Impact of warming on CO₂ emissions from streams countered by aquatic photosynthesis

Streams and rivers are an important source of CO₂ emissions. One important control of these emissions is the metabolic balance between photosynthesis, which converts CO₂ to organic carbon, and respiration, which converts organic carbon into CO₂ (refs,). Carbon emissions from rivers could increase with warming, independently of organic carbon inputs, because the apparent activation energy is predicted to be higher for respiration than photosynthesis. However, physiological CO₂ -concentrating mechanisms may prevent the increase in photorespiration, limiting photosynthesis with warming. Here we report the thermal response of aquatic photosynthesis from streams located in geothermal areas of North America, Iceland and Kamchatka with water temperatures ranging between 4 and 70 °C. Based on a thermodynamic theory of enzyme kinetics, we show that the apparent activation energy of aquatic ecosystem photosynthesis is approximately 0.57 electron volts (eV) for temperatures ranging from 4 to 45 °C, which is similar to that of respiration. This result and a global synthesis of 222 streams suggest that warming will not create increased stream and river CO₂ emissions from a warming-induced imbalance between photosynthesis and respiration. However, temperature could affect annual CO₂ emissions from streams if ecosystem respiration is independent of gross primary production, and may be amplified by increasing organic carbon supply.