Parameters of photosynthesis: definitions, theory and interpretation of results

A global assessment of carbon flux in the world ocean is one of the major undertakings of the Joint Global Ocean Flux Study (JGOFS). This has to be undertaken using historical in situ data of primary productivity. As required by the temporal and spatial scales involved in a global study, it can be conveniently done by combining, through appropriate models, remotely sensed information (chlorophyll a, temperature) with basic information about the parameters related to the carbon uptake by phytoplanktonic algae. This requires a better understanding as well as a more extended knowledge of these parameters which govern the radiative energy absorption and utilization by algae in photosynthesis. The measurement of the photosynthetic response of algae [ photosynthesis (P) versus it-radiance (E) curves], besides being less shiptime consuming than in situ primary production experiments, allows the needed parameters to be derived and systematically studied as a function of the physical, chemical and ecological conditions. The aim of the present paper is to review the sig nificance of these parameters, especially in view of their introduction into models, to analyze the causes of their variations in the light of physiological considerations, and finally to provide methodo logical recommendations for meaningful determinations, and interpretation, of the data resulting from P versus E determinations. Of main concern are the available and usable irradiance, the chloro phyll a-specific absorption capabilities of the algae, the maximum light utilization coefficient (cs), the maximum quantum yield (φm)( the maximum photosynthetic rate (Pm) and the light saturation index (EL). The potential of other, non-intrusive, approaches, such as the stimulated variable fluorescence, or the sun-induced natural fluorescence techniques is also examined.