In a harsh and unforgiving environment like the desert, it is hard to believe that any forms of life can survive, let alone flourish. Desert rocks are an example of this, inhabiting many dynamic and thriving life forms. The microorganisms living on desert rocks face extreme temperatures, intense UV light and a great deficiency of nutrients and water. Biofilms, desert varnish and lichens are proof of life enduring these arduous conditions.
Introduction to Biofilm
Biofilms were first described by Antonie van Leeuenhoek, but the modern day theory that describes the biofilm process was not developed until 1978. Today we understand that biofilms are universal and occur in both aquatic and non-aquatic environments. We now understand that biofilms are not just clumps of unstructured cells, they are not homogenous communities of cells that accumulate slime. Instead biofilms are compromised of complex communities of surface-associated cells enclosed in a polymer matrix that contain open water channels. (Donlan, Costerton) Biofilms are a group of microbial cells that group together and are irreversibly associated with a specific surface and are enclosed in a matrix composed of mostly polysaccharide materials. (Donlan) They are complex communities of microorganisms that attach to various surfaces.
Biofilms are microbial communities that very often are composed of multiple and various species that not only interact with their environment but also with one another. (Davey, O’toole) A biofilm’s basic structural unit is the microcolony. The biofilm architecture more specifically the arrangement of microcolonies, which are basically clusters of cells, relative to the placement of one another has profound implications for the purpose and function of these biofilms. (Davey, O’toole) The cells in these microcolonies are close in proximity thus establishing an ideal environment for quorum sensing, nutrient gradient, and the exchange of genes. Conjugation also occurs at a faster rate in biofilms than those conjugation rates between planktonic cells. (Donlan) “Since plasmids may encode for resistance to multiple antimicrobial agents, biofilms association also provides a mechanism for selecting for, and promoting the spread of, bacterial resistance to antimicrobial agents.” (Donlan)
The difference between microorganisms and biofilms is that microorganisms attach to surfaces and develop biofilms. These biofilm associated cells are different from other cells that are simply suspended because biofilm associated cells generate an extracellular polymeric substance (EPS) matrix. This EPS allows an ideal environment for exchange of genetic materials between cells. (Donlan) Bacteria that live in a biofilm usually have very different properties than free-floating bacteria of the same species. The EPS allows for the exchange of genetic material, allows the cells to interact and cooperate in various ways, it also allows for greater resistance to detergents and antibiotics and protection of the interiors of the community.
Where Biofilms are Found
As mentioned above, biofilms are universal. They are present in both aquatic and non-aquatic environments. In this article we will focus on biofilms that grow on rocks. One of the most commonly encountered biofilms grow on rocks in bodies of water such as rivers. The slimy layer on the rocks is a biofilm. The EPS is secreted by the cells that form a biofilm, is slimy and enables the cells to stay within close proximity to one another and protects them. Microorganisms that are able to form biofilms are extremely diverse, they have been found in very cold to very hot environments. One of the most common places biofilms are encountered are on rocks that have been submerged in water, at Yellowstone National Park there are biofilms that are slimy algae that are found around several of the hot springs, geysers, and warm streams located in the park. In some parts of the park because of ideal environments where the water is warm and rich in nutrients from the geysers the green algae which are biofilms are several inches thick! These biofilms that occur naturally in such environments are very important because they provide food and nutrients for organisms that are larger than biofilms, such as larvae of insects that are then eaten by larger fish. Biofilms do not only occur in solely aqueous environments. There are also biofilms that develop on rocks in the desert. These biofilms are known as “desert varnish”. (Cunningham, Lennox, Ross)
Desert varnish appear in arid desert conditions such as long periods of drought and sporadic periods of severe rain. Desert varnish is a dark, thin coat found on the surfaces of desert rocks. Because of the it's hostile environment, desert varnish house only a few xerophilic organisms. Some primary inhabitants are endospore forming bacteria and microcolonial fungi. These microorganisms have acquired survival skills in able to endure high temperatures, intense UV radiation and low nutrient supply.
Physical and Chemical Conditions
Desert varnish is mostly made up of clay minerals and Iron and Manganese oxides. These two oxides give the desert varnish its red or black colors, respectively. Endospore forming microbes, specifically Gram positive bacteria, are responsible for it's Manganese oxidation. These include Micrococcus, Planococcus, Arthrobacter, Deodermatophilus, and Bacillus. Dying bacteria secrete amino acids, also contributing to the varnish coating and biominerals.
Cell walls of gram positive bacteria contain oligosaccharides, teichoic acids and sugar lipids. These components interact with sand grains on rock surfaces. As it attaches, it secretes a sticky, viscous material called extracellular polymeric substances (EPS). This polysaccharide coat is important for biofilm formation, protection from harsh conditions and attachment onto rock surfaces. EPS also keeps essential metals and nutrients from escaping.
