Abstract
Photosynthesis and respiration cause distinct chemical microenvironments within cyanobacterial aggregates. Here, we used microsensors and a diffusion–reaction model to characterize gradients in carbonate chemistry and investigate how these are affected by ocean acidification in Baltic vs. Pacific aggregates ( Nodularia and Dolichospermum vs. Trichodesmium ). Microsensor measurements of O 2 and pH were performed under in situ and expected future p CO 2 levels on Nodularia and Dolichospermum aggregates collected in the Baltic Sea. Under in situ conditions, O 2 and pH levels within the aggregates covered ranges of 80–175% air saturation and 7.7–9.4 in dark and light, respectively. Carbon uptake in the light was predicted to reduce HCO 3 − by 100–150 μ mol L −1 and CO 2 by 3–6 μ mol L −1 in the aggregate center compared to outside, inducing strong CO 2 depletion (down to 0.5 μ mol L −1 CO 2 remaining in the center) even when assuming that HCO 3 − covered 80–90% of carbon uptake. Under ocean acidification conditions, enhanced CO 2 availability allowed for significantly lower activity of carbon concentrating mechanisms, including a reduction of the contribution of HCO 3 − to carbon uptake by up to a factor of 10. The magnification of proton gradients under elevated p CO 2 that was predicted based on a lower buffer capacity was observed in measurements despite a concurrent decrease in photosynthetic activity. In summary, we provide a quantitative image of the inorganic carbon environment in cyanobacterial aggregates under present‐day and expected future conditions, considering both the individual and combined effects of the chemical and biological processes that shape these environments.