Broad tolerance or narrow niches – ecophysiological responses of Icelandic benthic diatoms towards abiotic variables

The intertidal zone of the Icelandic coast is characterized by strong seasonal and diurnal changes in the abiotic variables light, temperature and salinity, which are additionally affected by the tidal range and climate change. Iceland’s coastal zones host many phototrophic organisms including benthic diatoms that may strongly contribute as primary producers to the ecosystem functioning. However, studies on the ecophysiological responses to abiotic changes are rare. To better understand the impact of these variables, the influence on photosynthesis, respiration and growth of eight benthic diatom strains ( Navicula gregaria , Navicula jahniae , Navicula pseudoperminuta , Nitzschia olafsvikensis, Tryblionella ytritungaensis , Seminavis sp. and two strains of Navicula vesturlandica ) was comparatively investigated. Representatives of the different species exhibited distinct responses to the tested abiotic variables (light, temperature and salinity) with optimum photosynthetic and growth rates occurring between 5 and 30 °C. All strains exhibited a high photo-physiological plasticity reaching maximum photosynthesis between 11.9 and 105.8 µmol photons m -2 s -1 with no indication of photoinhibition at the maximum tested photon fluence rate of 1400 µmol photons m -2 s -1 . Strains of different taxa also showed specific responses to salinity, showing stenohaline and euryhaline (1 to 45 (65) S A ) traits but with a common optimum for all species between 25 and 33 S A , reflecting the conditions of their natural coastal habitat. Overall, these findings demonstrate the considerable ecophysiological plasticity of benthic diatoms from Iceland, that are required to withstand the environmental fluctuations in their natural habitat and to climate change driven variability.