The photosynthetic responses to stocking depth and algal mat density in the farmed seaweed Gracilaria lemaneiformis (Gracilariales, Rhodophyta).

The branches and mass of Gracilaria lemaneiformis increase with growth season, and the thalli sink to deeper depths with increasing biomass density during maricultivation. The changing depth and algal mat density may affect the physiology of the algae. In the present study, the photosynthetic behaviors regarding different biomass densities in G. lemaneiformis thalli collected from different stocking depths were determined, to examine how photosynthesis of this farmed alga was affected by the growth depths and algal mat densities. Our results showed that the chlorophyll a (Chl a), carotenoids (Car), phycoerythrin (PE) contents, and irradiance-saturated maximum photosynthetic rates (P _max ) of the deeper layer-grown algae were significantly increased relative to the surface layer-grown algae. The P _max , apparent photosynthetic efficiency (α) and dark respiration rate (R _d ) of G. lemaneiformis thalli, were reduced, whereas the irradiance saturation points (I _k ) were increased, with the increasing algal mat density. We proposed that appropriate measures are needed to trade off the stocking depth and biomass density, in an effort to maintain a relative high photosynthetic productivity during G. lemaneiformis maricultivation.