Seth Washburn and Dr. Nolan Mangelson, Chemistry

Lichens consist of two organisms that live in a symbiotic relationship. The lichen photobiont, green alga, cyanobacteria, or a combination of both, provides carbohydrates from photosynthesis. While the mycobiont partner, a fungus, accounts for the majority of the lichen mass, and provides shelter and nutrients (St. Clair, 1998). Although lichens come in many different shapes and colors, all lichens are grouped into three main categories, depending on their growth type. Crustose lichens attach very tightly to their rock substrate and often appear as if they were painted on. Foliose lichens have a more leafy appearance and are attached to the substrate at their base through small rootlets called rhizines. Fruticose lichens tend to be more fibrous with a single point of attachment. They have a vertical growth pattern that can look similar to shrubs or many hang down from the attachment like Spanish moss.

Lichens have no root system to absorb needed nutrients. Instead, they quickly absorb valuable water from rain or dew, and within several minutes can jumpstart metabolic activity. They also collect air particulates and substrate particles such as rock, soil, or pollutants for mineral nutrients. They use active uptake systems, and ion exchange mechanisms to transport the targeted minerals into their cells, and even produce metabolites that serve to help break up the trapped particles so that valuable nutrients are easier accessed (Richardson, 1992).

Because lichens tend to collect a significant amount of their nutrients from the air, they are often used as bioindicators of pollution. When there is an increase in a particular pollutant, it has been found that the lichen population in the area also contains an increase of the same pollutant. It is thus possible to record, over time, the elemental concentrations of the lichens and establish an increase in the air contaminants (St. Clair, 1998).

In order to better perfect pollution analysis through lichen tracking, studies have been done relating the substrate of lichens to their element concentrations. This was done in the attempt to determine what portion of collected pollutants really came from the air and what might have been absorbed from the substrate. St. Clair (1998) documented a distinct difference between the concentrations of lichens of the same species that grew on two different substrates. Ross (1998) also did a multivariate analysis of lichens based on their substrate, which suggested similar trends.

It is also suspected that different lichen growth forms have different elemental concentrations. For example, St. Clair (1998) observed that for the same substrate, foliose lichens tend to contain higher element concentrations then fruticose lichens. This is thought to occur because the different growth forms have different methods of attachment and appear to have different efficiencies for capture of air particulate material. Both of these factors may effect element absorption rates.

To better understand the relationship between element concentrations and the various growth forms, over 400 lichens were collected from numerous national parks and reserves throughout the western United States by Dr. Larry St. Clair of the Brigham Young University Botany Department. These lichens were then analyzed for their element concentrations using Proton Induced X-ray Emissions (PIXE) by the PIXE research group located at Brigham Young University. The data acquired from PIXE was then ran through various computer programs to ensure accuracy and tabulated into spreadsheets. The information was then finally assembled into a large database, which contains over 400 lichens, 1200 different analyses, and 22 different element concentrations per analyses. In the database, each analysis was marked according to lichen growth form, species, substrate, and location. Due to the large amount of information, multivariate factor analyses was necessary and help from the Brigham Young University Center for Statistical Consultation and Collaboration Research was enlisted to aid in the operation of the needed statistical programs and methods.

It was at this point that a problem developed relating to the interpretation of the acquired data. As with all natural systems the element concentrations for the various lichen growth forms should have fallen into a bell shaped Gaussian curve. The center or hump of the curve relating to the average concentration for that particular growth form, which then tapers off on both sides. However, the curve observed from the database contained a second very sharp peak on the lower half of the curve. This second peak was impairing the statistical methods used, which gave unrealistic results. The second peak was a result of the PIXE analysis method, which has a limit of detection under which concentrations are not reliable. While entering the reported concentrations into the database, anytime the concentration fell below the limit of detection, the reported concentration was then substituted with the lower limit. This then resulted in the observed second sharp peak instead of a natural tapering.

To further work on this project, either the database must be revisited, and all those analyses where the lower limit was used, be somehow corrected, or different statistical methods must be used that won’t be biased by the second peak. Once again aid from the Brigham Young University Center for Statistical Consultation and Collaboration Research will be sought to find the best possible solution.

Once this is accomplished, I am confident the relationship between element concentration and lichen growth form, as well as species and location, will be better understood. This will help make quantitative pollution analysis more accurate by establishing the relative effects of growth form, species, and location on element concentration, and allow contributions from air pollution to be more effectively determined.

References

• Richardson, D.H.S (1992). Pollution Monitoring with Lichens (1st ed.). Great Britain: The Richmond Publishing Co. Ltd.
• St. Clair, Samuel B. (1998). The Influence of Substrate and Growth Form on Element Accumulation in Lichens Masters Thesis, Department of Botany and Range Science, Brigham Young University, Provo, Utah.
• Ross, Steven A. (1998). Multivariate Factor Analysis of a Rocky Mountain Lichen Data Set Honors Thesis, Brigham Young University, Provo, Utah.