Inorganic carbon utilization in marine angiosperms (seagrasses)

Abstract

The mechanisms by which marine angiosperms, or seagrasses, utilize external inorganic carbon (Ci) include, in addition to uptake of CO₂ formed spontaneously from HCO₃^–: (i) extracellular carbonic anhydrasemediated conversion of HCO₃^– to CO₂ at normal seawater pH, or in acid zones created by H⁺ extrusion, and (ii) H⁺-driven utilization (direct uptake?) of HCO₃^–. The latter mechanism was recently indicated for Zostera marina, Halophila stipulaceaand Ruppia maritima, and manifested itself as a sensitivity of photosynthesis to buffers, as well as a relative insensitivity to acetazolamide under buffer-free conditions, especially at high pH.

Seagrasses have until recently been viewed as having Ci utilization systems that are less ‘efficient’ than macroalgae, and this has, for example, led to the thought that future rises in atmospheric and thus dissolved CO₂ would have a stronger effect on seagrasses than on macroalgae. However, most of the experiments leading to such conclusions were carried out in the laboratory on detached leaves, and buffers were used to keep HCO₃^–/CO₂ ratios stable during Ci additions. The revelation that seagrass photosynthesis is sensitive to buffers as well as to physical perturbations, has led to new experiments in which initial pH values are set by appropriate HCO₃^–/CO₃^2–ratios, and/or O₂ measurements on leaf pieces are replaced with pulse amplitude-modulated fluorometry on whole, attached leaves, often in situ. Under such conditions, the photosynthetic responses of seagrasses to Ci match those obtained for macroalgae. Thus, the paradigm of ‘inefficient’ Ci utilization by seagrasses as compared with macroalgae may no longer be valid. Consequently, it seems that the generally observed high productivity of seagrass beds may have its background in very efficient, H⁺-driven, means of HCO₃^– utilization.