During bloom events, freshwater cyanobacteria often produce a variety of harmful toxins with devastating health, environmental and economic consequences. The paralytic shellfish toxins are a large group of neurotoxic alkaloids including saxitoxin (STX), which is the most potent identified to date. In Australia, STX production is strain dependent within the cyanobacterium Anabaena circinalis. The following study utilised two strains of cyanobacteria, A. circinalis AWQC131C (131C) and A. circinalis AWQC310F (310F), as model organisms; 131C is a saxitoxin-producer whilst 310F serves as a non-toxic control. We aimed to characterise 131C and 310F at the genomic and proteomic levels using genome sequencing and isobaric tags for relative and absolute quantitation (iTRAQ), respectively, in order to identify key differences in not only their secondary but, primary metabolic pathways.
Draft genome sequencing of 131C and 310F revealed a genome length of 4.4 Mbp and a GC content of 37%, and the number of encoded genes was predicted to be 4447 and 4443 for 131C and 310F, respectively. A scan of each genome revealed a total of 740 unique coding regions within 131C, and 651 within 310F. Interestingly, the proteomic profile of 131C was significantly different from 310F. Using iTRAQ, we found that under standard laboratory conditions, 131C was highly abundant in photosynthetic and metabolic proteins compared to the non-toxic control. This suggests a high C:N ratio and intracellular 2-oxoglutarate concentration and may be a novel site for posttranslational regulation of STX. Overall, 131C is potentially a high energy ecotype likely to inhabit the water surface. Conversely, 310F was more abundant in molecular chaperones and proteins that neutralise reactive oxygen species, indicating activation of cellular stress response. Therefore, 310F seems to be experiencing cellular stress under laboratory conditions and in the environment, may inhabit low-light areas below the water surface.
In conclusion, this study has provided an insight into fundamental differences between the toxic 131C and non-toxic 310F strains of A. circinalis. These findings will provide a platform for future experiments and hopefully pave the way to identify the cellular function of STX.