Light and temperature demands for photosynthesis and growth of two benthic diatoms from Potter Cove, a shallow bay in King George Island, South Shetland Islands (Antarctica), were determined in laboratory cultures. The species investigated were Gyrosigma subsalinum var. antarctica and the Antarctic endemic Odontella litigiosa. The algae exhibited different light requirements for growth and photosynthesis. G. subsalinum was adapted to low light conditions, with a high light utilisation coefficient (α) and low saturating photon irradiances for photosynthesis and growth (~25 μmol m−2 s−1 and 11 μmol m−2 s−1, respectively). Photosynthesis and growth in O. litigiosa were saturated at a photon irradiance of ~100 μmol m−2 s−1. Both benthic diatoms exhibited extremely low upper survival temperature (UST, 5–7 °C) and growth only occurred over a narrow range from 0 and 7 °C. Optimal temperature for growth (0 °C) was slightly lower than optimum temperature for photosynthesis (5 °C). The results suggest that G. subsalinum is well-adapted to the low light and constant low temperature conditions present in water depths below 10 m where it occurs. In contrast, O. litigiosa requires more light and is able to tolerate the higher temperatures typical of shallow waters and tide pools. The northern distribution limit of G. subsalinum is the 4 °C summer isotherm, i.e. at the northern boundary of the Antarctic region. In contrast, O. litigiosa could potentially also occur in the sub-Antarctic region up to the 7 °C summer isotherm.
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