Increased Toxicity of Karenia brevis during Phosphate Limited Growth: Ecological and Evolutionary Implications

Abstract

Karenia brevis is the dominant toxic red tide algal species in the Gulf of Mexico. It produces potent neurotoxins (brevetoxins [PbTxs]), which negatively impact human and animal health, local economies, and ecosystem function. Field measurements have shown that cellular brevetoxin contents vary from 1–68 pg/cell but the source of this variability is uncertain. Increases in cellular toxicity caused by nutrient-limitation and inter-strain differences have been observed in many algal species. This study examined the effect of P-limitation of growth rate on cellular toxin concentrations in five Karenia brevis strains from different geographic locations. Phosphorous was selected because of evidence for regional P-limitation of algal growth in the Gulf of Mexico. Depending on the isolate, P-limited cells had 2.3- to 7.3-fold higher PbTx per cell than P-replete cells. The percent of cellular carbon associated with brevetoxins (%C-PbTx) was ∼ 0.7 to 2.1% in P-replete cells, but increased to 1.6–5% under P-limitation. Because PbTxs are potent anti-grazing compounds, this increased investment in PbTxs should enhance cellular survival during periods of nutrient-limited growth. The %C-PbTx was inversely related to the specific growth rate in both the nutrient-replete and P-limited cultures of all strains. This inverse relationship is consistent with an evolutionary tradeoff between carbon investment in PbTxs and other grazing defenses, and C investment in growth and reproduction. In aquatic environments where nutrient supply and grazing pressure often vary on different temporal and spatial scales, this tradeoff would be selectively advantageous as it would result in increased net population growth rates. The variation in PbTx/cell values observed in this study can account for the range of values observed in the field, including the highest values, which are not observed under N-limitation. These results suggest P-limitation is an important factor regulating cellular toxicity and adverse impacts during at least some K. brevis blooms.

Osmotic stress does not trigger brevetoxin production in the dinoflagellateKarenia brevis

“…2). A decrease in salinity from 36 to 27 significantly increased the mean cell volume [F(1,31) = 478; P < 0.001, one-way repeated measures ANOVA], with an average increase of 17.4% within the first 3 min and a 9.7% increase after 48 h. As before, the salinity decrease had no effect on toxin content per cell [F(1,31) = 2.36; P = 0.153, one-way repeated measures ANOVA] ( (17,18).…”

“…Recent studies support the hypothesis that brevetoxins serve as grazing deterrents, which promote population survival by decreasing grazing mortality rates (12)(13)(14), consistent with a classic defensive role for this neurotoxin. Furthermore, very recently published experiments show that limitation of growth rate by nitrogen and phosphorus increases cellular brevetoxin concentrations by two-to threefold, consistent with the behavior of other grazing defense toxins in terrestrial plants and phytoplankton (15)(16)(17)(18). However, before these recent findings, brevetoxins were thought to be constitutively expressed, and the cellular function of these and other algal toxins was debated (19)(20)(21).…”

Positive feedbacks between bottom-up and top-down controls promote the formation and toxicity of ecosystem disruptive algal blooms: A modeling study

“…K.b. was also chosen because of the recent publication of quantitative relationships between nutrient (N and phosphorus [P]) limitation of specific growth rate and increased ratios of cellular toxins (brevetoxins) to cell carbon (C) in this species (Hardison et al, , 2013. These relationships provided the data needed to construct an NPZ model that directly examines the effect of increased cellular toxins under N-limitation of growth rate on the occurrence and toxicity of EDAB events.…”

Synthesis of a PbTx-2 photoaffinity and fluorescent probe and an alternative synthetic route to photoaffinity probes

“…An increase in brevetoxin production in K. brevis was also reported to occur under phosphate limited growth 40 . Brevetoxin content per cell were 2.3-7.3 fold higher in phosphorous limited cells in comparison to phosphorous replete cells.…”

“…Compound 34 could then be taken through the typical steps needed to construct the diazirine functional group to give 38. Once the diazirine is in place, the acetonide may be removed, followed by an oxidation of the benzyl alcohol to an aldehyde to provide photoaffinity label 40 …”