Every grain of sand is a met­ro­polis for bac­teria

A single sand grain harbours up to 100,000 microorganisms from thousands of species.

Just ima­gine, you are sit­ting on a sunny beach, con­ten­tedly let­ting the warm sand trickle through your fin­gers. Mil­lions of sand grains. What you prob­ably can't ima­gine: at the same time, bil­lions upon bil­lions of bac­teria are also trick­ling through your fin­gers. Between 10,000 and 100,000 mi­croor­gan­isms live on each single grain of sand, as re­vealed in a study by researchers from the Max Planck Institute for Marine Microbiology in Bremen. This means that an in­di­vidual grain of sand can have twice as many res­id­ents as, say, the city of Fairb­anks, Alaska!

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View of a sand grain under a fluorescence microscope: The green spots are stained bacteria, which have mainly colonized depressions on the grain.
View of a sand grain under a fluorescence microscope: The green spots are stained bacteria, which have mainly colonized depressions on the grain.

It has long been known that sand is a densely pop­u­lated and act­ive hab­itat. Now David Probandt and his col­leagues have de­scribed the mi­cro­bial com­munity on a single grain of sand us­ing mod­ern mo­lecu­lar meth­ods. To do this, they used samples taken from the south­ern North Sea, near the is­land of Hel­go­land, off the Ger­man coast.

The bac­teria do not col­on­ize the sand grains uni­formly. While ex­posed areas are prac­tic­ally un­col­on­ized, the bac­teria bustle in cracks and de­pres­sions. “They are well pro­tec­ted there”, ex­plains Probandt. < “When wa­ter flows around the grains of sand and they are swirled around, rub­bing against each other, the bac­teria are safe within these de­pres­sions.” These sites may also act as hid­ing grounds from pred­at­ors, who comb the sur­face of the sand grains in search of food.

Impressive diversity

However, the di­versity of the bac­teria, and not just their num­bers, is im­press­ive. “We found thou­sands of dif­fer­ent spe­cies of bac­teria on each in­di­vidual grain of sand”, says Probandt.

Some bac­teria spe­cies and groups can be found on all in­vest­ig­ated sand grains, oth­ers only here and there. “More than half of the in­hab­it­ants on all grains are the same. We as­sume that this core com­munity on all sand grains dis­plays a sim­ilar func­tion”, ex­plains Probandt. “In prin­ciple, each grain has the same fun­da­mental pop­u­la­tion and in­fra­struc­ture.” We can there­fore really dis­cover a great deal about the bac­terial di­versity of sand in gen­eral from in­vest­ig­at­ing a single grain of sand.

Sandy coasts are enormous filters

Sand-dwell­ing bac­teria play an im­port­ant role in the mar­ine eco­sys­tem and global ma­ter­ial cycles. Be­cause these bac­teria pro­cess, for ex­ample, car­bon and ni­tro­gen com­pounds from sea­wa­ter and flu­vial in­flows, the sand acts as an enorm­ous puri­fy­ing fil­ter. Much of what is flushed into the seabed by sea­wa­ter does­n't come back out.

“Every grain of sand func­tions like a small bac­terial pantry”, ex­plains Probandt. They de­liver the ne­ces­sary sup­plies to keep the car­bon, ni­tro­gen and sul­phur cycles run­ning. “Whatever the con­di­tions may be that the bac­terial com­munity on a grain of sand is ex­posed to – thanks to the great di­versity of the core com­munity there is al­ways someone to pro­cess the sub­stances from the sur­round­ing wa­ter.”

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