Without oxygen, Earth's early microbes relied on arsenic to sustain life

[Eugene]

Related: Modern arsenotrophic microbial mats provide an analogue for life in the anoxic Archean (Communications Earth & Environment)

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Source: University of Connecticut

SEPTEMBER 22, 2020

Without oxygen, Earth's early microbes relied on arsenic to sustain life

by Elaina Hancock, University of Connecticut

Much of life on planet Earth today relies on oxygen to exist, but before oxygen was present on our blue planet, lifeforms likely used arsenic instead. These findings are detailed in research published today in Communications Earth and Environment.

A key component of the oxygen cycle is where plants and some types of bacteria essentially take sunlight, water, and CO2, and convert them to carbohydrates and oxygen, which are then cycled and used by other organisms that breathe oxygen. This oxygen serves as a vehicle for electrons, gaining and donating electrons as it powers through the metabolic processes. However, for half of the time life has existed on Earth, there was no oxygen present, and for the first 1.5 billion years, we really don't how these systems worked, says lead author of the study and UConn Professor of Marine Sciences and Geosciences Pieter Visscher.

Light-driven, photosynthetic organisms appear in the fossil record as layered carbonate rocks called stromatolites dating to around 3.7 billion years ago, says Visscher. Stromatolite mats are deposited over the eons by microbial ecosystems, with each layer holding clues about life at that time. There are contemporary examples of microbes that photosynthesize in the absence of oxygen using a variety of elements to complete the process, however it's unclear how this happened in the earliest life forms.

Theories as to how life's processes functioned in the absence of oxygen have mostly relied on hydrogen, sulfur, or iron as the elements that ferried electrons around to fulfill the metabolic needs of organisms.

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Read more: https://phys.org/news/2020-09-oxygen-earth-early-microbes-arsenic.html

[tblue37]

1. Fascinating. K&R. nt

[NNadir]

2. I really doubt this speculation will hold up. n/t.

[Warpy]

3. i find it a bit odd, also

Anaerobes that derive most of their energy from sulfur have been found to exist in high arsenic concentration water and even reduce the concentration of arsenic. That's probably what this is based on and somebody who knows as little about metabolic pathways as he does about respiration wrote a misleading headline.

It might be a feature in some species of anaerobes. Whether or not they've found that exact species fossilized in stromatolites is up for debate, as is whether or not high arsenic concentrations in shallow water where these bugs lived were high enough to support them.

[eppur_se_muova]

4. Headline should have included the phrase 'may have'. Interesting speculation, but hardly proven.

It is important to note that what is being proposed here is the use of arsenic as an 'electron carrier' -- which might involve interconversion of arsenate(V) and arsenate(III) for example (though this system is NOT specified in this very brief article. ETA: see below). This is completely different from the claim made several years ago that bacteria found in Death Valley were incorporating arsenic into their DNA, where it served only a structural role (very dubious, and since refuted) and not an electron-transfer role.

Oh, look, the full article is available online ! And ref. 23 in that article describes the finding of light-dependent As(III)-->As(V) conversion ! I can only access the abstract, but apparently workers in this field had previously isolated bacterial genes for As(V) reductases, so this is not *quite* the radical proposal it might first appear.

Very interesting stuff, and much more plausible than it appeared at first blush.

[Javaman]

5. really interesting. what I would like to know is

how did early life transition from arsenic to oxygen?