Kingdom Monera

The simplest of all living things are the bacteria cells. They are classified in a kingdom called Monera, derived from the Greek word moneres, meaning single. Other kingdoms have single-celled organisms, too. But bacteria cells are unique mainly because they have no nucleus; in other words no nuclear membrane surrounds the chemicals containing their hereditary information.

Though individual bacteria cells are microscopic, you can still detect them with unaided eyes if you know the way their colonies manifest themselves. Some stain sediments and release malodorous gases. Others create colorful slimes, films or mats.

A film of Beggiatoa bacteria producing food at New Castle

They may not appear as lovely as other “higher” creatures, but their activities are interesting, not to mention essential to the life in the estuary.

The Decomposers
[aerobic & anaerobic]
So many organisms — countless plants, animals, algae, fungi and microbes — are continually growing and dying throughout the estuary. Somehow all of their remains must be dealt with. They must eventually be recycled. Essential nutrients like carbon, sulfur and phosphorus, which future generations of living things need to stay alive, are locked inside of them. Without any way to liberate these nutrients and convert them to the elementary forms that plants and algae can absorb, new food production in the estuary would grind to a halt.

The magnitude of plant material alone is immense. Only about 10 percent of tidal marsh plants, for example, are eaten while they are alive. Their nutritional value is realized only after they start decomposing, since their tough structural fibers are indigestible to most animals. Animals that feed on decomposing organic matter, like zooplankton or tiny invertebrates, consume only particle-size portions.

From this perspective, the work of decomposer bacteria starts to have more appeal.

Once organic materials are eroded by water currents, shredded by grazing animals, or fragmented as partially digested food material, colonies of various decomposers move in to finish off deconstruction. How do these little cells accomplish what big animals can’t? They use enzymes on their cell surfaces to break down solid organic matter into compounds, which they can absorb and digest. These enzymes can take on the tough cellulose walls of myriad plant cells, breaking them into simple carbohydrates. Proteins are broken into amino acids. Fats are broken into glycerol and fatty acids.

Decomposers attract a diverse community of various phytoplankton, zooplankton and decomposer fungi. The fungi also feed on the organic solids, while the zooplankton eat the released particles or other members of the community. This assortment of the living and the nonliving forms a nutritious mixture called detritus. Eventually they are all consumed by little detritivores like shrimp, amphipods and worms. The bacteria themselves are high enough in protein to be considered the most nutritious ingredient of this detrital soup.

The heavier decomposing organic materials drop to the bottom of the water column and end up within the sediments. You have probably been aware of their decomposition if you have ever smelled a rotten egg aroma near the tidal flats and the marshes at low tide. This is hydrogen sulfide, a by-product of the decomposition process in sediments with little or no oxygen. To live in such an environment decomposer bacteria must have anaerobic metabolisms to digest the organic material. In the same way we breathe oxygen and exhale carbon dioxide, anaerobic decomposers inhale sulfate and exhale this hydrogen sulfide.

Even within dense sediments there is no shortage of sulfate. It is a common element in seawater. A constant supply is also released by another group of anaerobic decomposers, the fermenting bacteria, as they deconstruct sulfur-containing proteins. These fermenters can break materials down only to a certain point. Then the sulfate breathers can come in and finish off decomposition.

The reason mudflats have a stronger low tide smell than sand flats is because mud is more accommodating to anaerobic decomposition. The finer particles trap more organic matter, they provide more surface areas for bacteria to cling to, and they tend to prevent oxygenated water from percolating through.

Black sediments indicate presence of hydrogen sulfide

The surface of a mud flat looks grayish or tan where water is able to penetrate it, replenishing oxygen faster than oxygen-breathing decomposers can consume it. When you dig down, sometimes even a few millimeters, the sediments are all black. This is where the anaerobic bacteria are at work. The resulting hydrogen sulfide reacts with certain minerals (black ferrous sulfide and gray pyrite) in the sediments, leaving the mud stained black. The very top of this black layer indicates where the anaerobic activity starts.

Though hydrogen sulfide is poisonous to most plants and animals, it directly benefits some food producer bacteria.

Food Producers
[photosynthetic & chemosynthetic]
Whereas photosynthetic plants use hydrogen molecules from water to produce food, some food producer bacteria use hydrogen molecules from the hydrogen sulfide to make their food.

A colorful, summertime community of food producers growing at the edge of a saltmarsh panne by Chapman's Landing in Stratham, NH. Green cyanobacteria grows over purple sulfurs. The white film around the edges is a Beggiatoa bacteria.

In the summertime look for purple sulfur bacteria creating films on sediments or clouds in stagnant pools, which are a pinkish or purplish color. Their red pigments absorb the sun’s energy, which drives photosynthetic food production.

Some bacteria from a group called Beggiatoa use another energy source for their food production. They release chemical energy by breaking the bonds of the hydrogen sulfide. This is a kind of chemosynthesis, since the food production is driven by chemical energy.

These Beggiatoa appear as white film, scum or dust on black sediments, rotting seaweed and other places rich in hydrogen sulfide. They stay on or near the surface for the oxygen, rather than the sunlight.

Cyanobacteria have green, yellow and blue photosynthetic pigments to absorb the sun’s energy. They use the hydrogen from water to fix carbon from carbon dioxide into sugars. As a result they release lots of leftover oxygen to their surroundings. They are also able to convert nitrogen gas to ammonia, a mineral form of nitrogen that plants must have to construct proteins.

The colorful pigments of some kinds of cyanobacteria are obvious to the naked eye. Delicate chains of Lyngbya form super-fine gossamer threads, which weave into thin microbial mats. They might have bubbles on their surface from all the oxygen produced through photosynthesis. Below them there may be layers of other bacteria colonies, like the purple sulfurs, thriving the presence of hydrogen sulfide.

What appear to be patches of tar coating the surfaces of intertidal rocks are actually Calothrix cyanobacteria colonies. To prevent dehydration at low tide they are encased in a jelly-like mass, which makes the rocks treacherous to walk on when wet. The blue-green of the pigments can be seen by scraping off part of the colony and observing it under a microscope strong enough to magnify cells.

Black film of cyanobacteria on intertidal rocks.

Other cyanobacteria float in the water column as part of the phytoplankton. Their colors are also invisible to the naked eye.

All of these photosynthetic and chemosynthetic bacteria are important food sources for many invertebrates, including protozoa and meiofauna living between the grains of sediment; mud eating worms and crustaceans; filter-feeding tunicates and bivalves; and rock-scraping snails and limpets.

For an interesting, easygoing book on discovering bacteria by identifying their field marks all around us, check out A Field Guide to Bacteria, by Betsey Dexter Dyer.

Copyright © 2006 Barbara Driscoll.