Photosynthesis and Fungi


Symbiotic relationships between fungi and phototrophic organisms were extremely influential in the colonization of land by algae and plant species. It is assumed that fungi helped the early phototrophs survive in the dry conditions of the terrestrial environment. These interactions led to permanent associations. Mycorrhizal symbioses and lichens are just two examples of beneficial relationships. These relationships involve a fungal component, which gains carbohydrates from its host. The roots of the plant provide these carbohydrates as energy and in return, the mycelium of the fungi improve the plant's water and mineral absorption capabilities. In lichens, the association is between autotrophic and heterotrophic partners. The alga or cyanobacterium (photobiont) produces carbohydrates through photosynthesis, which then is transferred to the fungus as food. In return, the fungus provides protection, a supply of moisture, and a substrate helpful in providing the most advantageous amount of light.

Mycorrhizae

Mycorrhizas are symbiotic associations between fungi and the roots of pants. "Myco," meaning fungal and "rhizal," meaning root, these associations were first described in 1885 by German botanist A.B. Frank. There are seven recognized types of mycorrhizas: vesicular-arbuscular mycorrhiza, ectomycorrhiza, orchid mycorrhiza, ericoid mycorrhizae, ectendomycorrhiza, arbutoid mycorrhiza, and monotropoid mycorrhiza (Paracer, Surindar & Ahmadjian 117). Some types aer more common than others, but they all have fungal hyphae that grow extensively through the soil around the roots and provide a greater surface area for absorption of nutrients.





One of the most common myccorhizal relationships is a vesicular arbuscular (VAM or AM) type. These types of symbiotic relationships are most often seen in herbaceous and woody seed plants, ferns and bryophytes. The fungi are present in the soil and infect the developing roots by growing between the cortical cells of the root and penetrating the cells. Unlike, ectomycorrhiza, the fungus does not form an outer sheath around the root, but the hyphae do grow out extensively from the root and into the surrounding soil. The hyphae that penetrate the cells form shrublike growths called arbuscules that fill most of the host cell. These structures form in response to compounds that are produced by the plant.


Mycelium of AM; http://www.agro-genesis.com/pics/cropscience_rhizogold1.jpg
Mycelium of AM; http://www.agro-genesis.com/pics/cropscience_rhizogold1.jpg

In a study done by Sandra Varga and Minna-Maarit Kytövita, they looked at the role of arbuscular mycorrhizae (AM) and potential sex-specific differences of host plants. They were interested in researching whether or not plant sexes differ in drought tolerance. AM fungal colonization provides higher drought resistence and so they hypothesized that male dioecious plants would gain more benefit from the fungal symbiosis. They concluded that both sexes benefited from the mycorrhizal colonization in terms of of biomass, relative growth rate, number of ramets, and shoot phosphorous concentration.


Sex-specific responses to mycorrhiza in a dioecious species (Varga and Kytovita)




Lichen


Fungi, often glanced over as mere mushrooms or mold, are surprisingly versatile eukaryotes that are able to engage in symbiotic relationships with plants. Not only can fungi incorporate themselves into plant roots to aid in their nutrient uptake, but they can also take in symbiotic algae, themselves becoming functional pseudo-plants.

Essentially, the fungus holds the algal photobiont up to the light and leeches the photosynthetic carbon products produced by the algae. The fungus forms a thallus around the algal layer, holding it in place with fungal hyphae, which also extract nutrients. The typical lichen thallus has an upper and lower cortex, sandwiching a medulla and algal layer. Lichen thalli come in three varieties: crustose, the most primitive type which lacks a lower cortex, foliose, which attaches itself to a substrate via rhizines, and fruticose, which is upright or hanging and can be highly branched.

The rate of photosynthesis depends on the amount of water within the lichen thallus. Saturation decreases the rate of CO­2 diffusion, lowering the photosynthesis rate. For this reason, lichens generally prefer not to live in continuously wet or humid areas such as rainforests, although they are remarkably versatile and have been found in almost every type of terrain on the planet, including areas polluted by industrial sulfur contamination.

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British Soldier Lichen - Billi-Jean.com


Surface Hydrophobicity Causes SO2 Tolerance in Lichens


Markus Hauck et al. wanted to determine whether lichen surface hydrophobicity caused sulfur dioxide resistance. Sulfuric acid is extremely toxic to lichen. Lichen that are more resistant to sulfur dioxide are able to survive in environments with more acidic precipitation containing sulfur dioxide.

Hauck et al. took 50 lichen species with known sulfur dioxide sensitivities and placed water droplets on the lichens' thalli surface. The contact angles of these water droplets were then measured to quantify hydrophobicity. Highly hydrophobic thalli had contact angles greater than 120 degrees, moderately hyrdophobic thalli had contact angles between 90-120 degrees, and hydrophilic thalli had contact angles less than 90 degrees. Hauck et al. also extracted the lichen thalli with acetone to remove any extracellular substances. Surface metabolites were measured from thalli with natural content and compared to lichen where these substances had been extracted.

The data clearly supports that sulfur dioxide tolerance in lichen is correlated with hydrophobicity of the thallus surface structure and coating of the cell walls with hydrophobic substances. However, these substances are not essential for hydrophobicity because some hydrophobic species are devoid of the substances.

Hauck et al. showed a significant correlation; however, the paper does not discuss the role of thallus morphology, photobiont choice or the nature of the hydrophobic substances on the thalli. There is still much to be understood about what makes lichens more sulfur dioxide tolerant and what aspects of the lichens' hydrophobicity contribute to their tolerance.


thallus.png
Courtesy of Lichen thallus - Dr. Craft's PowerPoint




Additional Photosynthesis & Fungi Symbioses

Mycorrhizal Fungi and Plant Associations