Carbon acquisition mechanisms in Halophila johnsonii and Thalassia testudinum

Mechanisms for carbon uptake in the small-bodied Halophila johnsonii and large-bodied Thalassia testudinum were compared using photosynthesis measurements (oxygen flux) with, and without, the extracellular carbonic anhydrase inhibitor acetazolamide (AZ) and TRIS buffer. Our results indicated T. testudinum and H. johnsonii both utilize external membrane-bound carbonic anhydrase to facilitate dehydration of HCO3− into CO2(aq). T. testudinum also utilizes an active proton (H+) pump to create localized H+ gradients within the boundary layer and it derives a larger portion of photosynthetic carbon from bicarbonate than H. johnsonii. Stable carbon isotope composition (δ13C) of T. testudinum and H. johnsonii leaves under high and reduced irradiance were also compared to evaluate photosynthetic carbon use. Isotope ratios of H. johnsonii were significantly more negative than T. testudinum at a high-light field site (−10.44 ± 0.26‰ vs −9.25 ± 0.25‰). In contrast, there was no significant difference in isotope ratios between species for plants maintained in a greenhouse under reduced photosynthetically active radiation. This suggests photosynthesis of T. testudinum is more carbon limited or more reliant on bicarbonate for photosynthesis than H. johnsonii under high-light conditions. Although photosynthesis of both species is expected to increase in response to greater CO2(aq) availability, ocean acidification and its CO2(aq) enrichment of seawater may have greater benefits for H. johnsonii compared to T. testudinum, whose photosynthesis relies more heavily on HCO3− use.