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

Photosynthetic inorganic carbon utilization by the seagrasses Posidonia oce anica (L.) Delile, Cymodocea nodosa (Ucria) Aschers., Zostera marina (L.) a nd Phyllospadix torreyi S. Watson was studied by manipulating the concentra tions of aqueous dissolved CO2 and HCO3- in seawater. At constant dissolved inorganic concentration (ca. 2.2 mM), photosynthetic rates were higher at low pH (i.e. 6-7; [CO2] = 1.2-0.21 mM) than at "normal" pH (i.e. 82. [CO2] = 0.016 mM) in all four species. Photosynthetic rates of all four species e xhibited saturation kinetics in response to [HCO3-] at constant. low [CO2] (0.016 mM), with saturating [HCO3-] between 1 and 1.5 mM; this demonstrates , as it is known for other seagrass species, a clear capacity to use HCO3- as inorganic carbon source for photosynthesis. Photosynthetic rates at satu rating [HCO3-] were 1.6 to 6 times lower than rates measured at high (simil ar to1 MM) [CO2]. Thus, photosynthesis of these species was limited by diss olved inorganic carbon (DIC) availability in normal seawater. No significan t differences were found among species in their affinity for HCO3- as deter mined by the half-saturation constant K-s. [CO2(aq)] enhancement of photosy nthesis was lower in the Mediterranean species (P. oceanica and C. nodosa) than the Pacific ones (Z marina and P. torreyi); in addition, P. oceanica s howed the highest HCO3--driven photosynthesis at saturating HCO3- concentra tion. These species-specific differences in inorganic carbon uptake kinetic s should be considered in any inter-specific analysis of plant features dep ending on carbon balance, e.g. the deep distribution limit or the sensitivi ty to future changes in [CO2(aq)]. However, CO2 limitation of seagrass phot osynthesis appears to be a common physiological feature of seagrasses that may have significant ecological and evolutionary implications. (C) 2001 Els evier Science B.V. All rights reserved.