Blue-fluorescence of Nadph as an Indicator of Marine Primary Production

Nicotinamide Adenine Dinucleotide Phosphate (NADPH) is the primary product of photosynthesis and can therefore serve as an indicator of biomass and photosynthetic activity. Pure NADPH which is the reduced form of NADP shows an absorption maximum at 340 nm and a maximum of emission at 460 nm. NADPH concentrations in terrestrial vegetation have already been studied since 1957 in great detail with optical methods. However, its potential as a biomass parameter of oceanic phytoplankton which can be assessed in situ and remotely with fluorescence spectroscopy has not yet been investigated. In this paper, we report on laboratory investigations of the blue-fluorescence spectrum in algal suspensions of Chlorella and Thalassiosira when excited with UV-A light. It is shown that cell densities of about 10 per litre as they are typically found under natural conditions are too low for precise detection of NADPH fluorescence, while concentrated samples with 10-10 cells per litre exhibit significant blue-fluorescence which can be related to NADPH. Inhibition of photosynthetic activity by addition of DCMU decreases the strength of blue-fluorescence remarkably. Since NADPH is an end product of photosynthesis, changes of PAR illumination levels should directly affect its concentration and hence the intensity of blue-fluorescence. However, no effect of illumination on blue-fluorescence could be observed in our study. Possible reasons of these observations are discussed, and perspectives for practical applications of the method used are proposed.