Back on my other site I wrote about making your own cactus/succulent mix, which isn’t relevant here. But in it, I very briefly mentioned the lithotrophs, which are.

By way of background, I wrote there on the use of sand over poultry grit:

Coarse Sand or Poultry Grit

Either is just as good as the other, so long as it is coarse. You want largish, rough particles to encourage good drainage of water. I use poultry grit as it is more convenient for me, but the soil microbiologist in me also thinks that the minerals in that grit may also be being made bio-available by the microbes present in the compost (specifically by the lithotrophs).

So what are microbial lithotrophs? They are the ‘rock eaters’, and the name comes from the Ancient Greek λίθος, líthos, ‘stone’; and τροφή, trophḗ, ‘nourishment’: ‘nourishment from stone’.

Microbial lithotrophs are relevant here and in any other plant/soil-related subject, as by “eating rocks” they help form soil and make plant nutrients such as iron, nitrate and sulfur available at all. It’s worth knowing about these under-appreciated and quite frankly unknown microbes!

Let’s backtrack a bit.

All life requires a flow of electrons. Biochemistry — the chemistry of life — drives that flow of electrons within an organism. This flow of electrons makes protons (H⁺ or hydrogen ions) available, which in turn drive energy production by that organism. That energy is stored as potential energy in a molecule called adenosine triphosphate, or ATP. ATP is used by literally every cell on the planet from the smallest microbe to the tallest tree to drive other biochemical processes.

Electrons come from electron donors, and all life can be split into two groups based on their electron donor.

Many bacteria and archaea, and all fungi, protozoa and animals including us are organotrophs. This word comes from the combining word organo-, ‘organic’ ; and τροφή, trophḗ, ‘nourishment’: ‘nourishment from organic (materials)’.

Our electron donors are organic, meaning they contain carbon — lipids, carbohydrates, and proteins. (And yes, these are also our carbon sources, making us heterotrophs as well.)

Everything else — the rest of the bacteria and archaea, as well as all algae and plants — are lithotrophs. Yes, plants are ‘rock eaters’ too! They, and algae, are classified as lithotrophs as their electron source is water (H₂O), which is inorganic (does not contain carbon).

But I want to focus on the microbial lithotrophs, which are more literally ‘eaters of rock’ than plants. Only microbes are able to utilise the electrons in minerals.

Rocks are aggregates of minerals, though some rocks such as limestone are mostly if not fully comprised of a single mineral [calcium carbonate (CaCO₃) in this case]. Common minerals found in rocks include the feldspars comprised of aluminium-silicon-oxygen (Al-Si-O) complexes; quartz comprised of silicon dioxide (SiO₂); and granite comprised of quartz and feldspars. Many beneficial elements such as calcium (Ca), potassium (K), sodium (Na), magnesium (Mg), manganese (Mn), and copper (Cu) are found in mineral form. Iron pyrites (FeS₂) is a mineral of both iron (Fe) and sulfur (S).

Elements in mineral form are inaccessible to plants and non-lithotrophic soil organisms, and thus of no value to them in that state.
However, elements in mineral form are very much accessible to microbial lithotrophs.

To describe the different metabolic processes is quite involved, but very simplistically, microbial lithotrophs ‘eat’ rocks by oxidising the minerals in them.
‘Oxidising’ suggests burning something in the presence of oxygen, but in chemistry, ‘oxidation’ merely means to lose electrons in a chemical reaction. And often oxygen isn’t even involved! Those ‘lost’ electrons are ‘received’ by the other substance in the reaction. (And chemists will say that that receiving substance is ‘reduced’. Oxidation and reduction always occur together, and these reactions are called redox reactions.)

Also very simplistically, when a microbial lithotroph oxidises a compound, elements and smaller molecules within that compound lose electrons to the lithotroph, which receives and uses them in biochemical pathways. The elements and small molecules, having lost electrons, become ions and thus bioavailable to plants and non-lithotrophs.

Going back to the poultry grit — the pinkish and tan-coloured pieces in the image below:

These are very much inorganic mineral particles, and any lithotrophs present in the compost component of this potting mix will almost undoubtedly avail themselves of the electrons therein, releasing calcium and other ions into the mix.

Thus I do feel that adding grit has a slight advantage over coarse sand, as while both aid in drainage, only the grit contains nutrients of value to the succulents. I say ’slight’ as this would be a slow-release operation on the part of the lithotrophs, and most people would fertilise their plants via other means anyway. I certainly wouldn’t push the use of grit over sand, and truth be told, it just makes me feel better as a soil microbiologist more than anything else, to create an environment somewhat reminiscent of a natural one. (But I will secretly smile should others feel the same way and do so too!)

Microbial lithotrophs are simply amazing to me. They are the bridge between inorganic and organic, being not the sole, but a very important means nonetheless, by which inorganic nutrients such as potassium, magnesium, sulfur, phosphate, nitrate, and manganese are made bioavailable to enter organic systems at all. Lithotrophs are also an important contributor to the formation of soil from rock, and vital for the recycling of inorganic material through the ecosystem.

The rock-eaters don’t need us, but we and all other life simply would not exist were it not for them.