Algal evolution in relation to atmospheric CO2: carboxylases, carbon concentrating mechanisms and carbon oxidation cycles

Algal evolution in relation to atmospheric CO2: carboxylases, carbon concentrating mechanisms and carbon oxidation cycles

Oxygenic photosynthesis evolved at least 2.4 Ga; all oxygenic organisms use the ribulose bisphosphate carboxylase-oxygenase (Rubisco)-photosynthetic carbon reduction cycle (PCRC) rather than one of the five other known pathways of autotrophic CO2  assimilation. The high CO2  and (initially) O2 -free conditions permitted the use of a Rubisco
with a high maximum specific reaction rate. As CO2  decreased and O2 increased, Rubisco oxygenase activity increased and 2-phosphoglycolate was produced, with the evolution of pathways recycling this inhibitory product to sugar phosphates. Changed atmospheric composition also selected for Rubiscos with higher CO2 affinity and CO2 /O2  selectivity correlated with decreased CO2 -saturated catalytic capacity and/or for CO2 -concentrating mechanisms (CCMs). These changes increase the energy, nitrogen, phosphorus, iron, zinc and manganese cost of producing and operating Rubisco-PCRC, while biosphere oxygenation decreased the availability of nitrogen, phosphorus and iron. The majority
of algae today have CCMs; the timing of their origins is unclear. If CCMs evolved in a low-CO2  episode followed by one or more lengthy high-CO2 episodes, CCM retention could involve a combination of environmental factors known to favour CCM retention in extant organisms that also occur in a warmer high-CO2 ocean. 

Prof. Mario Giordano
Laboratorio di Fisiologia delle Alghe e delle Piante
Dipartimento di Scienze della Vita e dell'Ambiente (ex Scienze del Mare)
Università Politecnica delle Marche
Via Brecce Bianche
Palazzina Scienze 2
60131 Ancona
Italy

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