Tiny helpers

A single tablespoon of garden soil contains billions of bacteria, according to the Encyclopaedia Britannica. That’s right: billions.

Furthermore, research conducted in the last two decades has revealed that bacteria inhabit every stratum of rock as deep as drills have penetrated. These hypersuccessful, single-celled beings are critically involved in all other life on earth: The number of E. coli in your gut exceeds the number of people who have ever lived. Science writers Dorion Sagan and Lynn Margulis note that bacteria account for 10 percent of the dry weight of a human body, while conservative estimates suggest that the total mass of the earth’s bacteria exceeds that of all other living matter combined.

As zoologist and geologist Stephen Jay Gould observed, the overwhelming dominance of bacteria on this planet completely contradicts any notion of evolutionary progress. Higher forms of life have been snuffed out while the bacteria endure.

But Sagan and Margulis point out that planetary ecology doesn’t depend on higher-level animals in any meaningful way. “All of the nutrients crucial to global life — oxygen, nitrogen, phosphorus, sulfur, carbon — return to a usable form through the intervention of microbes,” they write.

Gardeners, meanwhile, should enjoy a more intimate connection with bacteria than our brown-thumbed friends, since we witness microbes at work every day.

Perhaps the best known of these tiny garden helpers are the bacteria that enable plants to utilize atmospheric nitrogen. Every organism requires mineral nutrients to grow, and no mineral is more important than nitrogen. But while this element composes nearly 79 percent of the earth’s atmosphere, it’s in a form unavailable for chemical recombination. In the lingo of biochemists, the nitrogen needs to be “fixed,” or recombined into ammonium or nitrate, before it is useful to plants or animals.

Certain soil bacteria are very good at fixing nitrogen. When a thoughtful grower introduces a powdered inoculant into garden soil to boost nitrogen capture, the powder consists of dormant bacteria that stir to life in moist soil.

Legumes (the plant family that includes beans, peas and clover) are particularly adept at absorbing the airborne nutrient — but only with the help of nitrogen-fixing bacteria. Observant gardeners are familiar with the nodules that form on the roots of legumes, and it’s this symbiotic growth that makes these plants so useful for soil-building. By growing such crops in inoculated soil and then tilling the plant waste back into the ground, growers can markedly increase soil fertility.

In another phase of the nitrogen cycle, those of us who compost use bacteria to recycle garden waste. Bacteria and fungi degrade organic material, releasing fixed nitrogen for reuse by other organisms. Of course, the bacteria don’t require a compost bin to disassemble organic matter: The essential difference in a structured compost pile is that the freed-up nitrogen is less likely to return to the atmosphere.

Those with garden ponds also benefit from bacteria that break down waterborne nutrients (including fish waste) to make it available for lilies and other ornamentals. And given that bacteria are so good at breaking down organic compounds, it should come as no surprise that they’re also used to clean up oil spills.

Bacteria form symbiotic relationships with a multitude of other creatures. Symbiosis is a co-dependent relationship in which each participant both benefits and confers benefit. For example, we couldn’t digest the veggies we grow without the help of bacteria in our gut. Grazing animals depend on bacteria in their multiple stomachs to digest grasses.

An interesting sidelight to this digestive function is that about 30 percent of the methane in the atmosphere results from microbial action in animals’ digestive tracts. This prompted Ronald Reagan’s dismissive comment that humans couldn’t be held accountable for global-warming gases (of which methane is the most potent), because the most significant source is bovine flatulence.

As contemporary critics noted, however, Reagan overlooked the fact that animal husbandry has vastly increased the number of cattle, making cow farts very much a human-influenced commodity.

Another fascinating, if grotesque, microbial function involves beneficial nematodes (see “Becoming Biorational,” Aug. 9 Xpress). Nematodes are extremely tiny worms, some of which prey on certain insect pests. At least one variety of nematode enters its prey through the mouth, anus or spiracle (breathing organ) and moves into the insect’s blood cavity. There it ejects millions of bacterial symbionts from its intestine into the surrounding blood. Although the bacteria are harmless to the nematode, they kill the insect within hours.

Unable to gain entry to the insect’s blood cavity on their own, however, the bacteria depend on the nematode in order to launch an invasion. At the same time, the nematode releases an antibiotic that kills competing bacteria without harming its helpers. The bacteria also render the dead insect bioluminescent, and the glow invites other nematodes to join in the feast, ingesting many of the bacteria (which they then carry to other insects).

In a more intrinsic way, bacteria long ago formed an essential part of every plant in our gardens. The mitochondrion — the “energy factory” of all cells — started out as a symbiont that moved inside single-celled organisms. So did the chloroplast, which performs photosynthesis. While these bacteria benefited from the protection of the host’s cell wall, the host gained the ability to use sunlight to convert nutrients into sugars, the building blocks of life. Over eons, the bacterial DNA migrated to the cell nucleus and finally became an intrinsic part of what we know as green algae and plants. Thus bacteria made all higher life forms possible by impounding the solar energy needed to nurture food crops that sustain rabbits, gazelle, aphids, cheetahs, elephants, kangaroos, caterpillars — and gardeners.

The garden adage “Feed the soil, not the plant” means we’re now reciprocating by feeding the multiple billions of bacteria that have made our landscapes their homes. So does that make them pets?