Effects of nutrient limitation on the synthesis of N-rich phytoplankton toxins: a meta-analysis

Eutrophication has played a major role in the worldwide increase of
harmful algal blooms (HABs). Higher input of key nutrients, such as
nitrogen (N) and phosphorus (P), can stimulate the growth of harmful algal
species in freshwater, estuarine, and coastal marine ecosystems. Some
HAB-forming taxa, particularly several cyanobacteria and dinoflagellate
species, are harmful through the production of N-rich toxins that have
detrimental effects on the environment and human health. Here, we test how
changes in nutrient availability affect N-rich toxin synthesis in
cyanobacteria and dinoflagellates using a meta-analysis approach. Overall,
N-rich toxin content showed an increase with P limitation, while it tended
to decrease with N limitation, but we also observed substantial variation
in responses both within and across genera and toxin groups. For instance,
in response to N limitation, microcystin content varied from a 297%
decrease up to a 273% increase, and paralytic shellfish poisoning (PSP)
toxin content varied from a 204% decrease to an 82% increase.
Cylindrospermopsin, produced by N2-fixing cyanobacteria, showed no clear
direction in response to nutrient limitation, and cellular contents of
this compound may thus vary independently of nutrient fluctuations. Our
results confirm earlier reported stoichiometric regulation of N-rich
phytoplankton toxins, showing increased toxin content with an increase in
cellular N:P ratios, and vice versa. Thus, changes in N-rich toxin content
largely follow the changes in relative cellular N content. Consequently,
although nutrient limitation may limit bloom biomass and thereby bloom
toxicity, our results warn that P limitation can cause accumulation of
cellular toxins and thus lead to unexpected increases in bloom toxicity.