Q:How many individual organisms exist on Earth?
A: About 10,000,000,000,000,000,000,000,000,000,000.
A superb paper HERE on this topic.
Prokaryotes: The unseen majority
William B. Whitman*†, David C. Coleman‡, and William J. Wiebe§
Departments of *Microbiology, ‡Ecology, and §Marine Sciences, University of Georgia, Athens GA 30602
The number of prokaryotes and the total amount of their cellular carbon on earth are estimated to be 4–6 x 10^30 cells and 350–550 Pg of C (1 Pg = 10^15 g), respectively. Thus, the total amount of prokaryotic carbon is 60–100% of the estimated total carbon in plants, and inclusion of prokaryotic carbon in global models will almost double estimates of the amount of carbon stored in living organisms. In addition, the earth’s prokaryotes contain 85–130 Pg of N and 9–14 Pg of P, or about 10-fold more of these nutrients than do plants, and represent the largest pool of these nutrients in living organisms.