Why are microorganisms ubiquitous

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The biosphere supports astronomical numbers of free-living microorganisms that belong to an indeterminate number of species. One view 1 , 2 , 3 is that the abundance of microorganisms drives their dispersal, making them ubiquitous and resulting in a moderate global richness of species. But ubiquity is hard to demonstrate, not only because active species have a rapid turnover, but also because most species in a habitat at any moment in time are relatively rare or in some cryptic state 4.

Here we use microbes that leave traces of their recent population growth in the form of siliceous scale structures to show that all species in the chrysomonad flagellate genus Paraphysomonas are probably ubiquitous.

Paraphysomonas consists of 50 species, which can be distinguished by the morphology of their surface scales 5 , although oligonucleotide sequence small-subunit ribosomal RNA data indicate that the morphospecies are also genetically distinct 6.

The scales remain recognizable for several months after cell death, so looking at their remains in the sediment of a pond provides evidence of the preceding species succession. We identified and quantified all the scales and cell remains of Paraphysomonas species present, and used this information to reconstruct whole cells.

Our examination of We compared our data with information in 73 published surveys of Paraphysomonas species from biogeographic regions across the world. The pattern of relative abundance of species in Priest Pot is similar to the global one Fig. Species that are frequently recorded globally are also abundant in sediment from Priest Pot, and species that are rarely found globally are not abundant in Priest Pot.

Commonness data are ranked in order, decreasing from left to right. Species 1 and 2 are P. Further details are available from the authors. They are probably capable of population growth in a broad range of conditions, so they will more frequently find suitable conditions.

Finally, termination of population growth is accompanied by the production of resting cysts. It is widely believed that most microbial species have yet to be discovered, which follows from the general rule that, for each tenfold reduction in body length, the global number of taxa increases roughly fold 8.

As ubiquity will limit rates of local speciation and extinction 1 , the global number of species less than 1 mm long will be relatively small. Free-living bacteria sustain all the important ecosystem functions.

They are about three orders of magnitude more numerous than heterotrophic flagellates, so it is even more likely that they too are ubiquitous, and that the global richness of free-living microbial species is moderate.

Fenchel, T. Oikos 68 , — Google Scholar. Roberts, M. Evolution 49 , — Finlay, B. For sexual outbreeders, there is, at least in principle, a theoretically based species concept. But many protist groups are sexless or include sexless species; this applies, for example, to all naked amoebae and to many groups of flagellates. In these species, as in bacteria, evolution is clonal and the species concept is fundamentally arbitrary. For clonal organisms, there are no constraints to genetic divergence, and descendants from a cell will accumulate neutral mutations over time.

Moreover, isolation is not a prerequisite for adaptive radiation e. Considerable genetic variation and clusters of genotypes within a nominal species can therefore be expected, and these effects have been demonstrated for bacterial species Cohan Sequencing of genes such as ribosomal RNA genes from remote places could thus reveal patterns indicating some degree of geographical patterning, but the data available so far are very limited in the case of eukaryotic microbes.

Atkins and colleagues isolated a number of small heterotrophic flagellates from hydrothermal vents in the Pacific Ocean. All of the organisms they retrieved belonged to known and widespread nominal species. Gene sequencing revealed variation between populations, but there was no consistent geographic pattern when sequences were compared to sequences of isolates from elsewhere. The flagellate Cafeteria roenbergensis from a Pacific vent was genetically almost identical to a strain from a shallow-water habitat in Denmark, but it differed from a morphologically similar shallow-water isolate from the eastern coast of North America.

So far, however, there are too few data to draw any firm conclusion from this approach. Organisms differ in their degree of specialization, but all are to some extent confined to particular types of habitats. Some protists grow only within a narrow temperature range. It therefore occurs only in tropical regions; it has been found in Africa, South America, and Southeast Asia Dragesco Likewise, many foraminiferans have a pantropical distribution.

Other marine protists are confined to cold seas or to porous sea ice, but identical species occur in the Arctic and Antarctic regions Montresor et al. In the case of planktonic cold-water foraminiferans, gene flow between Arctic and Antarctic populations has been demonstrated Darling et al. One could also look at species that have unusually wide habitat niches.

Most protist species occur either in fresh water or in the sea, but some morphospecies appear in both habitats. Isolates of different euryhaline protists have shown that strains from marine and hypersaline habitats grow equally well at all salinities, from fresh water to three times oceanic salinity, whereas the freshwater isolates generally will not grow in full-strength seawater. This suggests that freshwater strains lose the ability to cope with high salinities, but it still supports the correlation between particular morphospecies and specific tolerance limits.

