Competitive superiority of N-fixing cyanobacteria when fixed N is scarce: Reconsiderations based on a model with heterocyst differentiation

Some cyanobacteria fix nitrogen (N) with specialized cells (heterocysts). Resource competition theory and some evidence suggest that such cyanobacteria can dominate when the supply of available dissolved N is low relative to that of phosphorus (P). To probe this notion, a recently introduced mathematical model of cyanobacteria that differentiate heterocysts was paired with a model of non-fixing algal competitors. With sufficiently low abiotic loss rates, dominance by cyanobacteria was predicted at low N:P supply ratios, coexistence for intermediate ratios, and dominance by algae at high ratios. Heterocysts do not reproduce, so differentiation is demographically costly, and at abiotic loss rates exceeding about 0.18 d − 1, cyanobacteria are predicted to be competitively excluded for all N:P supply ratios examined. The demographic cost of heterocyst production is also predicted to impact competitive dynamics in the presence of herbivores. Cyanobacteria are highly susceptible to apparent competition from algae, even when they are the less-preferred food of herbivores. Algae dominate at highest and lowest N:P supply ratios, and cyanobacteria at intermediate ratios, with two non-contiguous intervals of coexistence. In addition to these unexpected results, this study illustrates how planktonic community processes could determine whether nitrogen fixation contributes significantly to planktonic nitrogen supply.