What fossils tell us about prehistoric microbes
Kevin’s latest post is about how babies first acquire the suite of microbes (their microflora) that form the beginning of the community that will live within them for the rest of their life. This is one example of symbiosis (two organisms living together) involving microbes, but there are countless others. The list below is just a few of the better known examples.
- Nitrogen fixing bacteria (Rhizobia) in the roots of plants
- Photosynthetic bacteria living within corals
- Microbes in termite guts helping them digest cellulose
- Bioluminescent quorum sensing -also mentioned by Kevin recently–
- Bacteria allow the Hawaiian Bobtail squid to hunt at night without casting a shadow
The environment that I study (hydrothermal vents) is also rife with microbial symbioses, but these are slightly different because they are what is called chemoautotrophic. This means that they get their energy from electrons they garner from environmental chemicals rather than the sun (as in photosynthesis), but, like photosynthesizers, they get their carbon from an inorganic source (typically CO2) rather than an organic source. One of the most common inorganic energy sources in vent ecosystems is hydrogen sulfide, which is found in abundance at vents. Other chemicals such as methane, sulfur, hydrogen, ammonia, and certain forms of iron are also used. Over evolutionary time, some of the animals that developed these symbioses have lost their own feeding structures. Certain tube worms, for example, have no mouth, no gut, and no anus. They rely completely on the microbes in their trophosome (an internal microbe-holding organ) to provide them with all of their nutrients.
Last night I came across a very cool study from 2000, entitled where a case is made that a certain group of trilobites may have had chemoautotrophic symbionts. This would make them the oldest known organisms to have developed this type of partnership. You can read it for yourself, here, but the gist is as follows. This group of trilobites has been found in sedimentary rock layers that indicate they lived in a low oxygen, high sulfide environment. The fossil record shows that they lack the typical trilobite feeding structure (the hypostome), which they likely lost as the symbiosis made these structures unnecessary (why have a mouth if you don’t eat?), and is (according to the author) “difficult to explain any other way”. The fossils that were examined also show some body structures (extended pleural areas and multiplied thorasic segments) that are enlarged and duplicated, possibly as a space to cultivate the microbial symbionts. This is the type of adaptation that we see today in modern day organisms that host such symbioses.
Interpreting fossils is certainly a challenging task, and one such hypotheses are often challenging to disprove (lacking a time machine), but I think it is extremely cool that these types of interpretations can be made about ancient microbes and co-evolution, based on what we know about modern host organisms, and the fossil record.