14C labelling of algal pigments to estimate the contribution of different taxa to primary production in natural seawater

Several attempts have been made in the past to measure taxon-specific growth rates in natural phytoplankton populations in order to evaluate the conditions leading to success of individual taxa, to estimate the specific role of the various taxonomic components of algae in the food web and in nutrient cycling, and to explain succession. The method of Redalje and Laws, who studied the pattern of l4C labelling of Chi a (the pigment common to all microalgae), has now been adapted by following 14C incorpor­ ation into the carotenoids that are tags of taxonomic groups. The results obtained so far indicate that even in oligotrophic regions different species co-occurring in one sample usually incorporate 14C at a very different rate, which suggests that it is not only eutrophic pelagic plankton communities that are subject to rapid shifts in growth conditions. The pigment labelling method is still open to challenge from several directions. More research must be done to transform this approach into a technique that is undeniably accurate and reliable for the assessment of growth rates of algal taxa-o r to show when and under what circumstances it cannot be used. Special care should be taken to avoid radioactive contamination of pigments separated by HPLC, and isotope dilution during experiments. The methods that are available now can also be applied directly to studies of the turnover and eventual fate of the various pigments synthesized by algae, be they photosyntheticaliy active or photoprotective, e.g., in studies of photoadaptation, for example the response of algae to increased ultraviolet radiation. I4C labelling of both chlorophylls and carotenoids may even become a tool in studies of large-scale carbon cycling because a considerable part of phytoplankton carbon biomass in the sea and in sediments is associated with pigments or their degradation products.