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

Abstract

AbstractIncreasing levels of CO2in 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 CO2levels (eCO2), could increase toxin production and net photosynthesis (NP) rates in both terrestrial and aquatic diazotrophic cyanobacteria. Both toxin and non-toxin producing strains ofNostocandNodulariawere grown at present atmospheric levels (PAL) of CO2or near future elevated (eCO2) and net photosynthesis (NP) determined. Short term responses demonstrated CO2associated increases and decreases in NP, withN. harveyanaSAG44.85 showing little change in its NP at eCO2. Long term responses recorded increases in NP for all species in response to eCO2, except forN. harveyanaon day 7. Nitrogen fixation rates were significantly higher by approx. 10 fold in the aquaticNodulariaspecies compared to the terrestrialNostocspecies tested. Moreover, nitrogen fixation rates were not significantly higher at eCO2, except forN. harveyana. There was no direct correlation between increased nodularin production and eCO2in neither aquatic, nor terrestrial nodularin producing species, however there was a significant correlation between nodularin content and POC:PON ratio for the terrestrialNostocsp. 73.1 not observed for the aquaticNodularia spumigenaCCY9414.