SEASONAL PATTERNS OF PHOTOSYNTHESIS AND LIGHT-INDEPENDENT CARBON FIXATION IN MARINE MACROPHYTES

The contribution of light-independent carbon fixation (LICF) to the ov erall carbon gain and the seasonal patterns of maximum photosynthesis (P-max) and LICF were characterized in a broad taxonomic range of macr ophytes from Monterey Bay, California. P-max and LICF rates (nmol C.g filtered seawater(-1).min(-1)) varied among species and taxonomic grou ps examined and as a function of tissue type in the phaeophyte Laminar ia setchellii Silva (Phaeophyceae). On average, P-max values were high er in the Rhodophyta, whereas LICF rates were greater in the Phaeophyc eae. LICF rates were generally less than 5% of P-max in the marine mac rophytes studied and, as a consequence, cannot fully compensate for re spiratory carbon losses, which usually are greater than 10% of P-max. All species studied possessed the highest P-max and LICF rates when ir radiance levels were highest and decreased during periods of low incid ent irradiance. Seasonal patterns of P-max and LICF in most of the mac rophytes from the stenothermal environment ofMonterty Bay were strongl y correlated with photosynthetic photon flux rather than seawater temp erature. The concomitant decrease of LICF and P-max rates in all speci es examined argues against LICF playing a major role in carbon acquisi tion under light-limiting conditions as suggested previously. Rather, the strong positive correlation of P-max and LICF indicates the direct coupling of photosynthate (e.g. 3-phosphoglyceric acid) generation wi th production of substrates for LICF reactions. Our results also sugge st that LICF might be a useful indicator of photosynthetic metabolism in marine macrophytes.