The barrier to preying on "mirror-image" life is likely lower than presumed due to:
Lipids (triglycerides), largely achiral, offer immediate energy. In E. coli for example, they constitute ~10% of dry mass, yielding ~27% of its macronutrient calories[1]). This non-chiral caloric gain from mirror prey could sustain predators during adaptation to chiral components.
L-glucose metabolism has evolved at least twice despite its rarity in nature:
However, given that carbohydrates constitute a relatively minor fraction of the total caloric content in microorganisms (e.g., ~7% in E. coli[1:1]), the immediate selective advantage of metabolizing mirror-sugars might be less pronounced than for fats or proteins. That these adaptations emerged even under low selective pressure, suggests the evolutionary barrier to such adaptations might be lower than anticipated.
Amino acid racemases, enzymes interconverting L- and D-amino acids (e.g., for cell walls, neurotransmission), are widespread (Yoshimura & Esaki, 2003[4]). This pre-existing machinery could be adapted to process D-amino acids from mirror proteins, the largest caloric source in microorganisms (~55% dry mass, ~66% major macronutrient calories in E. coli[1:2]).
Achiral fat calories, existing L-sugar pathways, and D-amino acid racemases collectively suggest a lowered barrier to mirror-life predation. Fats offer initial sustenance, while pre-existing enzyme capabilities could enable adaptation to mirror-chiral molecules. While mirror organisms still likely have a large competitive advantage, these metabolic footholds suggest that a stable equilibrium might develop more readily than initially expected.
Co-authored by Gemini 2.5 Pro
Phillips, R., Milo, R., et al. Cell Biology by the Numbers. https://book.bionumbers.org/what-is-the-macromolecular-composition-of-the-cell/ ↩︎ ↩︎ ↩︎
Sasajima, K.-I., & Sinskey, A. J. (1979). Oxidation of l-glucose by a Pseudomonad. Biochimica et Biophysica Acta (BBA) - Enzymology, 571(1), 120-126. DOI: 10.1016/0005-2744(79)90232-8 ↩︎
Shimizu, T., Takaya, N., & Nakamura, A. (2012). An l-glucose Catabolic Pathway in Paracoccus Species 43P. Journal of Biological Chemistry, 287(48), 40448–40456. DOI: 10.1074/jbc.M112.403055 ↩︎
Yoshimura, T., & Esaki, N. (2003). Amino acid racemases: functions and mechanisms. Journal of Bioscience and Bioengineering, 96(2), 101-108. DOI: 10.1016/S1389-1723(03)90111-3 ↩︎
(I commented on the LessWrong post but figured it'd be useful to also comment here so that it is visible to people reading the post here.)
Hi—thanks for engaging with this! These points are discussed within the Technical Report on Mirror Bacteria, in Chapter 1 (which reviews opposite-chirality nutrient use) and Chapter 8 (which discusses predation specifically). Predation requires much more than simply the ability to catabolize small metabolites. From §8.5 (p. 184):
The evolution of protists and animals that prey on mirror bacteria appears more challenging. Protists and animals lack the enzymes required to degrade mirror proteins, sugars, nucleic acids, and lipids. They likely have limited or no ability to catabolize most mirror metabolites (Friedman & Levin 2012), and some D-amino acids are toxic to many organisms (Forsum et al. 2008; Friedman & Levin 2012; Yow et al. 2006; see also Box 1.2). These deficiencies could not be remedied through a handful of mutations, but would require the much slower evolution of novel proteins and catabolic pathways.
To give an illustrative example, C. elegans relies on many dozens of distinct lysozymes, glycosidases, proteases, phospholipases, nucleases, and other lytic enzymes to digest the macromolecules present in their bacterial prey (McGhee et al. 2007; Yilmaz & Walhout 2016). Digesting even a simple macromolecule like bacterial-derived glycogen requires intestinal amylase and α-glucosidase. Evolving a similar catabolic pathway for mirror glycogen would almost certainly require a similar or greater number of novel enzymes, and it seems questionable whether such adaptations could arise in nematodes even over millions of years.
For example, having a racemase that interconverts a D-amino acid into an L-amino acid isn’t enough, you also have to first breakdown mirror proteins into their constituent D-amino acids and then import the D-amino acids into the cells. Even beyond that there are a few challenges to overcome: D-amino acids can be toxic, and also racemases catalyze the interconversion of L/D-amino acids, so that would have to be carefully regulated to avoid causing a build-up of D-amino acids in the cell.
As a more minor point, the lipid membranes of bacterial cells don’t contain triglycerides (indeed, triglycerides are not capable of forming lipid membranes, rather, they are primarily used as specialized energy storage, usually in multicellular organisms; triglycerides are entirely absent from E. coli and most other bacterial species.) The constituents of bacterial lipid membranes are varied but chiral.
Wow, thank you for the kind and thorough reply! Obviously there is much more to this, I'll have a look at the report.
P.S.: I can't see your comment on lesswrong. If it doesn't show up later, I will link it so readers there also have the benefit of these corrections and references!