Comparative research on inorganic carbon acquisition by the macroalgae Ulva prolifera (Chlorophyta) and Pyropia yezoensis (Rhodophyta)

The red macroalga Pyropia/Porphyra is one of the most important marine crops in the world; Pyropia cultivation is severely affected by the green macroalga Ulva spp., a fouling organism. In this study, growth competition and acquisition of dissolved inorganic carbon (Ci) by Ulva prolifera and Pyropia yezoensis were investigated to understand the physiological characteristics of the two species. Competition experimental results showed that the relative growth rate of U. prolifera was not affected, whereas the growth rate of P. yezoensis was significantly inhibited. U. prolifera exhibited a competitive advantage when these two species were cultured together. The two algal species displayed high pH compensation points, suggesting that U. prolifera and P. yezoensis can photosynthesize by using HCO3 (-); the utilization ability of U. prolifera is stronger than that of P. yezoensis. The net photosynthetic rates of P. yezoensis and U. prolifera were significantly inhibited by the carbonic anhydrase (CA) inhibitors acetazolamide and 6-ethoxyzolamide indicating that CA is implicated in carbon-concentrating mechanisms (CCM). 4,4'-diisothiocyano-stilbene-2,2'-disulfonate (DIDS), another inhibitor that prevents direct HCO3 (-) uptake, did not significantly affect U. prolifera. Conversely, DIDS can sharply decrease the photosynthetic rate of P. yezoensis, particularly at high pH. U. prolifera and P. yezoensis also showed CO2-limited photosynthesis by which the half-saturating concentration of Ci exceeds that of seawater. U. prolifera did not show active HCO3 (-) uptake; instead, U. prolifera used HCO3 (-) via extracellular CA for photosynthetic carbon fixation. U. prolifera also utilizes extracellular CA-mediated HCO3 (-) absorption to a higher extent than P. yezoensis. By contrast, P. yezoensis shows active HCO3 (-) uptake and extracellular CCMs for photosynthetic carbon fixation. Therefore, these two algal species can survive in changing environments with high pH and low Ci concentration because of the action of CCMs.