A Metabolic Perspective on Nitrogen Source and Temperature in Microalgae

Abstract

The roles of nitrogen (N) and temperature in primary producer metabolism are becoming increasingly relevant for optimising algal culturing and for understanding how global warming and changing agricultural practices will influence crops. I argue that these factors could be combined into a single mechanistic framework based on the 'metabolic theory of ecology (MTE)', which has been successful in other fields. The hallmark of this approach is the lower temperature sensitivity of photosynthesis (P) compared with respiration (R), which reduces carbon allocation to biomass (carbon-use efficiency [CUE]) with warming. Notably, it has been reported that photorespiration associated with N assimilation could explain reduced thermal sensitivity of P in C3 organisms. If true, this means that the N source should change the thermal sensitivity of P and therefore shape the thermal responses of CUE and growth, providing a simple mechanistic framework for predicting the impacts of N and temperature on C3 organisms. To test this metabolic perspective, I cultured the tractable green alga, <i>Chlorella vulgaris </i>Beijer., and measured the rates of P and R alongside the rates of CUE and growth when provided with inorganic (nitrate ions or ammonium) or organic (glutamine) N sources across a temperature gradient. As expected, organic N increased the thermal sensitivity of P but not R, with an associated increase in CUE with warming. This work suggests that we could give this MTE-based perspective the 'green light' to inform optimal algal culturing conditions in industrial applications, and further work should investigate the possible impacts of agricultural organic N use in a warmer world.