• Alaska;
  • conchocelis;
  • culture;
  • irradiance;
  • marine algae;
  • photosynthesis;
  • Porphyra;
  • respiration;
  • salinity;
  • temperature

Photosynthesis and respiration of three Alaskan Porphyra species, P. abbottiae V. Krishnam., P. pseudolinearis Ueda species complex (identified as P. pseudolinearis” below), and P. torta V. Krishnam., were investigated under a range of environmental parameters. Photosynthesis versus irradiance (PI) curves revealed that maximal photosynthesis (Pmax), irradiance at maximal photosynthesis (Imax), and compensation irradiance (Ic) varied with salinity, temperature, and species. The Pmax of Porphyra abbottiae conchocelis varied between 83 and 240 μmol O2 · g dwt−1 · h−1 (where dwt indicates dry weight) at 30–140 μmol photons · m−2 · s−1 (Imax) depending on temperature. Higher irradiances resulted in photoinhibition. Maximal photosynthesis of the conchocelis of P. abbottiae occurred at 11°C, 60 μmol photons · m−2·s−1, and 30 psu (practical salinity units). The conchocelis of P. “pseudolinearis” and P. torta had similar Pmax values but higher Imax values than those of P. abbottiae. The Pmax of P. “pseudolinearis” conchocelis was 200–240 μmol O2 · g dwt−1 · h−1 and for P. torta was 90–240 μmol O2 · g dwt−1 · h−1. Maximal photosynthesis for P. “pseudolinearis” occurred at 7°C and 250 μmol photons · m−2 · s−1 at 30 psu, but Pmax did not change much with temperature. Maximal photosynthesis for P. torta occurred at 15°C, 200 μmol photons · m−2 · s−1, and 30 psu. Photosynthesis rates for all species declined at salinities <25 or >35 psu. Estimated compensation irradiances (Ic) were relatively low (3–5 μmol · photons · m−2 · s−1) for intertidal macrophytes. Porphyra conchocelis had lower respiration rates at 7°C than at 11°C or 15°C. All three species exhibited minimal respiration rates at salinities between 25 and 35 psu.