One Billion Shipping Containers

(July 10th, 2015) Have you ever wondered how much total DNA there is on Earth? Researchers in Edinburgh tortured their calculators and found a mind-blowing answer.

Thanks to James Watson and Francis Crick, the DNA structure is known since 1953. We understand how it replicates and how the hereditary information is coded, which has set the stage for massive advances in molecular biology over the past few years. Curiously, however, 60 years on and we still don’t know how much of the stuff is out there.

Following an idea rooted in his astrobiology background, Charles Cockell, Professor of Astrobiology at the University of Edinburgh, used DNA sequencing combined with biomass surveys to estimate the total amount of DNA in the biosphere. Basically, explains Cockell, the team worked out the number of cells of different organisms using total biomass and typical cell sizes. The total number was then multiplied by the average genome size and the results added up to get the total amount of DNA.

After all these calculations, the grand total came to 5.3 x 1031 megabase pairs (Mb) corresponding to about 5 x 1010 tonnes of DNA. If it was possible to stockpile all the DNA present in the biosphere, you would need one billion shipping containers to do so. The team also conducted some further calculations to confirm the original results, including estimates of DNA from soil and water. These values resulted in a rougher calculation of DNA, but crucially within the same order of magnitude.

Using technology as an analogy, Cockell calculated it would require 1021 of the world’s most powerful supercomputers to store all this information. In addition, assuming an average 30 bases per second transcription rate, the potential computational power of this “biosphere supercomputer” would need about 1022 times more processing power than any of our current supercomputers. It’s impossible to know exactly how much DNA is actually being transcribed at any time, but it’s safe to say that a human-made computer that matches this processing power is yet to be built.

Surprisingly, there was a remarkable similarity - within two orders of magnitude - between eukaryotes and prokaryotes, despite the fact that prokaryotes evolved 3 billion years earlier and exhibit a biomass two to five orders of magnitude higher. The authors suggested a larger genome size in eukaryotes to cover lost ground, allowing the development of a much more complex genetic machinery.

These estimates, however, contain a certain dose of guess work. There is substantial data about genome size, but “better information on the biomass of different organisms in different parts of the biosphere is the crucial missing information,” says Cockell. Overall, this analysis highlights the lack of crucial research looking at the biomass of different types of organisms, which is vital to improve estimates of DNA content present in the biosphere.

Above all, more than just producing a simple estimate of the total amount of DNA, Cockell and his team looked at these values as an alternative way to understand biodiversity. DNA is the storage molecule that ultimately encodes all living organisms, and the amount of DNA on Earth can be seen as a measure of its diversity. “We know about the structure of DNA, but just how much is there on Earth?” asks Cockell. “DNA is the fundamental molecule of information storage, so if you want to know how much information is in the biosphere, you should start by first knowing how much DNA there is.”

An information-based view of the biosphere may also provide a way to study the changing complexity of the biosphere in the future. “We could use it to predict where ‘information hot spots’ are and, rather than think about how many pandas or lions a given environmental change will affect, actually quantify the loss of information,” explains Cockell. “This is potentially more objective and may even allow for diversity loss assessments without actually knowing all the species in a given environment.”

Alex Reis

Photo: adriao

Last Changes: 08.21.2015