Diazotrophy modulates cyanobacteria stoichiometry through functional traits that determine bloom magnitude and toxin production

Harmful cyanobacterial blooms are an increasing threat to water quality.
The interactions between two eco-physiological functional traits of
cyanobacteria, diazotrophy (nitrogen (N)-fixation) and N-rich cyanotoxin
synthesis, have never been examined in a stoichiometric explicit manner.
We explored how a gradient of resource N:phosphorus (P) affects the
biomass, N, P stoichiometry, light-harvesting pigments, and
cylindrospermopsin production in a N-fixing
cyanobacterium, Aphanizomenon. Low N:P Aphanizomenon cultures
produced the same biomass as populations grown in high N:P cultures. The
biomass accumulation determined by carbon, indicated low N:P Aphanizomenon
cultures did not have a N-fixation growth tradeoff, in contrast to some
other diazotrophs that maintain stoichiometric N homeostasis at the
expense of growth. However, N-fixing Aphanizomenon populations produced
less particulate cylindrospermopsin and had undetectable dissolved
cylindrospermopsin compared to non-N-fixing populations. The pattern of
low to high cyanotoxin cell quotas across an N:P gradient in the
diazotrophic cylindrospermopsin producer is similar to the cyanotoxin cell
quota response in non-diazotrophic cyanobacteria. We suggest that
diazotrophic cyanobacteria may be characterized into two broad functional
groups, the N-storage-strategists and the growth-strategists, which use
N-fixation differently and may determine patterns of bloom magnitude and
toxin production in nature.