Earlier this month, I talked about my research into sediment chemistry and the role of sediment microbes in the cycling of carbon. A key part of my research provided additional validation into the original model of sediment electron transfer pioneered by Robert Aller and others in the 60’s and 70’s.
A bit of background: organisms make energy for themselves by passing energetic electrons in sugars down a protein chain, releasing parcels of energy along the way. To picture this, think of a bucket of water being passed along a line of gardeners, each using some of the water for a small plant in front of them. At the end of the line, the ‘used up’ electron (an empty bucket) is dumped onto an electron acceptor – the chemical ‘bin’. In ‘higher’ organisms, that chemical bin is oxygen – we’re aerobic.
Different kinds of bacteria can also use nitrates, certain metal ions and sulfates in this role, instead of oxygen. However, oxygen is the most energy efficient, while the alternatives give less energy – picture moving the chemical bin from the end of the line of gardeners to halfway along the line. When the bucket reaches the bin and is thrown away, there’s unused water left in it – a potential resource that couldn’t be tapped.
Because the bacteria who don’t use oxygen get less energy from their electron pathway, they can’t compete with the aerobic bacteria. So, the bacteria live in layers, starting with oxygen-users at the top and others in order of their energy efficiency deeper in the sediment, where oxygen isn’t available. Picture a set of filters, with the oxygen getting caught in the top filter, then nitrate, then metals, and finally sulfate – at each layer, there’s hungry bacteria using the chemicals stuck in the filter to dump electrons on.
As explained in my other blog post, my experimental results backed up this idea, showing how the simple framework holds fairly true even in complex sediments. The patterns aren’t always layers – it can be a veritable mosaic of lines and dots – but the general principle holds true.
However, a recent study has provided a glimpse of evidence that some microbial communities may be capable of bypassing such spatial restraints by the use of nanowires, which transport the ‘waste’ electrons to the surface, where oxygen is available as an electron dump. It’s like being able to water your garden with the metaphorical bucket of water, then throw the bucket over your neighbour’s fence for them to deal with. Neat trick if you can pull it off! The results are only suggestive, so far – they haven’t been able to observe the nanowires in real sediments, so they can’t confirm if their description of what’s taking place is accurate.
If it is, it’s pretty remarkable! We rely on cables to transfer electricity and all kinds of signals, and to think that populations of single-celled bacteria have beaten us to the punch… I wonder if they ever made the transition to Wi-Fi?