Last month NASA published evidence for ancient life on Mars. If true, it would have been like the bacteria on Earth which get their energy from iron chemistry without needing oxygen from the air. It felt strangely close to home as iron bacteria on Earth are present in oxygen-starved seasonal ponds and streams, and become particularly visible this time of year, including in Century Wood.
The Perseverance Rover has been on the red planet since 2020. It has found some of the iron compounds which on earth would normally be the result of bacteria that obtain their energy from reactions of iron ions. They’ve now found minerals present which are incompatible with the high temperature processes which are otherwise know to form these compounds without the involvement of life back on Earth. So it’s difficult to explain how the iron compounds formed without Martian life.
The iron reactions NASA is talking about involve the oxidation of organic compounds and the reduction (“de-oxidation”) of iron ions, and result in iron sulphide and iron phosphate. However, the ones I see in Century Wood are probably the opposite type of iron bacteria: ones that use the oxidation of iron compounds into iron oxide, that is, rust. In the most common case, the oxygen atoms come from splitting the water molecules (H2O) and the reactions happen in stagnant water without oxygen gas dissolved in it.
In the first photo you can see a small pond in part of one of our old boundary ditches and the bed of it is covered in rust-orange iron oxide deposits from these bacteria. The second photo is in a seasonal pond that appears when the water table rises in winter. There are no signs of iron oxide but instead there is an oily sheen on the water. It looks like petrol but it’s actually solid “mats” floating on the surface and you can break them into small pieces with a stick. As it’s a liquid, petrol would just flow round the stick and reform a solid layer without gaps. You can see the woodland floor here is covered in dead leaves, and so when the seasonal pond appears, the rotting vegetation takes up all the dissolved oxygen.
As well as being the result of fascinating natural processes, these signs are useful markers of low-oxygen stagnant water and low-pH acidic soils.