Does growth rate affect diatom compositional response to temperature?

Temperature is one of the environmental factors that most strongly influence the life of microalgae, from the enzymatic level up to the modification of biosynthetic and degradation potential and, thus, cellular composition. When exposed to a change in environmental milieu two main responses can be attained by cells: acclimation or homoeostasis. Recent studies testing the impact of other environmental factors indicated that the choice between these strategies is related to growth rates, i.e. cells dividing at a faster rate showed inclination to acclimation while cells dividing at a slower rate were more prone to homoeostasis. These findings motivated us to further explore the extent to which these compositional response modes are applied under varying temperature. Two representative marine diatoms, one centric, Thalassiosira pseudonana, and one pennate, Cylindrotheca fusiformis were investigated. Species were cultured at 15°C, 20°C and 30°C and at two growth rates. Compositional response was analysed with respect to elemental stoichiometry (carbon, nitrogen, silica, phosphorus, sulphur, manganese, iron, zinc and copper) and the major organic pool ratios (carbohydrate:lipid, carbohydrate:protein and protein:lipid). While T. pseudonana exhibited an acclimation response to temperature, C. fusiformis proved to be strongly homoeostatic. In the case of T. pseudonana, our data showed that the variations in composition were consistent with the growth rate dependence hypothesis for many micronutrients, but not for carbon and nitrogen contents, and C:N ratio. These species-specific differences in response mode highlight the diversity in ecophysiological strategies of diatoms, which needs to be considered when predicting climate change responses.