Light respiration by subtropical seaweeds

Photosynthetic responses of Halimeda 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 Q₁₀P_gross values confirmed the effect of temperature only under nutrient enrichment scenarios. Besides the above, bicarbonate (HCO₃⁻) absorption was assessed by the content of carbon stable isotope (δ¹³C) 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 δ¹³C revealed this species uses both, CO₂ and HCO₃⁻ forms of C_i relying on a CO₂ 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 ¹³C isotope into aspartate in light and dark treatments also confirmed photosynthetic and non-photosynthetic the HCO₃⁻uptake.