I posted some excerpts from a paper in progress a while back (How many species of bacteria are there? and Bacteria vs. Animals). Here another post on the question “Is every bacterium everywhere?” (then I will stop and just link to the published version!).
Through the narrow sea strait between the Indonesian islands Bali and Lombok, past Borneo and Sulawesi on towards the Philippines runs the Wallace line, named after the co-conceiver of the theory of evolution through natural selection Alfred Russell Wallace. The fauna west of this invisible line is Asiatic, and includes for example tigers and orang-utans, whereas the fauna east of this line shares Asiatic and Australian characteristics, missing most of the Asiatic megafauna species. To understand why such biogeographic boundaries exist, it is necessary to look into the geological past. During the last Ice Ages, seawater levels were dramatically lower and the islands on the Asiatic side of the line were connected to each other as well as to mainland Asia. However, deep waters separated this landmass from more easterly islands such as Lombok and Sulawesi. Without land bridges, these islands could only be colonized by animals capable of surviving journeys over sea or through the air. The debate on whether there exist equivalents of the Wallace line for bacteria is well over a century old (1) and has still not been resolved. One important reason for this lack of progress is that biogeographic patterns change depending on the taxonomic or genetic resolution used (2). For example: different monkey species occur on both sides of the Wallace line and so when only the broad classification ‘’primate’ would be used, no biogeographical patterns would be apparent. Only when looking at the level of species, patterns would emerge, for instance showing that the Crab-eating macaque is the only primate species occurring on either side of the Wallace line. Moreover, it is likely that high resolution sequencing data would reveal subtle genetic differences between populations of a species inhabiting different islands on one side of the Wallace line. As the methods used to differentiate bacteria are often relatively crude, biogeographic differences thus could be overlooked.
One other problem with the study of bacterial biogeography is that different researchers use different definitions of the term ‘biogeography’. In its broadest sense, biogeographic patterns could be said to exist when finding different species in different geographical locations. In a narrower sense, biogeographic patterns are recognized only in the subset of cases where geographically distinct, but environmentally very similar locations harbour different species. In this latter case, barriers to dispersal must have prevented a single species to colonize all suitable habitats (3). In its broad definition, bacterial biogeography is uncontroversial: different environments select for different bacteria. In its narrower definition, things are not so clear. The very small sizes of bacteria combined with their large population sizes are likely to make dispersal by water or wind highly effective, and presumably not much hindered by potential barriers such as seas, mountain ranges or just sheer distance. Indeed, there are no known cases of bacteria that are restricted to one geographical location, whereas there are many documented cases of bacterial species (be it in forest soils, hot springs or the open ocean) that occur globally. This is not to say that there is no geographic structuring within these species: isolates that are geographically more distant often are on average also genetically more distant. To some extent, the differences must be due to chance, with different populations experiencing different mutations with small effects on fitness that persist without spreading to other geographic populations. To a large extent, these genetic differences must be due to adaptation to local differences in habitat. It is hard to figure out which ecological differences between locations are relevant to bacteria as there are many and they can be subtle (some could even be related to the biogeographic differences in animals and plants). The effects of habitat on bacterial diversity are therefore likely to be underestimated (2).
1. O’Malley MA. 2007. The nineteenth century roots of ‘everything is everywhere. Nat Rev Microbiol 5:647-651.
2. Hanson CA, Fuhrman JA, Horner-Devine MC, Martiny JB. 2012. Beyond biogeographic patterns: processes shaping the microbial landscape. Nature Reviews Microbiology 10:497-506.
3. Martiny JB, Bohannan BJ, Brown JH, Colwell RK, Fuhrman JA, Green JL, Horner-Devine MC, Kane M, Krumins JA, Kuske CR, Morin PJ, Naeem S, Ovreas L, Reysenbach AL, Smith VH, Staley JT. 2006. Microbial biogeography: putting microorganisms on the map. Nat Rev Microbiol 4:102-112.