Strains of the marine ciliate Uronema U. We have so far implicitly discussed only free-living species. The distribution of host-specific symbiotic microbes is, of course, limited to the distribution of the host species. Furthermore, total symbiont population sizes are, depending on the abundance of the host, probably often small in an absolute sense, and so the arguments put forward here may not apply.

However, even symbionts may have cosmopolitan distribution. The dinoflagellate Symbiodinium is a phototrophic endosymbiont in a variety of corals, bivalves, and some other marine invertebrates. Recent studies have shown that Symbiodinium comprises a number of genotypes that differ with respect to temperature tolerance and preferences for light intensity. However, the different genotypes do have a cosmopolitan distribution LaJeunesse Returning to free-living forms, it is evident that some genetically based adaptations to different habitats may occur within nominal protist species.

Whether there is any real geographical structuring of such genetic variation within nominal species is difficult to disprove. Among macroscopic organisms, however, restricted geographic distribution applies not only to genetic variation within species but also to variation among species, families, and orders.

With the exception of species introduced by humans, representatives of the macrofauna of Australia are nearly all endemic to the continent. In comparison, Esteban and colleagues recorded 85 ciliate species from a crater lake in Tasmania; all but one already known from Africa had previously been found in Europe. Every year a few new species of protists are discovered, usually when previously ignored habitat types are investigated. Such newly discovered species are initially defined as endemic.

However, experience has shown that such species are usually rediscovered in similar habitats worldwide—as soon as someone looks for them. Both sites had been subject to previous floristic and faunistic studies by other researchers; we analyzed these data along with the results of our own more recent surveys.

The species list for these sites is not complete, because we lack the taxonomic expertise to identify some groups e. Also, it is still possible to find additional species especially among protists not previously recorded at the sites. So far approximately species have been recorded from the freshwater site and approximately from the marine site.

These numbers are not quite comparable, in that the freshwater site includes amphibious species, whereas the marine data are confined to organisms that are permanently beneath the water surface. In both cases, most organisms are small. At the marine site, for example, species are heterotrophic or phototrophic protists most of these being either ciliates or flagellates.

Of the remaining species, belong to the meiofauna animals less than 1 mm long; mainly nematodes, rotifers, gastrotrichs, turbellarians, and some crustaceans and to the macrofauna mainly molluscs, crustaceans, polychaetes, coelenterates, and fish ; we also found 19 species of macroalgae and 2 species of vascular plants. The size spectra of the two sites are similar figure 2. Clearly, small organisms less than 1 mm dominate in terms of species numbers, supporting the general notion that there are many more species of small organisms than of large ones.

Determining the global distribution of large plants and animals is a relatively easy task. Regrettably, this is not so for small organisms, because of undersampling in many parts of the world. The vast majority of recordings of protists and meiofauna derive from Europe and North America, with many fewer from most other parts of the world. Data on marine meiofauna, for example, are almost absent from Australasia, where the protist biota has so far been covered by only a handful of workers.

Data are also scarce from South America, Southeast Asia, and Africa, although they are somewhat more complete for limnic than for marine organisms. A few species that could be identified only to the genus level had to be omitted from the analysis.

Furthermore, we had to use a pragmatic definition of cosmopolitan species as species occurring in at least two oceans or two biogeographical regions and in both Northern and Southern Hemispheres. Because of undersampling of small species, their degree of cosmopolitanism may be underestimated.

Figure 3 shows the relationship between body size and the fraction of species with cosmopolitan distribution macrophytes are omitted. The freshwater data suggest a sharp transition at a body size of approximately 1 mm, below which species tend to have a cosmopolitan distribution, whereas the marine data suggest a more gradual change.

This difference, however, may be an artifact reflecting the incomplete records of marine meiofauna from the Southern Hemisphere. Aquatic macrophytes deviated from this pattern: For the limnic species measuring less than 10 cm, approximately 50 percent had a cosmopolitan distribution.

In the case of the marine macroalgae, the relationship with size was not evident because of the limited number of species, but approximately half of the species had cosmopolitan distribution.

We are unable to explain why macrophytes apparently have a wider distribution than similarly sized animals. Both study sites represent shallow-water biota in a temperate climate zone. It is therefore not impossible that a similar study set in, for example, a tropical region would look somewhat different, showing a smaller fraction of cosmopolitan species. This does not, however, affect the difference between small and large organisms.

