For many scientists, a species' success is measured by sheer numbers. In that case, the most successful species known to man is a type of bacterium known as S-A-R-11, or SAR-11 for short. Scientists estimate that there are two-hundred and forty times a billion billion billion SAR11 cells floating around in the oceans. Now that makes six-billion humans sound like a mere handful.
SAR11 bacteria are known for their ability to transform one substance into another, which is why they are such an important part of the Earth's chemical cycles. Although scientists are still uncertain about SAR11's specific role, it appears to produce carbon dioxide using the oxygen and carbon from organic matter that's derived from photosynthesis. Scientists speculate that SAR11 plays a major role in the way the ocean's surface acts as a giant carbon pump that removes carbon dioxide from the atmosphere.
The catch is that SAR11 is what is known as an uncultured organism, which means that scientists haven't been able to cultivate it under laboratory conditions. This requires scientists to develop pretty indirect genetic methods just to be able to study SAR11 cultures. Now that this technology is available, scientists are sequencing the SAR11 genome in order to figure out exactly what kinds of carbon it uses, and why it's so successful at ocean living.