Photosynthetic responses ofHalimeda scabra(Chlorophyta, Bryopsidales) to interactive effects of temperature, pH, and nutrients and its carbon pathways

In this study, we evaluated the interactive effects of temperature, pH, and nutrients on photosynthetic performance in the calcareous tropical macroalga Halimeda scabra . A significant interaction among these factors on gross photosynthesis ( P gross ) was found. The highest values of P gross were reached at the highest temperature, pH, and nutrient enrichment tested and similarly in the control treatment (no added nutrients) at 33 °C at the lowest pH. The Q 10 P gross values confirmed the effect of temperature only under nutrient enrichment scenarios. Besides the above, bicarbonate (HCO 3 − ) absorption was assessed by the content of carbon stable isotope (δ 13 C) in algae tissue and by its incorporation into photosynthetic products, as well as by carbonic anhydrase (CA) inhibitors (Acetazolamide, AZ and Ethoxyzolamide, EZ) assays. The labeling of δ 13 C revealed this species uses both, CO 2 and HCO 3 − forms of C i relying on a CO 2 Concentration Mechanism (CCM). These results were validated by the EZ-AZ inhibition assays in which photosynthesis inhibition was observed, indicating the action of internal CA, whereas AZ inhibitor did not affect maximum photosynthesis ( P max ). The incorporation of 13 C isotope into aspartate in light and dark treatments also confirmed photosynthetic and non-photosynthetic the HCO 3 − uptake.