PSAT Reading Practice Question 507
Question: 507
Methanosphaera stadtmanae, the first single-celled (archaeal) commensal (i.e., two
organisms have a relationship wherein one benefits, and the other has no harm nor ben-
efit) organism to have its genome sequenced, is an anaerobic, non-moving, sphere-shaped
organism that inhabits the human gastrointestinal tract. Of all methanogenic (methane-pro-
05ducing) Archaea, Methanosphaera stadtmanae has been found to have the most restrictive
energy metabolism as it can generate methane only by reduction of methanol with H2 and is
dependent on acetate as a carbon source. These unique energy conservation traits are what
make Methanosphaera stadtmanae beneficial to its human host and not an opportunistic
pathogen.
10Methanosphaera stadtmanae's genome lacks 37 protein-coding sequences present in
the genomes of all other methanogens. Among these are the protein coding sequences for
synthesis of molybdopterin, which is required for the enzyme catalyzing the first step of
methanogenesis from CO2 and H2, as well as for the synthesis of the CO dehydrogenase/
acetyl-coenzyme A synthase complex. This explains why Methanosphaera stadtmanae cannot
15reduce CO2 to methane nor oxidize methanol to CO2. While this is the typical path of methanogenesis
for many archaeal methanogens, it is not the path for Methanosphaera stadtmanae.
Methanogenic Archaea are naturally occurring components of the human gut micro-
biota. The two original methanogenic species belonging to the order Methanobacteriales,
Methanobrevibacter smithii and Methanosphaera stadtmanae, were identified over 30 years
20ago by the detection of methane in the breath, and eventually isolated from fecal samples.
Methanosphaera stadtmanae, one of the major archaeal inhabitants of the gut, is able to
thrive in the human digestive system because methanol is a product of pectin degradation
in the intestine by Bacterioides species and other anaerobic bacteria. Methanosphaera stadt-
manae reduces methanol produced by the anaerobic bacteria with H2 present to produce
25methane. Production of methane in this manner is beneficial to the human host because of
energy conservation. Methanogens, like Methanosphaera stadtmanae, also play an important
role in digestion by improving efficiency of polysaccharide fermentation by helping to
prevent accumulation of acids, reaction end products, and gaseous hydrogen. It is thought
that Methanosphaera stadtmanae's energy conserving methanogenesis process is one of
30the ways it helps in maintaining homeostasis (biological equilibrium) within the human gut
microbiota.
Homeostasis of the human gut microbiota is a delicate balance, and if disrupted can
cause serious issues for humans. One of these issues is the growing number of cases of IBD
(Inflammatory Bowel Disease). IBD is a term used in the medical field to describe conditions
35of the gastrointestinal tract that have chronic or recurring immune responses and inflamma-
tion. Methanosphaera stadtmanae's commensal role with the human can be disrupted when
other bacteria in the highly immunologically active intestinal tract stop performing their normal
processes. While the details of all of the processes that bacteria perform are not completely
known, it is understood that Methanosphaera stadtmanae reacts to the adverse effects
40by inducing the release of proinflammatory cytokine TNF in peripheral blood cells. By releasing
this, Methanosphaera stadtmanae produces a four-times stronger response than any
other methanogen of the gut microbiota. This response causes increased inflammation in the
gastrointestinal tract, and can only stop when balance within the gut microbiota is restored.
One method physicians have found to help restore the homeostasis of the gut microbiota
45is the administration of archaebiotics. Archaebiotics colonize in the gastrointestinal tract to
help restore balance by eliminating and controlling bacteria or archaea that disrupted the
balance in the first place. Archaebiotics also help by keeping commensal methanogens, like
Methanosphaera stadtmanae, so that they can continue to perform their necessary role of
methanogenesis. It is imperative to maintain methanogenesis so that proper digestion and
50energy conservation can happen for the human.
Understanding the role Methanosphaera stadtmanae plays in the human gastrointestinal
tract has been extremely important in the advancement of understanding IBD, as well as the
development of treatments. Identification of additional archaeal and bacterial species will
continue to help develop the field so that scientists and physicians can better understand how
55different organisms work with each other or against each other.
| Pathway | Complete/Incomplete/Absent | Intermediates |
| Glycolysis (Embden-Meyerhof) | Incomplete | Missing glucose and D-glucose 6-phosphate |
| Entner-Doudoroff (Semi-phosphorylative Form) | Both Absent | Missing all intermediates |
| Pentose Phosphate | Incomplete | Missing all intermediates except D-ribulose-5-phosphate and D-ribose-5-phosphate |
| Pyruvate Oxidation | Complete | All intermediates present |
| Citrate Cycle (Glyoxylate Cycle) | Both Incomplete | Missing citrate and isocitrate as intermediates. Pyruvate feeds into the Citrate Cycle via oxaloacetate. Glyoxylate Cycle only contains oxaloacetate and malate. |
| Reductive Citrate Cycle | Incomplete | Missing all intermediates except oxaloacetate and malate |
| Calvin Cycle | Incomplete | Missing Erythrose-4P, Sedoheptulose 1,7P, Sedoheptulose 7P, and Ribulose 1,5P |
| Methanogenesis | Complete | All intermediates present (can only use methanol and H2 to produce methane) |
| Reductive Acetyl-CoA | Incomplete | Missing all intermediates except 5,10-Methylene-THF and THF (come in from a different pathway) |
Figure 1. The above chart lists the known mechanisms that bacterial and archaeal organisms use for energy purposes. The chart lists if M. stadtmanae has the necessary intermediates present in the human digestive system to have a functional energy pathway given a particular mechanism.
Figure 2. The intermediates in the black boxes are ones that M. stadtmanae cannot use or does not have. The intermediates in the gray boxes are ones that M. stadtmanae does use or has.
According to the passage as a whole, the overall role of Methanosphaera stadtmanae with respect to human health is
Correct Answer: B
Explanation:
(B) The author states that Methanosphaera stadtmanae is helpful to humans because it conserves energy and aids in digestion (lines 44–50). He also says that it likely contributes to IBD in some individuals. Therefore, M. stadtmanae plays an important and helpful role in the body. However, if homeostasis is not maintained, it can also have negative effects in the body. Therefore, it is primarily beneficial, choice (B). It isn't uniformly positive, choice (A), because of its role in IBD. It isn't somewhat harmful, as in choice (D), because the author explicitly states in the first paragraph that M. stadtmanae is beneficial to humans. It's necessary for energy conservation and digestion.