Along with microbacteria, black, yeast-like fungi called microcolonial fungi (MCF) influence desert varnish formation. MCF are chemoorganotrophs, using organic compounds as a source of energy. In addition, MCF have microsporines, melanin and carotinoids that shield them from UV radiation.
Desert varnish are capable of preserving microbial fossilization, which may include bacterial and fungal casts. These fossils are primarily found on the surface of rock varnish. Desert varnish, however, are easily eroded by lower pH conditions (acid rain) or a weathering polysaccharide coat. Therefore, most preserved structures are lost. Current research are taking use of desert varnish biosignatures as a potential record of environmental processes, such as climate change and global warming.
Other research include using desert varnish as evidence for the colonization of extraterrestrial life forms. Rock coatings and possible biosignatures on rock surfaces may exist on other meteorites, particularly on Mars. Varnish-like structures on rocks found at the Mars pathfinder landing site have furthered this curiosity.
Lichens are unique organisms that consists of a symbiotic relationship between fungal filaments and algae or cyanobacteria. Algae and cyanobacteria are known as photobionts, while the fungal component of lichen is called a mycobiont. Algae are photosynthetic and provide the fungus with carbohydrates which can be used to produce energy. Cyanobacteria on the other hand, fix nitrogen or carbon which the fungal can use to synthesize amino acids and carbohydrates respectively. The fungus can also be considered a parasite because it provides no dehydration protection or nutrients for the photobionts.
Lichens can inhabit a diverse environment ranging from deserts, and tundras all over the world covering an estimated 8% of all land. Lichens can be found in deserts such as Saudi Arabia, Australia, and North America. In these environments lichens are situated on tree barks, leaves, ground, and rocks.
As stated earlier lichens come in many form, each with different roles and adaptations that suit their environment. Many of these lichens can occupy the same niche but over time the lichen that has the best adaptations for the environment will have a higher growth rate and a lower death rate, and out live the poorly suited lichen. Lichens can even overgrow and kill mosses that are in their niche by excreting toxins. Lichens are also known to produce useful antibodies and dyes that have been harnessed by humans. For instance lichens with cyanobacteria are less adaptive to live in deserts because they require more water then the arid climate can provide. Due to this lichens with various types of algae are better suited to inhabit desert rocks because they need less water to survive.
Lichens can survive in extreme desert heat. In the desert Lichens can survive up to -60 º to 55 º Celsius which is -76 to 131º Fahrenheit(Hawksworth). The water content for lichens can vary from 2%-300% dry weight(Douglas). The low water content lichens are ideal for desert rock lichens because deserts are known for their minuscule or non-existent precipitation. Due to the lack of precipitation, lichens obtain most of their water from the atmosphere and morning dews. When not enough water is obtained from the atmosphere the lichen turns off metabolic processes and go dormant, which can last several years. However when a small amount of water is obtained from morning dews the lichen comes out of its dormant stage and resumes physiological activities(Douglas). Compared to cyanobacteria, algae are more adaptive at low water levels and can occupy desert niches better.
Since Lichens are composed of a symbiotic relationship between fungus and photosynthetic algae there can be many combinations of lichens. It is estimated that 85 percent of the photobiont are eukaryotic algae while 10 percent are cyanobacteria and 5 percent are a combination of cyanobacteria and eukaryotic algae. Some examples of alga are trebouxia, Pseudotrebouxia, and Trentepohlia while some examples of cyanobactera are Nostoc and Scytonema. The fungi that are in the symbiotic relationship can be Ascomycota, Basidiomycota, or Conidial fungi (Douglas).
The relationship between the fungal and the alga or cyanobacteria can be considered to be parasitic because the fungal benefits from the photobionts but the photobionts do not benefit from the fungal. Studies using radioactively labeled carbon have shown that photobionts retain only 20% of the carbon they fix, while they transport 80% of the fixed carbon to the fungus(Douglas). Algae release the carbon they fix in the form of glucose, while cyanobacteria release carbons in the form of polyhydric alcohols. These forms of carbon can then move on to provide energy for the photobionts and the mycobionts.
Cyanobacteria, particularly Nostoc, are unique from alga because along with fixing carbon they also fix nitrogen into ammonia. Most of the ammonia is actually used by the fungus to synthesis necessary amino acids.
Changes to Environment
Most lichens change the diverse environment they live in. They can form fruticose which looks like shrubs, foliose which are like leaves, crustose which are like a crust, on the surfaces they are in contact with (Hawksworth). Lichens are also known to weather rocks through chemical and physical methods. Chemical weathering of rocks is due to the secretion of oxalic acid converted from insoluble carbonates and silicates. Physical weathering of rocks is done by contracting and swelling of the body of the lichen. Swelling of the body of the lichen happens during wet periods, while contracting happens during the dry periods of the lichen. The changes in size of the lichen help those that are embedded in cracks of rocks to break the rock apart.
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Edited by [Priya Patel, Fatima Khan, Luanne Cabiling], students of Rachel Larsen