It Takes Two, Baby: Fungus and Algae

Lichens can easily be found on the side of trees(http://rutgers-leslie.blogspot.com/2010/04/lichen-on-your-trees.html)

Introduction

Lichens are the most well known examples of symbiosis between fungi and plants. Classifications of lichens include some species of mushrooms, slime molds, and some members of the Zygomycota [4]. However, the mutualistic relationship in lichens is basically defined by its association between a fungus and an alga that develops into a unique morphological form that is distinct from either partner [1]. The fungus is called a mycobiont and the algal part is referred to as the phycobiont [1]. The life cycle begins when these components combine, and the fungal filaments enclose to develop into the algal cells. This algal cells provide the lichen with its physical structure and shape. As a result of fungal reproduction, the apothecium produces spores to continue the life cycle again[5]. Like other green plants, it relies on photosynthesis to produce food. The alga contributes food supply through photosynthesis, while fungus protects the alga from losing moisture, harmful solar radiation, and provides the alga with water and nutrients[4]. Although lichens are resilient enough to survive and exploit hard substrata like rocks and tree trunks, they still remain sensitive to air contaminants of the environment.   are so tough that they can survive on hard surfaces such as exposed bare rock surfaces, they are also very sensitive to modern elements introduced to the environment, like sulfur dioxide and other air pollutants [4]. Lichens grow in three forms: foliose or leaf-like, fruticose, that are branched or bushy in appearance, and crustose, which grows a thallus or thin crust upon the surface[2]. Although lichens have the ability to grow in a number of diverse areas, they are most prominent in the Arctic tundra and Antarctic [5]. The global distribution of lichens depends on physical factors like light availability, humidity, substratum nature and chemistry, and sulfur dioxide levels in the air [3]. 

Cross section view of lichen 

                                      (http://www.saburchill.com/ans02/chapters/chap010.html)

Description of the Relationship

 Lichens are dualistic organisms, composed of a fungi and an alga growing together to form single body[2]. The fungal component is often a species of Ascomycetes, although a few of the Basidiomycetes will sometimes grow with green algae such as Protococcus, Cryptococcus, or Trebouxia, or blue green algae such as Gloeocapsa, Nostoc, or Stigonema [5]. The lichen’s method of reproduction can be fungal or asexual[5]. Lichens fungally reproduce by growing fruiting bodies and spores. These spores can produce additional fungi, however, the alga does not get the opportunity to reproduce at all. To continue to life cycle, the new fungus has to either find an algal partner or it perishes [5]. The lichen reproduces asexually by producing soredia, which are small specialized fragments of thallus consisting of fungal tissue. The soredia forms in the parent thallus, grows out through the surface, and are carried by the wind and rain to new environments as small bits of tissue [2]. Lichens are not considered one single organism, but are rather a combination of two organisms that share an intimate relationship[2]. Although fungi and algae that produce lichens can exist in nature alone, many lichens include a fungus that cannot survive with its algae counterpart and takes on a whole new morphology as a separate entity[2]. Based on the anatomy of the lichen, there is an obvious mutualistic symbiosis between the phycobiont and mycobiont [4]. The alga produces the food material and the fungus protects alga from drying out, acquiring too much solar radiation, physically injury, while providing it with water and minerals[4]. Scientists believe the relationship developed because neither component could obtain all the necessary nutrients for survival until they interacted[4].

Crutose lichen(http://www.perspective.com/nature/fungi/lichens.html)

Cost/Benefit Analysis

Although fungi and algae only prefer moist and wet environments that do not receive direct sunlight, and cannot survive outside these conditions, Lichens have the ability to grow all over the world [4]. These even include arctic and hot, dry desert areas where not many organisms are able to survive[4]. The alga contributes food supply, and the fungus protects the alga from losing moisture, harmful solar radiation, and provide the alga with water and nutrients [4]. Also, the fungus is able to grow on bare rock and other surfaces where other plants cannot because because the fungus is able to take a firm hold where most plant roots are unable to penetrate[6]. Neither the fungus nor the alga have the ability to solely survive in such hostile environments[6]. However, working as a unit, they can successfully compete with other plants for light and space [6]. Because of the absences of roots, lichens use air as a the primary source of elements[8]. This, however makes them especially vulnerable to industrial air contaminants such as sulfur dioxide[6]. This vulnerability is also correlated with the energy needs of the mycobiont. As the dependency between the mycobiont and phycobiont increases, so does the lichen's vulnerability to air pollution[7,8]. The phycobiont may also use its metabolic energy to repair cellular structures that would otherwise be used towards the maintenance of the photosynthesis process[7,8]. Deviations from equilibration between the mycobiont and phycobiont can lead to complete breakdown of the symbiotic relationship[7]. Therefore the decline in the lichen population may not only be attributed to air contamination, but also from a symbiont that receives more nutrient supplies over the other[8]. The lichen association is in tight knit mutualism in which neither the fungus or algae can reproduce without the balanced efforts of it symbiotic partner.

 

Different faces of the lichen

                                            http://www.youtube.com/watch?v=dHpruP_AaKc

 References

1. http://www.ucmp.berkeley.edu/fungi/lichens/lichenlh.html
2. "Lichens." World of Microbiology and Immunology. 2003. Encyclopedia.com. (April 25, 2012).                     http://www.encyclopedia.com/doc/1G2-3409800348.html

3. http://www.hokoon.edu.hk/download/biology/Lichen/lichen_e.pdf

4. http://www.botany.hawaii.edu/faculty/wong/BOT135/Lect26.htm

5. http://wardsci.com/images/art/Lichens.pdf

6. http://www.saburchill.com/ans02/chapters/chap010.html

7.http://www.absoluteastronomy.com/topics/Lichen
8.http://bwindiresearchers.wildlifedirect.org/category/lichens/