O2 Dropping Faster than CO2 Rising

http://www.i-sis.org.uk/O2DroppingFasterThanCO2Rising.php

Implications for Climate Change Policies

New research shows oxygen depletion in the atmosphere accelerating since 2003, coinciding with the biofuels boom; climate policies that focus exclusively on carbon sequestration could be disastrous for all oxygen-breathing organisms including humans Dr. Mae-Wan Ho

"First, O2 is there principally because of carbon storage time,its rate of drop currently is ~10 ppm, but it could well swing further downwards."

CO2 increase is just 1.8 ppm per annum.

Therefore reducing the decline in O2 by 50% to 5 ppm per annum using photosynthesis will stop the increase in CO2 level of atmosphere.

Evolution of Plants and Oxygen in Atmosphere

http://sci.waikato.ac.nz/evolution/plantEvolution.shtml

Interesting graphic of evolution of plants and increase in oxygen level in atmosphere.
Prokaryotes (Cyanobacteria,etc.) do not seem to have contributed much to the O2, its only after evolution of Eukaryotes (Green Algae, Diatom Algae, etc.) that the O2 in air started to increase and land plants have contributed the most.