Ancient atmospheric CO2 and the timing of evolution of secondary endosymbioses

We postulate that the evolutionary success of these diverse algal groups characterized by secondary endosymbiotic origin of plastids was directly connected to changing atmospheric CO2 in the late Palaeozoic. Atmospheric CO2 levels were reduced to historic lows, probably lower than today's atmosphere, during the Pennsylvanian and Permian periods. This most Likely resulted in dissolved inorganic carbon (DIC) becoming the limiting factor in photosynthesis in these ancient oceans. Fossil and molecular data suggest that a number of algar groups with secondary endosymbioses evolved between 260 and 285 Ma during the minimum in atmospheric CO2. It has been hypothesized that these algae were able to more efficiently utilize DIC because their chloroplasts were contained within an acidic compartment where DIC was largely in the form of CO2. In this paper we postulate that secondary endosymbioses arose continuously from the time of evolution of the chloroplast (about 2000 Ma). However, these secondary endosymbioses were quickly eliminated because they possessed no selective advantage over existing phytoplankton in waters high in DIG. It was not until the ancient atmospheric CO2 minimum that secondary endosymbioses were selected for, because these algae were able to utilize the low DIC more efficiently and outcompete existing algae. Under these favourable conditions, a number of secondary endosymbioses evolved and survived, and it is their ancestors that constitute most of the eukaryotic phytoplankton in today's oceans.