Atmospheric CO₂availability does not equate to increased nodularin production in diazotrophic cyanobacteria, but does induce varying responses in net photosynthesis and N₂fixation rates

Abstract

Increasing levels of CO 2 in the atmosphere are suggested to favour increased incidences of cyanobacterial blooms in water bodies, with a potential concomitant increase in toxin production. As nitrogen fixing cyanobacteria are independent of nitrate and ammonium, this pilot study investigated whether elevated atmospheric CO 2 levels (eCO 2 ), could increase toxin production and net photosynthesis (NP) rates in both terrestrial and aquatic diazotrophic cyanobacteria. Both toxin and non-toxin producing strains of Nostoc and Nodularia were grown at present atmospheric levels (PAL) of CO 2 or near future elevated (eCO 2 ) and net photosynthesis (NP) determined. Short term responses demonstrated CO 2 associated increases and decreases in NP, with N. harveyana SAG44.85 showing little change in its NP at eCO 2 . Long term responses recorded increases in NP for all species in response to eCO 2 , except for N. harveyana on day 7.Nitrogen fixation rates were significantly higher by approx. 10 fold in the aquatic Nodularia species compared to the terrestrial Nostoc species tested. Moreover, nitrogen fixation rates were not significantly higher at eCO 2 , except for N. harveyana. There was no direct correlation between increased nodularin production and eCO 2 in neither aquatic, nor terrestrial nodularin producing species, however there was a significant correlation between nodularin content and POC:PON ratio for the terrestrial Nostoc sp. 73.1 not observed for the aquatic Nodularia spumigena CCY9414.