Thermal Niche Differentiation in the Benthic Diatom Cylindrotheca closterium (Bacillariophyceae) Complex

Coastal waters are expected to undergo severe warming in the coming decades. Very little is known about how diatoms, the dominant primary producersin these habitats, will cope with these changes. We investigated the thermal nicheof Cylindrotheca closterium, a wide-spread, benthic marine diatom using 24 strains collected over a wide latitudinal gradient. A multi-marker phylogeny in combination with a species delimitation approach shows that C. closteriumrepresents a (pseudo)cryptic species complex, and this is reflected in distinct growth response patterns in terms of optimum growth temperature, maximum growth rate and thermal nichewidth. Strains from the same clade displayed a similar thermal response, suggesting nicheconservation between closely related strains. Due to their lower maximum growth rate and smaller thermal nichewidth, we expect the polar species to be particularly sensitive to warming, and, in the absence of adaptation, to be replaced with species from lower latitudes.