Is That a Martian Meteorite in Your Creek,
or Just A Mat of Iron Oxidizing Bacteria?

by
Larry Miller, Ph.D.
Oceanographer
U.S. Geological Survey

The western watersheds of the Santa Cruz Mountains are rich in varied and diverse habitats. Gazos Creek transects many miles of this terrain in its course from the mountains to the sea. The creek is influenced along the way by geological and biological processes and itself influences the surrounding environment; cutting through rock, transporting materials, nurturing plants and creating habitat. This article describes some observations about the smallest living things and the base of the food chain in Gazos Creek -- the bacteria.

Figure 1. Larry Miller points to mat of bacteria (rust-red color) in shallows of Gazos Creek.

Ponds and streams harbor an incredible diversity and number of microorganisms. A teaspoon of typical stream water may contain as many as 10 million bacteria. Organisms that rely on outside sources of organic nutrients for their supply of carbon and energy are called heterotrophs, and, like us, most bacteria are heterotrophic.

Microorganisms can use oxygen as we do for respiration or, when oxygen is absent, they can use other compounds (for example nitrate or sulfate dissolved in water) to help burn their food source to produce carbon dioxide and energy. Many bacteria use compounds other than oxygen to burn organic carbon. In fact, bacteria thrived for 1.5 billion years before the earth's atmosphere even had oxygen. It has been proposed that oxidized forms of the elements arsenic and iron may be evidence for the presence of life on Mars during its early history.

Bacteria are extremely opportunistic organisms; while it can be relatively easy to burn organic carbon using oxygen, there are many situations where a competitive advantage can occur by using an alternative source for oxidizing power. A competitive advantage allows some species to thrive in environments where they would otherwise be excluded or inhibited. One such example can be seen along the south fork of Gazos Creek where the footbridge crosses the creek. Here, along the bank of the creek, is an excellent example of both the diversity of microbial habitats within a single reach of the creek and the abundance of growth that can occur in a natural setting (Figure 1).

The source of carbon for heterotrophic activity in this environment is dissolved organic carbon in the creek water and in water seeping from the creek bank. This dissolved carbon was once living material, mostly in the form of plants and soil bacteria. If you look around yourself at this site in Gazos Creek, organic biomass (both living and dead) dominates your view.

Figure 2.

The bright orange material along the stream bank is a mat of bacteria that are specialized in the use of dissolved iron for energy. These bacteria are poised to take advantage of the transformation of the reduced form of iron (ferrous iron) to the oxidized form of iron (ferric iron) that occurs just where the water seeps out of the creek bank. It is here that soil water, which has had all of its oxygen removed by heterotrophic processes, re-encounters the oxidizing environment of the atmosphere (21% oxygen). It is not clear whether the bacteria facilitate iron oxidation or if the process is purely chemical, but in either case the bacteria are able to gain energy from the reaction.

One product of the reaction is iron oxide, more commonly known as rust, which accumulates in and around the bacteria and gives the mat its orange color. It is difficult to distinguish cells of bacteria from particles of iron oxide, even using a powerful microscope.

In order to tell the two apart, microbiologists use a fluorescent stain that interacts only with living material. By applying a focused source of ultraviolet light and using optical filters to select certain wave lengths of emitted light, we can see and then count the numbers of bacteria in a small sample on a slide (Figure 2). It is then a matter of scaling the counts to obtain the concentration of viable bacteria in the environment.
Growth of the mat persists as long as there is a supply of reduced (ferrous) iron and dissolved organic carbon. Interestingly, reduced iron may also be supplied by a bacterial process, iron reduction, associated with organic-rich layers in the sedimentary rocks of the Gazos Creek watershed. Because reduced iron is unstable in the presence of atmospheric oxygen, the bacteria position themselves exactly at what is called the oxic/anoxic interface. The bacterial mat is somewhat self-sustaining; while it is growing, the mat also consumes oxygen and therefore increases the extent of the oxic/anoxic interface, creating more habitat for itself.

The mat of iron bacteria could accumulate for some time, disappearing when the high flow associated with winter rains washes it away downstream. At that point, the mat breaks apart and becomes food for the creatures living in and near the creek. Some of the bacteria may find suitable habitat downstream and begin forming a new colony or mat of iron bacteria.

It is not known how the carbon or energy balances of Gazos Creek are influenced by microbial processes like iron oxidation and reduction and it will be useful and interesting the learn about the organisms that make up the base of the streams food web.