Rates of respiration in the light measured in marine phytoplankton using an ¹⁸O isotope-labelling technique

For seven marine phytoplankton species, rates of net and gross O₂ production (measured by the ΔO₂ and ¹⁸O techniques) and ¹⁴C assimilation were determined in laboratory incubations. The ¹⁴C experiments were conducted in culture medium that was either N₂-purged, air-purged or unpurged. In all species, the rates of respiration in the light are observed to be similar to, or greater than, the rates in the dark. For several species, respiration rates increased in the light but there was no Warburg effect (enhancement of ¹⁴C productivities in N₂-purged cultures), indicating that photoenhancement of respiration was not due to photorespiration. In a few species, significant rates of photorespiration were detected with the ¹⁴C method, corresponding to 20 to > 100% of the rates of respiration in the light. The ratio of light respiration to production showed variable changes when phytoplankton were exposed to greater light intensities. These results suggest that phytoplankton do not necessarily increase rates of respiration when placed in an environment with increased irradiance.