Potential mechanisms of coexistence between two globally important pseudo-nitzschia (Bacillariophyta) species

To understand the mechanisms leading to coexistence and exclusion, it is essential to establish information on the nutritional needs of species. We focused on the frequently coexisting Pseudo-nitzschia species, P. delicatissima and P. galaxiae, capable of forming blooms and producing domoic acid. We employed monoculture experiments to determine growth kinetic parameters important for understanding resource use (i.e. maximum specific growth rate, half-saturation coefficients for growth and cell quotas), and we coupled mixed-culture experiments and numerical modelling to explore the role of resource competition relative to unknown factors, such as allelopathy. Experimental results showed that both species had a high requirement for nitrogen (N) and low requirement for phosphorus (P), consistent with field observations of Pseudo-nitzschia blooms in N-rich conditions. The model accurately predicted the outcome of competition; P. galaxiae outcompeted P. delicatissima when considering only resource competition, but the population trajectories were better predicted when allelopathic effects were added. Since the competitive exclusion of P. delicatissima by P. galaxiae in our laboratory experiments is not consistent with observations of coexistence in the natural environment, the model was further modified to explore realistic ranges of population loss factors, such as sinking, demonstrating how coexistence is possible when these are considered