Re: How much oxygen do land plants and marine algae add to the atmosphere?

The short answer is that land plants and marine phytoplankton (algae)
contribute _all_ of the oxygen to the atmosphere.  Before the evolution of
photosynthesis (the biological process which generates oxygen), there was
essentially no oxygen in the earth’s atmosphere.  About 2 Billion years ago
photosynthesis evolved but it wasn’t until about 430 million years ago that
oxygen rose to near current levels (21% of the atmosphere).  In any given
year, respiration, oxidation and combustion only consume a small fraction
of the oxygen that has built up in the atmosphere (less than 1 part out of
2500), which is balanced by the generation of “fresh” oxygen by
photosynthesis.  Roughly half the oxygen generated each year comes from
land plants and half from phytoplankton.  

A full budget for oxygen in the atmosphere (just like a budget for a
business, it tracks where all of the oxygen is stored and “spent”) can be
found in the article: What atmospheric oxygen measurements can tell us
about the global carbon cycle by RF Keeling and others in the March 1993
issue of Global Biogeochemical Cycles.