Tolerance of benthic diatoms from temperate aquatic and terrestrial habitats to experimental desiccation and temperature stress

Soils differ from aquatic sediments in environmental characteristics such as moisture availability and temperature fluctuations, and it is therefore believed that soil-inhabiting diatoms have a broader tolerance range to these stresses than aquatic diatoms. To test this hypothesis, we assessed the survival capacities of vegetative cells of 34 benthic diatom species from terrestrial and aquatic habitats in Belgium when exposed to desiccation and temperature stress. Six different stress conditions were studied: gradual heating up to +30 degrees C and +40 degrees C, abrupt heating to +40 degrees C, freezing. to -20 degrees C and desiccation with and without preconditioning at +30 degrees C. All six conditions resulted in a significantly decreased survival of cells compared to control conditions. Desiccation killed all tested strains, freezing was survived by only three species and abrupt heating was significantly more lethal than gradual heating, suggesting a generally high sensitivity of vegetative diatom cells to these three stress factors. While tolerance to temperature extremes (+40 degrees C and -20 degrees C) was to a large extent species-specific, habitat-specific differences in cell survival were also detected. Only terrestrial species survived freezing, and aquatic diatoms were less tolerant to gradual heating to +40 degrees C, both pointing at a higher tolerance of terrestrial diatoms to temperature extremes. Moreover, in two species with both aquatic and terrestrial isolates, only the terrestrial strains survived +40 degrees C. We conclude that vegetative cells of benthic diatoms (1) are very sensitive to desiccation, freezing and abrupt heating and (2) have a habitat-dependent tolerance to temperature extremes. The consequences of these observations for the dispersal capacities and the subsequent biogeographical patterns of diatoms are discussed.