A Hot-Spring Prokaryote and the Limits of 16S rRNA
A soil sample from a hot spring (74 C, pH 3) yields a unicellular organism, strain HS-7. Under the microscope it is 1.2 micrometres long, has no nucleus or membrane-bound organelles and divides by binary fission. It grows in a sealed culture with no oxygen and releases methane. Its cell wall contains no peptidoglycan, and its membrane lipids have ether linkages and branched hydrocarbon chains. Sequencing of its 16S rRNA gene gives 88 percent identity with the closest described species, Methanocaldococcus jannaschii, and 79 percent with Escherichia coli. A classmate argues that because the organism is a prokaryote it must be a bacterium, and that 88 percent identity is far too low for it to sit in the same genus as M. jannaschii; she proposes placing strain HS-7 in a new kingdom of bacteria.
1. Identify the domain to which strain HS-7 belongs and justify your answer using three independent lines of evidence from the setup.
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Strain HS-7 belongs to the domain Archaea. No single character is decisive, so the answer rests on convergence of several lines:
- Cell wall chemistry: a wall lacking peptidoglycan fits Archaea; bacterial walls almost always contain peptidoglycan (the exception, mycoplasmas, lack a wall entirely rather than having a different polymer).
- Membrane lipids: ether-linked, branched hydrocarbon chains are the archaeal pattern; bacteria and eukaryotes use ester-linked, unbranched fatty acids. This is a biochemical character independent of the rRNA data.
- Metabolism: methane production is confined to methanogenic archaea.
- Molecular data: 88 percent identity with an archaeon is much closer than 79 percent with E. coli; the gene used is the standard prokaryotic barcode.
The extremophile habitat is consistent with, but not evidence for, the identification, because bacteria also live in hot springs. A good answer explicitly notes that the cell-wall and lipid data are independent of the sequence data, which is why together they are convincing.
2. Evaluate the classmate's proposal to create a new kingdom of bacteria for strain HS-7.
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The proposal is not logically consistent with current classification. Under the three-domain system, Bacteria is a domain, and kingdoms sit inside domains, so a new kingdom of bacteria would be a subgroup of Bacteria, not a home for an archaeon. If HS-7 is an archaeon, the correct place to argue about is genus or family within Archaea.
The 88 percent value needs interpretation rather than knee-jerk novelty. rRNA genes evolve slowly and are used precisely because they can be compared across domains; 88 percent is low, so HS-7 is probably a new genus, maybe a new family, but it is still clearly within Archaea because it is far closer to the archaeon than to E. coli. Importantly, there is no universal percentage that defines a genus; genus boundaries are conventions agreed by taxonomists, not natural cut-offs.
A strong evaluation also notes that a single locus is thin evidence for a high-level taxon: the classmate would need multilocus sequence data, genome-wide average nucleotide identity, and phenotypic characters before proposing any new rank.
3. Explain why the biological species concept is difficult to apply to strain HS-7 and to prokaryotes generally.
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The biological species concept defines a species as a group of organisms that can interbreed to produce fertile offspring. Applying it requires controlled crosses and a test of reproductive isolation, which is not possible here:
- Prokaryotes reproduce asexually by binary fission, so there is no routine interbreeding and no fertile-offspring test.
- Horizontal gene transfer (transformation, conjugation, transduction) moves genes between lineages, so genetic boundaries are leaky and a gene tree from one locus may not match the organismal history.
- Many prokaryotes cannot be cultured, so mating or competition experiments cannot be done.
In practice, microbiologists use sequence similarity thresholds, average nucleotide identity of whole genomes, and ecological or phenotypic distinctness. These are operational criteria, and their thresholds are human decisions. The answer should therefore conclude that HS-7 can be described as a distinct lineage using molecular and biochemical evidence, but it is not a species in the same sense as a bird or a mouse.