Inorganic carbon sources for seagrass photosynthesis: an experimental evaluation of bicarbonate use in species inhabiting temperate waters

Abstract

Photosynthetic inorganic carbon utilization by the seagrasses Posidonia oceanica (L.) Delile, Cymodocea nodosa (Ucria) Aschers., Zostera marina (L.) and Phyllospadix torreyi S. Watson was studied by manipulating the concentrations of aqueous dissolved CO 2 and HCO 3 − in seawater. At constant dissolved inorganic concentration (ca. 2.2 mM), photosynthetic rates were higher at low pH (i.e. 6–7; [CO 2]=1.2–0.21 mM) than at “normal” pH (i.e. 8.2; [CO 2]=0.016 mM) in all four species. Photosynthetic rates of all four species exhibited saturation kinetics in response to [HCO 3 −] at constant, low [CO 2] (0.016 mM), with saturating [HCO 3 −] between 1 and 1.5 mM; this demonstrates, as it is known for other seagrass species, a clear capacity to use HCO 3 − as inorganic carbon source for photosynthesis. Photosynthetic rates at saturating [HCO 3 −] were 1.6 to 6 times lower than rates measured at high (∼1 mM) [CO 2]. Thus, photosynthesis of these species was limited by dissolved inorganic carbon (DIC) availability in normal seawater. No significant differences were found among species in their affinity for HCO 3 − as determined by the half-saturation constant K s. [CO 2(aq)] enhancement of photosynthesis was lower in the Mediterranean species ( P. oceanica and C. nodosa) than the Pacific ones ( Z. marina and P. torreyi); in addition, P. oceanica showed the highest HCO 3 −-driven photosynthesis at saturating HCO 3 − concentration. These species-specific differences in inorganic carbon uptake kinetics should be considered in any inter-specific analysis of plant features depending on carbon balance, e.g. the deep distribution limit or the sensitivity to future changes in [CO 2(aq)]. However, CO 2 limitation of seagrass photosynthesis appears to be a common physiological feature of seagrasses that may have significant ecological and evolutionary implications.