Biofilms in Nature
Biofilms are Everywhere
Biofilms are not exclusively laboratory phenomena, nor are they found only in habitats altered by human intervention such as polluted streams, ships hulls and urinary catheters. Biofilms also form in pristine natural habitats such as this alpine lake (Figure 1). In fact, for every bacterial cell found in the liquid phase of a stream or lake, there may be as many as 1000 to 10,000 cells attached to the surface of the rock substrate.
Biofilms are an important life-link in many natural communities. These organisms form the basis for food webs that nourish larger organisms such as insect larvae, which are consumed by fish, that are in turn consumed by birds like eagles. Biofilms attached to particles of contaminated soils and aquatic sediments can help degrade soil-bound contaminants that occur from accidental chemical releases into the environment.
Plants commonly have microbial populations associated with their external tissues. One beneficial type of plant-microbe relationship occurs in the rhizosphere between the plant roots, root hairs and a complex microbial community. Plant roots secrete significant amounts of sugars, amino acids, vitamins and plant hormones that serve as nutrients for microbes to grow on root hairs. This microbial growth may facilitate the plant's ability to absorb nutrients from the soil.
Biofilms Tolerate All Sorts of Environments
Biofilm communities are ubiquitous. They are found in every habitat in which water, nutrients, and a surface are found. From the frozen deserts of the Antarctic, to the depths of the ocean, and to the interstices of rock buried thousands of feet below the earth's surface, biofilms have been found in abundance. Biofilms grow in rain forests and in deserts, as "desert varnish." They have been found at the bottom of the ocean as early colonizers of new deep-sea vents and living on glaciers in the Antarctic.
Bacteria that live in very hot or very cold environments are called extremophiles. Yellowstone National Park in the United States is home to an amazing array of colorful communities of extremophiles. In fact, the entire globe—both above and below ground—is "seeded" with the bacteria that form biofilms, and bacterial communities flourish, disperse or become dormant depending on changing environmental conditions.
Estimates indicate that more than half of the earth's biomass is composed of biofilm. Imagine this: Greater than 98% of all bacteria are found in biofilms and more than 50% the earth's biomass is biofilm. This suggests that biofilms are the dominant communities on planet earth.
Biofilms Are Among Earth's Oldest Residents
The best estimates indicate that the earth is about 4.5 billion years old. This is not a number than can be determined directly from earth rocks. Age determination of several types of meteorites indicates ages for the solar system converging on that 4.5 billion year old figure. The oldest earth rocks are on the order of 3.8 billion years old and this is based upon radiometric data; it is worth noting that some of these rocks contain minerals that are even older.
The earliest evidence of organisms in the fossil record comes in the form of stromatolites, thought to be fossilized remnants of cyanobacterial biofilms (Figure 3) (See note). Stromatolites are composed of thin layers, thought to be produced by the upward growth of cyanobacteria living in microbial mats or photosynthetic biofilms. The oldest of these fossils are dated at 3.5 billion years old, once again based upon radiometric dating.
The modern analogs of these ancient stromatolites can be found today growing in shallow hyper-saline tide pools such as those at Shark’s Bay in western Australia and in other sites worldwide including Highborne Cay in the Bahamas. The latter is the only known site in which stromatolites are currently forming in waters of the open ocean rather than in hyper-saline tide pools. These laminated structures are formed by entrapment of sediment and by internal precipitation of calcarious materials due to the metabolic activities of the microbial consortium of photosynthetic cyanobacteria, and sulfate cycling anaerobic bacteria. Optical and electron microscopic investigations reveal a close similarity between the laminations of “living stromatolites” and those found as fossils hundreds of millions of years old.
For two billion years after they first appeared on Earth (3.5 billion years ago), photosynthetic biofilms produced biomass in prodigious amounts with oxygen as a by-product (Reid et al. 2003). During that early period the oxygen was almost instantly tied up in chemical reactions with the components of a highly reducing atmosphere and marine environment, including vast concentrations of ferrous (Fe+2) iron of marine hydrothermal vent origin. As oxygen was generated it combined with ferrous iron rising from the ocean depths and reacted to form insoluble hematite (Fe2O3) and magnetite (Fe3O4). The production of these oxidized iron minerals has traditionally been thought to be responsible for the origin of the so-called Banded Iron Formations (BIF) (Figure 4). Recently, the story of the BIF formations has become much more complex, and more interesting.
Banded Iron formations are precambrian sedimentary deposits of alternating layers of iron rich hematite and magnetite with layers of silica in the form of quartz or chert. Whether these oscillations represent seasonal changes or some other cyclical phenomenon is unclear. In any case, these deposits are phenomenally rich in iron (15-30%) and are the source of much of the earth’s commercial iron manufacturing industry.
Classically, the banding has been attributed to periods of oxygen abundance due to cyanobacterial photosynthesis during which sedimentation of iron rich material occurred alternating with periods of lower oxygen concentration during which silica was predominantly deposited. More recently a variety of other mechanisms, some microbial and some photochemical or physical have been proposed to account for the BIFs.
The snowball earth concept that, at one or more points, the earth has been covered with ice to a depth of several kilometers limiting the amount of oxygen in seawater is one such explanation and other biologists claim that the ferric iron deposits may have been produced as a result of the activity of anoxigenic photosynthetic bacteria acting in the ocean to oxidize the ferrous iron completely in the absence of oxygen. As is often the case in science the answer awaits further research and possibly the development of new research methods. In the meantime we should probably be content to withhold judgment and practice one of the most important qualities of the scientist, the willingness to say “I don’t know.”
Note: Although it is thought that stromatolites of cyanobacterial origin are the earliest fossilized evidence of life on earth, they must have had precursors. Oxygenic photosynthesis, that is photosynthesis in which water is split (Photolyzed) yielding hydrogen used to reduce Carbon dioxide to carbohydrate and oxygen as a byproduct is a rather sophisticated sort of photosynthesis. Non-oxygenic photosynthesis in which the hydrogen is obtained from other sources such as H2S presumably preceded oxygenic photosynthesis but has left fewer traces in the fossil record.
