Use of light and inorganic carbon acquisition by two morphotypes of Zostera noltii Hornem

Abstract

The affinity for dissolved inorganic carbon (DIC) and the mechanisms to use HCO 3 − as a source of DIC for photosynthesis were investigated in two morphotypes of Zostera noltii Hornem. Both morphotypes were collected at Ria Formosa lagoon (Southern Portugal) at two different levels in the intertidal. Affinity for DIC at saturating photon fluence rate (PFR), estimated as photosynthetic conductance for CO 2 ( g p(CO 2) ), was reduced by 75% in the Z. noltii plants adapted to shade conditions (lower intertidal) in comparison to the sun morphotype (45×10 −6 and 182×10 −6 m s −1, respectively), indicating that the plants acclimated to sun conditions (higher intertidal) had a higher capacity to use HCO 3 − as DIC source for photosynthesis. Since external carbonic anhydrase activity was negligible and a large inhibitory effect was produced by Tris buffer addition, this HCO 3 − use was attributed to the operation of H + ATPases creating low pH zones in periplasmic space. The photosynthetic CO 2-flux supported for this mechanism was calculated to be 53 μmol O 2 m −2 s −1 in sun morphotype, about 80% out of maximum photosynthesis rate. In order to determine the possible photosynthetic energy cost of the HCO 3 − use, the effect of decreasing light on photosynthetic rates and g p(CO 2) was estimated. Photosynthetic conductance decreased in both morphotypes at non-saturating PFR. This dependence of g p(CO 2) on PFR indicated the existence of a positive interactive effect between DIC and PFR which was more pronounced in the shade morphotype since the ascending slope of O 2 evolution vs. PFR curves at limiting PFRs was reduced from 7.2 to 2.3 mmol O 2 mol photon −1 at 4 and 0.5 mol m −3 of DIC, respectively.