Is the tetrasporophyte of Asparagopsis armata (Bonnemaisoniales) limited by inorganic carbon in integrated aquaculture?1

Seaweeds cultivated in traditional land‐based tank systems usually grow under carbon‐limited conditions and consequently have low production rates, if no costly artificial source of inorganic carbon is supplied. In integrated aquaculture, the fish effluents provide an extra source of dissolved inorganic carbon (DIC) to seaweeds due to fish respiration. To evaluate if the tetrasporophyte of Asparagopsis armata (Harv.) F. Schmitz (the Falkenbergia stage) is carbon limited when cultivated with effluents of a fish (Sparus aurata) farm in southern Portugal, we characterized the DIC forms in the water, assessed the species photosynthetic response to the different DIC concentrations and pH values, and inferred for the presence of a carbonic anhydrase (CA)–mediated mechanism. Results showed that A. armata relies mainly on CO2 to meet photosynthetic needs. Nevertheless, from pH 7.5 upward, the CO2 supply to RUBISCO seems to derive also from the external dehydration of HCO3– mediated by CA. The contribution of this mechanism was essential for A. armata to attain fully saturated O2‐evolution rates at the natural seawater DIC concentration (2–2.2 mM) and pH values (∼8.0). We revealed in this study that seaweeds cultivated in fish‐farm effluents benefit not only from a rich source of ammonia but also from an important and free source of DIC for their photosynthesis. If supplied at relatively high turnover rates (∼100 vol · d−1), fish‐farm effluents provide enough carbon to maximize the photosynthesis and growth even for species with low affinity for HCO3–, avoiding the artificial and costly supply of inorganic carbon to seaweed cultures.