F-m) can be used to test the physiological status of Spirulina cultures. The results indicate that the saturating pulse fluorescence technique, when used in situ, is a powerful tool for asse ssment of the photosynthetic characteristics of outdoor cultures of Sp irulina.