Effects of increased atmospheric CO2 and N supply on photosynthesis, growth and cell composition of the cyanobacterium Spirulina platensis (Arthrospira)

The consequences of the addition of CO2 (1%) in cultures of S. platensis ar e examined in terms of biomass yield, cell composition and external medium composition. CO2 enrichment was tested under nitrogen saturating and nitrog en limiting conditions. Increasing CO2 levels did not cause any change in m aximum growth rate while it decreased maximum biomass yield. Protein and pi gments were decreased and carbohydrate increased by high CO2, but the capab ility to store carbohydrates was saturated. C:N ratio remained unchanged wh ile organic carbon released to the external medium was enhanced, suggesting that organic carbon release in S. platensis is an efficient mechanism for the maintenance of the metabolic integrity, balancing the cell C:N ratio in response to environmental CO2 changes. CO2 affected the pigment content: P hycocyanin, chlorophyll and carotenoids were reduced in around 50%, but the photosynthetic parameters were slightly changed. We propose that in S. pla tensis CO2 could act promoting degradation of pigments synthetised in exces s in normal CO2 conditions, that are not necessary for light harvesting. Ni trogen assimilation was significantly not affected by CO2 and it is propose d that the inability to stimulate N assimilation by CO2 enrichment determin ed the lack of response in maximum growth rate.