Temperature and light requirements for growth of two diatom species (Bacillariophyceae) isolated from an Arctic macroalga

In the present study, two abundant epiphytic diatom taxa were isolated from the assimilation hairs of the brown macroalga Chordaria flagelliformis collected in the Arctic Kongsfjorden (Spitsbergen, Norway), established as unialgal cultures and their growth rates determined under controlled photon fluence rate and temperature conditions. Using morphological (light and scanning electron microscopy) and SSU rRNA gene data both isolates (ROS D99 and ROS D125) were identified as members of a Fragilaria–Synedropsis clade. The molecular data of ROS D99 and ROS D125 were not identical to any other published sequence. While ROS D99 has been identified as Fragilaria barbararum mainly due to the SEM characteristics, ROS D125 could not be definitely identified although morphological data speak for Fragilaria striatula. Both diatom species showed similar growth rates at all temperatures and photon fluence rates tested. They grew well between 0 and 15°C with optimum temperatures of 12–14°C, but did not survive 20°C. Therefore, compared to Antarctic diatoms both taxa from Kongsfjorden can be characterised as eurythermal organisms. Increasing photon fluence rates between 2 and 15 μmol m⁻² s⁻¹ were accompanied with an almost twofold increase in growth rates, but photon fluence rates >15 μmol m⁻² s⁻¹ did not further enhance growth pointing to low light requirements. From these data optimum, minimum and maximum photon fluence rates and temperatures for growth can be assessed indicating that both diatoms are well acclimated to the fluctuating environmental conditions in the Arctic habitat.