How many species of bacteria are there?

I am currently working on a manuscript on the ‘Secret Lives of Bacteria’ together with my ECEHH colleague Will Stahl-Timmins. Will is a graphic designer and science communicator who does a lot of great work making potentially ‘dry’ science accessible in the form of graphics, see his blog Seeing is Believing for an overview of his work. In this paper, we discuss a range of ‘big’ questions in ecology and evolution  in a (hopefully) accessible way to highlight a) that bacteria are very cool and b) that bacteria are fundamentally important to our lives. Some questions that are discussed are: Can bacteria be multicellular? Do bacteria get old? Do bacteria have sex? Can bacteria be altruistic?

One question I want to highlight here in this post is: How many species of bacteria exist on our planet? This is a big question if ever there was one, for (at least) two reasons. The first reason is that we do not have a widely accepted definition of what a species is. You kinda have to agree on what you’re counting before you actually start counting of course. Actually, in a way it is lucky that microbiologists are still so constrained by methodology because it means that we define bacteria very pragmatically, namely on the basis of differences in the 16S RNA marker gene (see this old post for some background). The second reason is that, whatever definition you use, the number of species is so high that any sample taken contains only a tiny fraction of the total diversity, making reliable estimation very difficult. I came up with a back-of-the-envelope calculation of bacterial species diversity using data on comparatively better understood (sorry entomologists!) insects. To quote from the manuscript:

“The estimated total number of insects on the planet is 1,000,000,000,000,000,000 (10¹⁸) or one quintillion. The number of currently described insect species is nearing 1,000,000 (10⁶) or one million (this is not a precise estimate as there are no official records of species numbers). The estimated number of insect species is on the order of 10,000,000 (10⁷) or ten million (estimates range from two to thirty million). The estimated total number of bacteria on the planet is 1,000,000,000,000,000,000,000,000,000,000 (10³⁰) or one nonillion. This estimate is obtained from calculating the density of cells from samples taken from a wide variety of habitats, followed by multiplication with the total mass of each habitat (the calculation for insect number is done similarly). Large proportions of all bacteria are found in the sparsely populated but immensely large biomes of the deep subsurface and deep seabed sediments. The total number of described bacterial species is very low, on the order of 10,000 (10⁴) or ten thousand (a recent list of taxonomically approved names lists 13.537 bacterial names). The total number of bacterial species can be estimated applying the insect ratio of species to individuals. This gives the staggering estimate of 10,000,000,000,000,000,000 (10¹⁹) or a ten quintillion bacterial species. This number is very many orders of magnitude higher than the ‘guesstimates’ of millions or tens of millions of bacterial species used by some microbiologists. Because insects and bacteria are unlikely to speciate in the same way (for instance due to differences in biogeography) and because of the large error margins for the numbers used, this exercise is foremost an illustration of the fact that the number of bacterial species is likely to be very large, rather than a serious attempt at estimating bacterial species richness.”

So please note the disclaimer; the only thing this example suggests is that perhaps we need to think a little bit bigger than we tend to do when it comes to bacterial diversity. I will in a later post highlight some of the graphs Will has been working on.

Michiel

 

P.S. I notice that this post gets the most views of all, so thought it would be worth to add this update. I would like to repeat that the calculation above should not be taken too seriously (see one of the comments below for one reason why); it serves only to highlight the fact hat bacterial diversity is ENORMOUS. A very exciting modelling paper is this one, which predicts a species richness of up to 10¹², which lies smack in the middle of my ridiculously high estimation and this ridiculously low estimation (which does not exceed the number of described species). If you are interested in bacterial speciation, you are welcome to check out an Opinion paper I wrote about this topic, available here. Also, it might be that this paper will see the light of day after all, as the effort to get it published might not be worth it. I might post some parts of it on the blog instead.