Salinity tolerance in biogeographically different strains of the marine benthic diatom Cylindrotheca closterium (Bacillariophyceae)

Abstract

In coastal zones, salinity is commonly a rather local environmental factor that can be highly variable. Future climate change scenarios indicate for many coastal regions besides warming also changes in current salinity regime due to less precipitation, higher evaporation, or more freshwater run-off, resulting in decreasing or increasing saline conditions. The soft bottom of shallow water coastal zones is typically inhabited by benthic diatoms which cover as photosynthetic biofilms extensive areas of sediments. Cylindrotheca closterium is a cosmopolitan and abundant taxon of such communities, and is widely used as a benthic diatom model system. Nevertheless, comprehensive ecophysiological data on the salinity tolerance of this ecologically important benthic species are still missing. Therefore, the main goal was to investigate the salinity tolerance in 6 strains of C. closterium from 5 different clades and from marine to brackish habitats. An analysis of growth as function of salinity allowed the evaluation of tolerance limits, growth optima, and acclimation abilities of individual strains. The data documented isolate-specific growth response patterns and rather broad tolerance widths among the six strains of C. closterium, which point to strong genotypic differentiation. The results of the phylogenetic network analysis indicate several well separated grades within C. closterium, thereby supporting the earlier suggestion of a cryptic species complex.