The exchange of electrons in sedimentary bacteria has been shown previously via nanowires. This has been shown to control the metabolisms of the prokaryotes via dumping of electrons to metals. However Pfeffer et al (2012) have discovered a novel species which couples two reactions from different sections of the sediment.
Oxygen reduction happens within the oxic (surface oxygen rich sediment) layer of the sediments, whereas sulphide oxidation happens in the sub-oxic (deeper oxygen deficient sediment) layer. Recently it has been discovered that electrical currents connect the two layers of sediments and that around half the sedimentary oxygen reduction is fuelled by electrons transported from deeper within the sediment.
In this experiment sediment samples were taken from Denmark, profiled and then incubated for three to four weeks before the top 20 mm of sediment was extracted and washed gently. This revealed a lattice of highly filamentous bacteria which were identified using 16s rRNA sequencing to be Desulfobulbaceae. Subsequent experiments were done using FISH and it was identified that these bacteria lived within the oxic and sub-oxic layers of sediment and fragments up to 1.5 cm were dissected out.
They then proved that it was the filamentous bacteria that were facilitating the exchange of electrons through the sediment using cores which included filters. Electrical conductivity was observed with a pore size of 2µm, which allowed migration or vertical growth of bacteria. However smaller pore sizes (≤0.8µm) didn’t show any electron transport between layers.
Interestingly as a control for sedimentary particles, a core was incubated with glass microspheres instead of a 5-mm sediment layer, and the filamentous desulfobulbaceae could be seen (using FISH) to be growing in the microsphere layer.
The structure of the Desulfobulbaceae was then studied using a combination of SEM, TEM, AFM and EFM microscopy. The bacteria showed uniformed ridges running the length of the cell which were encased in a 'collective' outer membrane, Ie. A membrane which surrounded individual cells and also bridged the gaps between bacteria to surround adjacent cells. This is hypothesized to act as an insulator for the conductive structures that run down the ridges of the bacteria. The authors propose that these bacteria are both oxiding sulphide in the deeper filaments and reducing oxygen in the upper filaments by acting as micro-cables capable of transferring electrical charge through the oxic and suboxic layers of sediments.
I chose to review this paper as it shows a novel bacteria exhibiting a relatively new process of 'electron transfer' in the sediments, however it has not previously been shown that the filamentous bacteria can act as living micro cables spanning two distinct layers of the sediment allowing for the exploitation of two major energy sources.
Hi Myles, I enjoyed your post. However I am not sure I understand a few points.
ReplyDeleteFirstly, do these micro wires transport electrons from one cell to another of the same species, or is it of the same cell or even between different species? If so are these wires protrusions of the cell, like sex pilli, or are the specialised differentiated cells (like heterocysts, endospores etc)?
Secondly, the membrane which surrounds multiple cells, is this secreted by a specialised cell or by all the participating cells?
And lastly, what exactly is the purpose of electrical transfer from oxic to sub-oxic? Is it just for respiration or can the novel bacteria communicate through this in any way? (maybe an adaption of quorum sensing?)
Thanks in advance, Harri
Hey Harri, sorry for not making it clear!
ReplyDeleteThe transport of electrons is between bacteria of the same species. The electrons are being transferred up the grooves on the filamentous bacteria (the grooves are similar to a xylem in a plant the outer membrane being the lignin) not across pili (although in other species of bacterium this has been shown!)
The authors do not mention where this outer membrane comes from, but I would suggest that as the bacteria grow the outer membrane is expanded as part of normal cell division, as there are no specialised cells in the filaments that this paper studied.
The advantage of the electrical transfer is hypothesized that, for example, compared to other sulphide oxidising bacteria in the sub-oxic sediment, these Desulfobulbaceae can use spare electrons from this process to fuel oxygen reduction further up in the sediment, meaning it can utilise two energy sources and lead to faster growth than bacteria which can only use one source.
hope that answers you questions, Myles
Thanks Myles, that's a very interesting concept! So if the electrons are going down through the sediment to the sub-oxic, donated by the oxic, do the sub-oxic cells do anything for the oxic in return? This seems like a very one sided agreement.
ReplyDeleteHarri
The bacteria in the suboxic layer are providing the electrons for the bacteria in the upper layers, not the other way around! As these bacteria are all the same species, perhaps they are also sharing resources through these hollow tubes (like permanent pili)? The authors mention that the outer membrane which encompasses all the cells within the filament could be to reduce leakage, similar in function to the structure of cyanobacteria that Mariscal in 2007 (Continuous periplasm in a filamentous, heterocyst-forming cyanobacterium) who also hypothesized the reduction of leakiness in the exchange of nutrition. Perhaps the bacteria in the sediments also share nutrients?
ReplyDeleteIf they did it would certainly help in times of nutrient stress of oxygen/sulphide. Also if spare metabolites are passed down through the sediment along the filaments, this strategy would help the colony to out compete bacteria within both sections of the sediment.
Myles