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A 1970s shag carpet underwater
Communities big enough to see
One microbe is typically one to ten microns wide, far narrower than a human hair, which is about 70 microns across. On the sea floor they sometimes hang out together in communities big enough to see. The best known are microbial mats, found at deep sea hydrothermal vents and natural gas seeps.
Where the yellow comes from
The yellow comes from sulfur. The microbes take in a sulfurous compound, breathing it as much as eating it, as one way to harness energy. The sulfur they produce ends up trapped inside or outside their cells.
Microbes that build chimneys
Some mats go further and deposit minerals, building chimney-like structures. A whole bunch of these chimneys turned up off Los Angeles in 2018. Ordinary vent chimneys grow from the metal dust coming out of a vent, and these were built by microbes.
How do microbes build chimneys?
At methane seeps, some microbes eat methane using sulfate from seawater. That reaction produces bicarbonate, which raises the alkalinity of the surrounding water. Calcium in seawater then combines with the bicarbonate and precipitates as calcium carbonate. Over time the mineral builds crusts and chimneys around the microbes.
How do microbes get energy from sulfur?
Sulfide-oxidizing bacteria, including the filament-forming Beggiatoa, combine hydrogen sulfide with oxygen or nitrate to release energy. The first step produces elemental sulfur, which many of them store as granules inside the cell. Those granules scatter light, which gives mats their white or yellow look. When sulfide runs short, the cells can burn the stored sulfur further.
Why do mats grow at vents and seeps?
Both vents and seeps push chemicals like sulfide and methane up from below the sea floor. Seawater above is rich in oxygen, so where the two meet there is a thin zone with fuel on one side and oxygen on the other. Microbes that can use both position themselves in that narrow gradient. A mat is the visible layer that forms where the zone reaches the sea floor.
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