The role of dissolved silicate in the growth kinetics of diatoms in natural Black Sea phytoplankton assemblages

Small diatoms are indicators of new production and major contributors to carbon export and biogeochemical cycling in tropical, subtropical, and temperate regions of the World Ocean. Changes in dissolved silicate concentration (DSi) may substantially alter plankton biodiversity and ecosystem functioning, because silicon availability strongly regulates their growth and competitive interactions. We investigated how contrasting DSi availability affects growth kinetics, morphology, and community structure of natural Black Sea phytoplankton using three enrichment experiments under non-limiting nitrogen and phosphorus supply. In two pulse-addition experiments, dominant diatom species exhibited distinct growth responses well described by a modified Monod model. Maximum specific growth rates of four dominant species (Skeletonema costatum, Thalassionema nitzschioides, Leptocylindrus danicus, and Cylindrotheca closterium) ranged from 0.56 to 1.33 day−1, while real silicate half-saturated growth constant varied from 0.96 to 1.78 µM, indicating pronounced interspecific differences in silicon requirements. Thalassionema nitzschioides displayed contrasting kinetic parameters between experiments, suggesting physiological acclimation to prior silicon conditions. In a replenishment experiment, sustained high DSi (7 µM) promoted strong diatom dominance, whereas low DSi (1 µM) favored non-silicifying taxa. Morphological analyses of T. nitzschioides revealed larger cells with thicker frustules under lower DSi, whereas smaller cells with higher surface-to-volume ratios were observed under higher DSi.Our results demonstrate that silicon limitation depends not only on DSi concentration but also on species-specific growth kinetics, physiological plasticity, and community context. The obtained results demonstrated the importance of stoichiometry for the growth and competitive properties of the four species, which was reflected in the annual dynamics: the summer peak of biomass was formed at the highest DSi:DIN and DSi:P ratios.