Comparative Proteomics Reveals That a Saxitoxin-Producing and a Nontoxic Strain of <i>Anabaena circinalis</i> Are Two Different Ecotypes

In Australia, saxitoxin production is restricted to the cyanobacterial species <i>Anabaena circinalis</i> and is strain-dependent. We aimed to characterize a saxitoxin-producing and nontoxic strain of <i>A. circinalis</i> at the proteomic level using iTRAQ. Seven proteins putatively involved in saxitoxin biosynthesis were identified within our iTRAQ experiment for the first time. The proteomic profile of the toxic <i>A. circinalis</i> was significantly different from the nontoxic strain, indicating that each is likely to inhabit a unique ecological niche. Under control growth conditions, the saxitoxin-producing <i>A. circinalis</i> displayed a higher abundance of photosynthetic, carbon fixation and nitrogen metabolic proteins. Differential abundance of these proteins suggests a higher intracellular C:N ratio and a higher concentration of intracellular 2-oxoglutarate in our toxic strain compared with the nontoxic strain. This may be a novel site for posttranslational regulation because saxitoxin biosynthesis putatively requires a 2-oxoglutarate-dependent dioxygenase. The nontoxic <i>A. circinalis</i> was more abundant in proteins, indicating cellular stress. Overall, our study has provided the first insight into fundamental differences between a toxic and nontoxic strain of <i>A. circinalis</i>, indicating that they are distinct ecotypes