Excerpts from Jim Conrad's
Naturalist Newsletter

BIOLOGICAL SOIL CRUST in dried-out salt marsh

from the May 10, 2015 Newsletter issued from Río Lagartos, on the north-central coast of Yucatán, MÉXICO

Here at the end of the dry season most ponds and lagoons that during the winter hosted interesting populations of herons, egrets, ducks, rails and the like, now are dry. After the water disappeared, for weeks a soft, sticky mud remained, but eventually the mud's exposed surface turned grayish and crusty, and cracked into brittle, irregular, leaflike patches that shriveled and curled at their edges, as shown above.

Ecologically, these brittle patches of leafy crust are important, for they are what's known to science as "biological soil crust," or "microbiotic crusts," or sometimes just "biocrust."

In southwestern Texas we looked at biological soil crust covering and protecting vast acreages of arid land {following entry}. At that time we learned that biological soil crusts are complex communities of interrelating and interdependent species of cyanobacteria, algae, fungi, lichens, mosses, liverworts, and many other kinds of microorganisms.

Here, when our marshs' cyanobacteria, algae, fungi, etc. dry out as the dry season gets underway, they form reproductive structures such as spores. When rains return, the propagules germinate into new organisms, most of which will photosynthesize and be eaten by tiny animals such as copepods, water-fleas and other zooplankton, which will be eaten by larger animals such as aquatic insect larvae and tiny fish, which will be eaten by the larger animals we like to watch.

Therefore, the leafy flakes of biocrust covering our dried-out marshes nowadays represent the base of an ecological pyramid that accomplishes its full expression in our very lively rainy-season marshes.

Though the world's forests often are thought of as serving as the main "carbon sinks" keeping carbon out of the atmosphere and thus diminishing problems with global warming, studies show that in areas where higher plants are scarce, as in our frequently inundated salt-marshes, biocrusts are a major component of carbon dioxide uptake by the whole ecosystem. On a global scale, in 2012, Wolfgang Elbert and others in a work published in Nature Geoscience, calculated that carbon assimilation by non-flowering ground covers contributes around 4.5% of that provided by all land vegetation.

Moreover, across the planet, non-flowering plants, or "cryptogams" such as the algae and cyanobacteria in our marsh biocrust, conduct almost half of the Earth's biological nitrogen fixation -- the conversion of atmospheric nitrogen, unusable to higher plants, into usable forms absolutely necessary for plant growth.

So, these crusty, scab-like patches that look so messy and accidental on the beds of our dried-out marshes deserve recognition and protection. Protection, because in many arid environments livestock, offroad vehicles and fires are destroying vast areas of biocrust.

from the February 9, 2014 Newsletter issued from the Frio Canyon Nature Education Center in the valley of the Dry Frio River in northern Uvalde County, southwestern Texas, on the southern border of the Edwards Plateau, USA

Biological Soil Crust on exposed roadcut face

The above picture shows a colony of Placidium Lichen on a vertical roadcut face. A kind of granular looking, crunchy feeling crust coats the roadcut's exposed dirt and small gravel. In many of the world's arid zones, this kind of humble-looking crust is the most important ecosystem. In general terms it's referred to as biological soil crust -- sometimes more technically as "cryptobiotic crust" -- and among its main ecological functions are to retard erosion, produce oxygen through photosynthesis, and contribute organic matter to an otherwise fairly sterile soil. Over vast stretches of desert, organisms in the biological soil crust are the main or only photosynthesizers.

Biological soil crusts are complex communities of interrelating and interdependent species of cyanobacteria, green algae, lichens, mosses, liverworts, and many kinds of microorganisms. The crust community creates a favorable environment for seed germination and for insects and other soil organisms to live in.

Crust organisms such as lichens and dried mosses can be killed even by relatively cool, fast-moving grass fires, but the cyanobacteria often survive, helping grasses, shrubs, and other crust organisms regenerate faster than they would on barren ground.

The bad news is that biological soil crusts are extremely fragile, being susceptible to crushing and trampling. Once damaged, it may take many years to return -- time during which several feet of sediment may be washed or blown away. Hikers and horseback riders destroy crusts when they leave trails through crust zones, but their destruction is little compared to that caused by grazing livestock, sand buggies and other offroad vehicles.

One of the most dispiriting features of the situation is that -- similar to the belief in this valley that Ashe Junipers are invasive and use so much water that they should be killed -- in many ranching areas biological soil crust is deliberately destroyed because it "competes" with grass desired by cattle, or "prevents" grass from growing.

from the October 11, 2015 Newsletter issued from Hacienda Chichen Resort beside Chichén Itzá Ruins, central Yucatán, México

Now during the rainy season we can see what happens to dried-out biocrust when puddles form. The organisms soak up water and become soft, even mushy, foamy or slimy, and form a floating scum like that shown below

BIOLOGICAL SOIL CRUST floating on water

The scum's different colors and textures confirm that it's made up of different kinds of organisms. Its surface is bubbly because many of the organisms forming the covering are photosynthesizing, generously spewing oxygen into the atmosphere.

I love the way the water in these scum-topped puddles is so clear, with the floating material forming dark shadows on the mud below, like cloud shadows on Earth. I bet that a microscope would show this waterlogged biocrust teeming with creeping, sailing, spiraling microorganisms.