This post is a follow-up to this one on the discovery of a bacterial phylum 20 metres underground, and about the significance of this.

Fair warning: this post will be full of generalisation and speculation, for the simple reason that a) not much is actually known about this phylum (all that is known is deduced from genetic analysis of DNA extracted from the soil at that depth), and b) I don’t know the environment of that soil (pH, temperature, composition, permeability, that kind of thing).

One very noteworthy point is that there are bacteria that deep at all — "life” and “twenty metres down” are not two concepts most people consider as coexisting.

It is also important to know that many substances are carried by water down a soil profile into groundwater which can be very, very deep. (This site states that groundwater can be “thousands of feet below the surface”. Two thousand feet is almost 610 metres.) These substances can be anything — simple nutrients such as calcium and nitrate; manmade chemicals such as herbicides, other pesticides and fertilisers; or heavy metals and other industrial waste such as oils and microplastics.

How many of these reach groundwater depends on both substance amounts and the properties of the soil profile they travel through. For example, many substances will adhere to clay particles or organic matter en route down the profile. But this is complicated by soil temperature and/or pH along the way possibly altering some substances chemically, which may then affect the amount of adsorption positively or negatively.

Some substances — even pesticides and industrial waste — are broken down by microbes, but again soil pH and/or temperature may dictate to what extent this occurs.

Soil temperature and/or pH may degrade a substance outright into other substances, making them more, or less, adsorbable or biodegradable.

It’s pretty complicated stuff, and any substance passing through a soil profile not filtered out beforehand, so to speak, by physical, chemical, or biological means, is likely to end up in, and contaminate to some degree, groundwater.

Which brings us back to this new phylum.

What is interesting is that this phylum so deep down is known to be active — again, many people would not realise just how inactive microbes can be in situ. Many microbes actually exist most of the time in a dormant state, and often as hardy, super-tough long-lasting spores.

All those images and (sped-up) videos of rapidly growing bacteria “everyone” is familiar with are highly deceiving — those are of medically-important bacteria growing in ideal conditions of warmth, food and oxygen in a laboratory. If they really grew like that “in the wild” we’d be overwhelmed by the sheer numbers and bulk of them.

I couldn’t find any temperature data on the soil the CSP1-3 phylum inhabits, but this 2003 study measured temperature in a borehole in Nicosia, Cyprus, and found it to be a consistent 22.5 °C between 15 and 50 metres in late December (their winter). If soil temperatures globally also don’t fluctuate at these depths, then this temperature may be indicative of what the new phylum experiences too.

This also is worth noting, as this makes them psychrophiles: microbes able to metabolise, grow, and reproduce at low temperatures. (Some psychrophiles can do these at minus 20 °C!)

While +20 °C doesn’t sound particularly cold, bear in mind that we, for example, maintain an internal temperature of 37 °C. We’d die should our core temperature drop to 20 °C, as enzyme activity would either cease to function outright, or be so slow as to not sustain life, as our enzymes work optimally at (surprise!) 37 °C.

Enzymes are needed for metabolism, growth, and reproduction, and are very much temperature-dependent to function. This is why foodstuffs tend to keep longer in the fridge — the enzyme activity of most food-spoiling microbes has slowed if not stopped altogether at 4 °C.

One notable exception is when refrigerated meat spoils — the bacteria in this case are also psychrophiles with enzymes enabling them to grow, metabolise and reproduce at 4 °C. This reproduction and growth manifests as a distinctive visible slime, and the by-products of their metabolism manifest as a distinctive odour.

So, metabolically active bacteria at cool temperatures does signal the possibility of their ability to metabolise and thus minimise substances entering groundwater.

Also worth noting is that there is every chance that there is little to no oxygen at this depth, making these bacteria either anaerobes (unable to survive in the presence of oxygen) or facultative anaerobes (tolerating the presence of oxygen but not using it).

This in itself isn’t amazing, as our gut microbes are also anaerobes or facultative anaerobes. (Anaerobes can also be found in the mouth of all places, protected by plaque from all that oxygen!) But what does make this interesting is that anaerobic environments make for some pretty interesting metabolic pathways — fermentation is but one such example.

Fermentation has economic significance to us for without it there’d be no cheese, wine or beer, but many other anaerobic pathways have far more important environmental significance. Exploring the chemistry here is just too involved and complex for one post, but please take as read that anaerobic pathways are very important in the recycling of carbon, nitrogen, iron and sulfur through the biosphere, lithosphere, hydrosphere and atmosphere.

Some anaerobic pathways also make some heavy metals less bioavailable, and have huge potential in the bioremediation of contaminated industrial sites. Some anaerobic bacteria can even metabolise ionic mercury and cadmium (both highly toxic) and radioactive uranium and vanadium to less dangerous forms.

So, anaerobic bacteria at depths where potential pollutants may exist also signals the possibility of their ability to minimise substances entering groundwater.

Thus to summarise the significance of deep-soil bacteria, whether this phylum or any other, is this:

• they are found where potential groundwater contaminants can be found;
• they are known to be active, meaning that they are metabolising, growing and reproducing at those depths;
• they are likely to be psychrophiles, with enzymes functional at temperatures lower than would normally denature potential contaminants chemically
• they are likely to be anaerobes, or facultative anaerobes, and thus likely to have biochemical pathways able to metabolise potential contaminants aerobic organisms cannot

And. going out on a complete limb here, there is a good chance that the reason this phylum is metabolically active at all is because these ever-present potential contaminants are proving to be a rich food source for them. Who knows, maybe a century ago they too were mostly dormant like any other soil microbe, and it’s only the modern industrial age which has changed their environmental response?

Now this would be wonderful beyond belief if so, for it would show that nature is far more resilient than we give credit for, and able to rise to any challenge through the power of microbes!