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Beaches are canvases for fine-grain sand art

Are they ink blots from a Rorschach test, satellite maps or abstract drawings by an artist sketching in charcoal? When the tide goes out, black streaks, lines, squiggles, and shapes drawn on the otherwise-neutral canvas of sand grains readily catch both my eye and imagination. Sand on a beach may look about the same from day to day but the grains are constantly on the move. Thousands of breaking waves per day translate to sand grains being pounded, stirred, and shifted, such that layer by top layer of wet, hard-packed sand is eventually pulled to the surface. Meanings of the designs revealed from the black deposits are up to my imagination, but in reality, the artists of these one-of-a-kind ephemeral creations are specialized bacteria that process sulfur. I’ll back up a minute here and explain.

Tan sand is composed mainly of quartz and feldspar. The watery cushion surrounding each sand grain, abundance of light and plentiful oxygen support a plethora of lifelike worms, crabs and snails. However, dig a toe down several inches below the surface to find a dark underworld. Though close to the surface, sand density is such that light and oxygen levels fall off quickly. Nevertheless, sand on our beaches is rich in organic matter that comes from kelp and other seaweed that regularly wash onto the beach. Numerous outfall locations also discharge sewage onto the sand. The sulfur particles and compounds accumulate in the sand, where bacteria with specialized capabilities process the sulfur for their own energy needs.

Waste from this processing comes in the form of hydrogen sulfide, a colorless, poisonous, flammable gas with the characteristic odor of rotten eggs. Some of this gas is released into the atmosphere (that’s the distinctive “smells-like-the-ocean” scent) but some reacts with metal ions, a common presence in the water, to produce metal sulfide particles, which do not dissolve in water. The metal sulfides account for the dark brown or black sand-drawn sketches brought to the surface, courtesy of ocean motion.

But this explanation does not speak to why metal sulfides concentrate as they do instead of appearing as evenly layered sheets across the sand plain. The answer lays in the sand environment itself, which is not homogeneous, and the nature of the microbes, which tolerate only the strictest of conditions. As such, the sulfur-eating bacteria concentrate in microenvironments in the sand, as opposed to evenly populating across a specific depth range in the sand.

Our planet has evolved many routes to keep the finite supply of elements (like sulfur) cycling throughout Earth’s living and nonliving environments. In the case of sulfur, it has been proposed that rock deposits of elemental sulfur found worldwide are derived from the actions of ancient microorganisms. It is certainly true that in today’s world, microbes are mostly responsible for cycling and recycling Earth’s finite elements. In doing so, they not only directly perpetuate their populations but also secure their long-term existence by keeping Earth’s systems in balance. We are finally catching on to what microbes have always practiced: Give to and take from the environment in equal measure, and always recycle. Imagine that.

— Judith Lea Garfield, naturalist and underwater photographer, has authored two natural history books about the underwater park off La Jolla Cove and La Jolla Shores. Send comments to