Interactions between inorganic phosphate (Pi) assimilation, photosynthesis and respiration in the Pi‐limited green alga Selenastrum minutum*

Abstract

ABSTRACTInorganic phosphate (Pi)‐limited chemostat cultures of the green alga Selenastrum minutum were employed to investigate interactions between Pi assimilation, respiration and photosynthetic processes. Changes in net and gross gas exchange rates indicated that O2 evolution decreases during photosynthetic Pi assimilation. Room temperature and 77K Chi a fluorescence measurements revealed that this photosynthetic suppression is correlated with a transition from state 1 to state 2. Substantial photosynthetic Pi uptake rates occur in the presence of DCMU and KCN. Additionally, the cellular ratio of ATP:NADPH increases following Pi enrichment, suggesting that the ratio of cyclic to linear electron flow is enhanced in response to the high energy requirements of Pi uptake. Net starch degradation was observed during photosynthetic Pi assimilation and the cellular pool size of 3‐phosphoglycerate increased; however, gross gas exchange parameters and cellular metabolite pool sizes indicated that mitochondrial respiration plays a smaller role during Pi assimilation in the light than it does in the dark. These observations were used to formulate a model depicting possible interactions between photosynthetic electron flow, photosynthetic and respiratory carbon metabolism and metabolite exchange between the chloroplast, cytosol and mitochondrion during photosynthetic Pi assimilation.