Before photosynthesis evolved, Earth's atmosphere had no free oxygen (O 2 ). Photosynthetic prokaryotic organisms that produced O 2 as a waste product lived long before the first build-up of free oxygen in the atmosphere, perhaps as early as 3.5 billion years ago. The oxygen they produced would have been rapidly removed from the atmosphere by weathering of reducing minerals, most notably iron. This 'mass rusting' led to the deposition of iron oxide on the ocean floor, forming banded iron formations. Oxygen only began to persist in the atmosphere in small quantities about 50 million years before the start of the Great Oxygenation Event. This mass oxygenation of the atmosphere resulted in rapid buildup of free oxygen. At current rates of primary production, today's concentration of oxygen could be produced by photosynthetic organisms in 2,000 years. In the absence of plants, the rate of oxygen production by photosynthesis was slower in the Precambrian, and the concentrations of O 2 attained were less than 10% of today's and probably fluctuated greatly; oxygen may even have disappeared from the atmosphere again around 1,900 million years ago These fluctuations in oxygen concentration had little direct effect on life, [citation needed] with mass extinctions not observed until the appearance of complex life around the start of the Cambrian period, 541 million years ago. The presence of O 2 provided life with new opportunities. Aerobic metabolism is more efficient than anaerobic pathways, and the presence of oxygen undoubtedly created new possibilities for life to explore. Since the start of the Cambrian period, atmospheric oxygen concentrations have fluctuated between 15% and 35% of atmospheric volume. The maximum of 35% was reached towards the end of the Carboniferous period (about 300 million years ago), a peak which may have contributed to the large size of insects and amphibians at that time. Whilst human activities, such as the burning of fossil fuels, have an impact on relative carbon dioxide concentrations, their impact on the much larger concentration of oxygen is less significant. (From wiki)
My guess is algae. Grass and trees have support structures which are heavy, but don't produce oxygen. I believe that algae have a more efficent photosynthesis than cyanobactera, but I'm not sure about that. So my best bet is algae, second best is cyanobacteria.
From what I read, algae are the preferred choice of NASA for future concepts of self-sustaining colonies in space.
I assume oxygen production by algae is temperature dependent then it would be expected norther region would be more dependent on tree for oxygen production and by the equator more on alae , any comment ?
