Air pollution has become a topic that is increasingly researched and talked about, and understandably so. It can have catastrophic effects on ecosystems, especially urban ecosystems, therefore negatively impacting human health as well as the surrounding environment. A symbiotic relationship between fungi and algae, lichens are key organisms when it comes to indicating air quality, pollution levels, and overall ecosystem health (National Park Service). Because they collect all of their necessary nutrients from the air, they are particularly sensitive to pollutants. The absence of lichens can indicate poor air quality or the presence of particularly toxic pollutants. Not only are they wonderful for indicating the health of an ecosystem, but they also serve as food to many organisms and clean the air, making them especially beneficial to the places they are found (National Park Service).
Despite lichens being ecologically important, not much is known about them and not much research has been done since the late 1900s/early 2000s. In 1997 a study was done in the Aniwaniwa River Valley of North Island, New Zealand, by Green et al. They looked at net photosynthesis (NP) of nine species of rain forest lichens in response to differing levels of light - from shaded areas in the forest through open ground sites, and NP of Pseudocyphellaria coronata in response to water content (WC) at constant light. They found that species found in the open areas had much higher light saturation (the amount of light needed for max net photosynthesis) than species found in and on the edge of the forest, and that as light increased, NP increased quickly until it reached light saturation when it tapered off and remained constant (figure 2). The authors noted that lichens dry out quickly once in the sun, so the lichens in the open habitat could photosynthesize at high light levels, but they were only active for 50% of the measurements. They suggest that the structure of the open habitat lichens (fruticose) may help by providing shade for itself. For P. coronata (found in half shade), they found that at low WC and high WC, NP and light saturation decreased with NP decreasing at many light amounts measured (figure 6). This suggests the importance WC plays for the metabolic and photosynthetic abilities of the lichen.
Figure 1: A (on the left) figures from Green et al. 1997 showing the response of NP to incident photosynthetically active photon flux (PPFD or amount of light as I have been referring to it) at optimal WC and 15C. B (on the right) figures from Green et al. 1997, showing NP in response to differing WC at constant light levels (also measured in PPFD).
Continuing on from this paper, Bjorn, Tallulah, and I are looking at lichens on the lamp posts of south campus. One aspect of the lichens we are measuring is the directionality distribution (north, east, south, west). Because of the dependency of lichens on light and water, as found by Green et al. (1997), the lichens we are looking at will want to be at light saturation, but also have an optimal WC. If our lichens are able to photosynthesize at a high light saturation and low WC, they may be found on the sunnier (south) sides of the poles, but if WC content plays as important a role it did for Pseudocyphellaria coronata, our lichens will likely be found on sides that spend less time in direct sunlight.
References:
National Park Service. (2018). Lichen - The little things that matter. https://www.nps.gov/articles/lichen-and-our-air.htm
T. G. A. Green , B. Büdel , A. Meyer , H. Zellner & O. L. Lange (1997) Temperate rainforest lichens in New Zealand: Light response of photosynthesis. New Zealand Journal of Botany, 35:4, 493-504, DOI: 10.1080/0028825X.1987.10410173
It seems like this will be a pretty interesting experiment given the findings of the literature. What do the authors of Green et al. 1997 mean when they suggest that open habitat lichens are "providing shade for itself?" Additionally, does your study plan to examine one particular species of lichen on south campus lampposts, or any and all lichens observed? Looking forward to hearing about your findings!
ReplyDeleteThere are three main forms of lichen thalli (almost like the limbs of the lichen), one is thinner and adheres tightly to what it's growing on and the other two are thicker and rise of the substrate surface a little bit. I think they mean that because the thallus stick out, as opposed to lying flat, it can shade itself a little.
DeleteLichen species are really hard to identify, so we are identifying all of them by color and thallus form - we have 7 categories right now.