The trend shown in figure 3 is real, demonstrating that small organisms have a wider distribution than larger organisms and that the smallest ones apparently are distributed worldwide. The global body-size distribution of aquatic organisms figure 4 is very different from the distribution in the pond and in the marine shallow-water habitats that we studied figure 2.

Compilations of the global inventory of species are uncertain. On the one hand, many species may remain undescribed; on the other, many taxonomic groups are burdened by synonyms species that have been given two or more names , thus inflating the number of nominal species.

Also, different taxonomic experts differ in their estimates of the global number of species for particular taxa. Nonetheless, it is evident that among the roughly , species of heterotrophic aquatic organisms named so far, the majority measure more than 1 cm, and less than 10 percent are protozoa. Probably only some 10, to 15, living protists including phototrophic forms have so far been discovered and named.

As a consequence, for any given area, the species present will represent an increasing fraction of the global species pool with decreasing body size figure 5. Again, this compilation probably underestimates this relation because, given sufficient time and effort, there is no doubt that an additional number of small organisms and especially protists can be found at the two sites we studied, whereas our survey of macroscopic organisms is probably almost complete.

Also, the particular sites we chose—a eutrophic pond and a sandy, shallow-water area with fluctuating salinities and temperatures—represent a limited subset of the local biota. Had we extended the marine sampling site to adjacent deeper offshore waters with a stable high salinity or, in the freshwater case, included a neighboring oligotrophic lake, we undoubtedly would have recovered an even larger fraction of the global pool of small organisms.

Figure 6 shows the species—area curves for some selected marine groups. The value of z diminishes with the decreasing size range of organisms belonging to different taxonomic groups. For the macrofaunal groups, the curves tend to be upward concave, reflecting the cumulative addition of new habitat types and climatic zones and—when going from the North Atlantic to the world's oceans—the inclusion of new faunal provinces with endemic faunas.

One of the most striking patterns of macroecology is the latitudinal diversity gradient Rosenzweig , the increase in species numbers when moving from higher to lower latitudes.

Hillebrand and Azovsky showed that the effect of the latitudinal diversity gradient decreases with decreasing body size and almost vanishes for protists. A simple implication of our findings is that those who are interested in microbes need not travel to exotic places to find interesting creatures to study: Most microorganisms can be found at the local seashore or lake—or, for that matter, in a garden pond.

The results also may illuminate mechanisms that determine local and global biodiversity and community structure. Recent understanding of community structure and diversity emphasizes dispersal and extinction as statistical phenomena rather than as results of special species interactions Lawton Strictly speaking, the neutral models apply only to guilds of species limited by a single resource, and they imply that all species are ecologically identical.

These models further assume constant probabilities of per capita reproduction, dying, and migration to a neighboring site.

Nevertheless, they provide predictions of patterns of diversity, relative species abundance, and species—area curves that resemble the findings for real biotic communities. The requirement of ecological identity of species in which intensities of intra- and interspecific competition are identical and predatory interactions are absent is rarely met in the real world. Microorganisms are ubiquitous. Microbiologists have located them almost everywhere on the planet. Roundworms , for example, are more abundant animals, native even to Antarctica.

Considering the ubiquity of microorganisms, finding microorganisms is not hard except for the fact that they can only be seen under microscopes. Bacteria, fungi and other single-celled organisms have been discovered in ordinary areas like the bathroom in your house, for example as well as in extreme locations like hydrothermal vents deep in the ocean.

Ubiquity means something that appears literally everywhere. It's hard to imagine the scope of the ubiquity of microorganisms especially since we cannot see them. But every imaginable surface in the world is covered in microorganisms. The table next to you, your shoes, your phone and even your skin are all covered by communities of microorganisms. Try a ubiquity lab in your class or on your own! Take swabs of various surfaces and transfer them onto agar growth plates.

Store them in the lab and check back in a couple days. You'll see hundreds of colonies of bacteria, fungi and other microorganisms growing on every plate no matter where the swab was taken from. Bacteria are extremely common microorganisms.

Although they are known for causing serious diseases like pneumonia, meningitis and toxic shock syndrome, only 3 percent of bacteria are actively harmful to people or animals. The human body itself has about trillion bacteria with most living on the skin and inside the digestive system. Harmless bacteria on the skin protect themselves from other microbes by releasing toxic proteins.