Coralline photosynthetic physiology across a steep light gradient.

Abstract

Coralline algae (CA) are globally distributed and fulfil many important roles within coastal ecosystems. In this study, photosynthetically active radiation (PAR) measured for 616days at 2 and 10m in a temperate subtidal kelp forest in southern New Zealand provided context to photosynthesis vs. irradiance relationships for, and pigment concentrations of, an articulated coralline alga, Arthrocardia sp. and a crustose coralline species assemblage within the Hapalidiales order. The maximum photosynthetic rate Pmax of the Arthrocardia sp. (20.38 ± 2.38µmol O2. gDW-1 h-1) was significantly higher than the Pmax of crustose coralline spp. (3.72 ± 0.74µmol O2. gDW-1 h-1) at the same 2m stratum. Pigment concentration of Arthrocardia sp. was significantly higher than that of crustose coralline spp. at the same depth, while pigment concentration of crustose coralline spp. at 2 and 10m were not significantly affected by depth. The photosynthetic characteristics of these coralline algae represent a shade acclimated organism with low saturation irradiance (all Ek < 100µmol photons m-2s-1). Despite sevenfold difference in average daily dose between 2 and 10m there was no significant effect of depth on the photosynthetic performance of crustose coralline algae measured. The lack of evidence for acclimation to low light could be because periods of clear water provide enough light to maintain photosynthesis, lower energetic requirements of species found at depth or constraints on the synthesis of photosynthetic pigments at greater depth.