Can chlorophyll fluorescence be used to estimate the rate of photosynthetic electron transport within microphytobenthic biofilms?

Abstract

Chlorophyll fluorometry has frequently been used to estimate the photosynthetic elec- tron transport rate (ETR) within oxygenic organisms. One of the requirements of this method is that the absorptivity of the photosynthetic system is known. In the specific case of microphytobenthos within biofilms, it is known that cells migrate vertically over relatively short time periods. This can radically alter the absorptivity of the photosynthetic biomass as a whole and, potentially, could result in highly inaccurate values of ETR being calculated. In this study, both modulated, integrating fluo- rometry and high resolution imaging of fluorescence have been used to investigate the error intro- duced into the calculation of ETR by the vertical migration of cells. Estimates of ETR derived from flu- orescence data were compared with rates of primary production measured using a 14 C-radiotracer method. The effect of fluorescence from photosystem I (PS I) on the calculated value of ETR was also assessed. Overall, these data suggest that PS I fluorescence can introduce significant errors into the estimation of ETR from diatom cultures and in the estimation of ETR from individual cells at the biofilm surface, when using high resolution imaging of chlorophyll fluorescence. Measurements made on intact biofilms under incident light showed an extremely poor correlation between esti- mated ETR (derived from fluorescence data) and primary production (measured by 14 C incorpora- tion). The simplest explanation for this result is that the downward migration of cells decreased the amount of light absorbed by the photosynthetic biomass, which led to substantial errors in the calcu- lation of ETR. The clear implication is that conventional (integrating) fluorometers cannot be used to determine rates of ETR from intact, migratory biofilms and that any realistic estimation of ETR within intact biofilms is likely to involve high resolution imaging of fluorescence.