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SEASONAL PATTERNS OF PHOTOSYNTHESIS AND LIGHT‐INDEPENDENT CARBON FIXATION IN MARINE MACROPHYTES 1

The contribution of light‐independent carbon fixation (LICF) to the overall carbon gain and the seasonal patterns of maximum photosynthesis (Pmax and LICF were characterized in a broad taxonomic range of macrophytes from Monterey Bay, California. Pmax and LICF rates (nmol C.g filtered seawater−1.min−1) varied among species and taxonomic groups examined, and as a function of tissue type in the phaeophyte Laminaria setchellii Silva (Phaeophyceae). On average, Pmax values were higher in the Rhodophyta, whereas LICF rates were greater in the Phaeophyceae. LICF rates were generally less than 5% of Pmax in the marine macrophytes studied and, as a consequence, cannot fully compensate for respiratory carbon losses, which usually are greater than 10% of Pmax. All species studied possessed the highest Pmax and LICF rates when irradiance levels were highest and decreased during periods of low incident irradiance. Seasonal patterns of Pmax and LICF in most of the macrophytes from the stenothermal environment of Monterey Bay were strongly correlated with photosynthetic photon flux rather than seawater temperature. The concomitant decrease of LICF and Pmax rates in all species examined argues against LICF playing a major role in carbon acquisition under light‐limiting conditions as suggested previously. Rather, the strong positive correlation of Pmax and LICF indicates the direct coupling of photosynthate (e.g. 3‐phosphoglyceric acid) generation with production of substrates for LICF reactions. Our results also suggest that LICF might be a useful indicator of photosynthetic metabolism in marine macrophytes.