We postulate that the evolutionary success of these diverse algal groups ch
aracterized by secondary endosymbiotic origin of plastids was directly conn
ected to changing atmospheric CO2 in the late Palaeozoic. Atmospheric CO2 l
evels were reduced to historic lows, probably lower than today's atmosphere
, during the Pennsylvanian and Permian periods. This most Likely resulted i
n dissolved inorganic carbon (DIC) becoming the limiting factor in photosyn
thesis in these ancient oceans. Fossil and molecular data suggest that a nu
mber of algar groups with secondary endosymbioses evolved between 260 and 2
85 Ma during the minimum in atmospheric CO2. It has been hypothesized that
these algae were able to more efficiently utilize DIC because their chlorop
lasts were contained within an acidic compartment where DIC was largely in
the form of CO2. In this paper we postulate that secondary endosymbioses ar
ose 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 water
s high in DIG. It was not until the ancient atmospheric CO2 minimum that se
condary endosymbioses were selected for, because these algae were able to u
tilize the low DIC more efficiently and outcompete existing algae. Under th
ese 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.