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.     

THE INVASIVE GENUS ASPARAGOPSIS (BONNEMAISONIACEAE,RHODOPHYTA): MOLECULAR SYSTEMATICS,MORPHOLOGY, AND ECOPHYSIOLOGY OF FALKENBERGIA ISOLATES1

The genus Asparagopsis was studied using 25 Falkenbergia tetrasporophyte strains collected worldwide. Plastid (cp) DNA RFLP revealed three groups of isolates, which differed in their small subunit rRNA gene sequences, temperature responses, and tetrasporophytic morphology (cell sizes). Strains from Australia, Chile, San Diego, and Atlantic and Mediterranean Europe were identifiable as A. armata Harvey, the gametophyte of which has distinctive barbed spines. This species is believed to be endemic to cold‐temperate waters of Australia and New Zealand and was introduced into Europe in the 1920s. All isolates showed identical cpDNA RFLPs, consistent with a recent introduction from Australia. Asparagopsis taxiformis (Delile) Trevisan, the type and only other recognized species, which lacks spines, is cosmopolitan in warm‐temperate to tropical waters. Two clades differed morphologically and ecophysiologically and in the future could be recognized as sibling species or subspecies. A Pacific/Italian clade had 4–8° C lower survival minima and included a genetically distinct apomictic isolate from Western Australia that corresponded to the form of A. taxiformis originally described as A. sanfordiana Harvey. The second clade, from the Caribbean and the Canaries, is stenothermal (subtropical to tropical) with some ecotypic variation. The genus Asparagopsis consists of two or possibly three species, but a definitive taxonomic treatment of the two A. taxiformis clades requires study of field‐collected gametophytes.