One of the things that strike me ever since I came to Singapore is that it is truly a Garden City. ” No roads has been left uncovered and no vacant land within the city left unplanted with shrubs ( W.Y.Chin and C.Richard, 1986).” Thanks to those big trees on campus I am able to survive the intense heat of the sun. There is one interesting feature on the tree that you might have observed before as well.
Above: An algae covered tree in PGP Residences. Below: “Rusty” trees seen outside Raffles Hall of Residences.
After a closer examination, the orangy stuffs that look powdery from far, actually appear to be quite rough. Some patches looks like a rough orange carpet covering the bark, while some looks somehow like overcompressed breadcrumbs. Now it looks like some fungus or lichen. Since this question has been bugging me, I decided to find out the identity of the rusty patches.
So…. It appears that the rusty orange patches are actually green algae! It’s not uncommon that algae lives attached to the trees, but what surprised me more was that it is actually classified under GREEN algae! Green algae, phylum Chlorophyta, now belongs to a well-established monophyletic group, Viridiplantae, in which the terrestrial plants are also included. This species of green algae comes from the order Trentepholiales and the genus of filamentous green algae, Trentepohlia. Algae under this order are usually subaerial algae which are adapted to live on substratum like natural rocks, concrete walls, plastic nets, tree barks, fruits and leaves (Rindi F. et.al., 2009). Some of the algae under this order are actually lichenised-algae. Trentepholiales attached to the subtrata via filamentous hyphae and reproduce via spores.
Now why is it in the same family as the green algae while it appears to be orange? Algae of order Trentepohliales produce large amount of carotenoids (which is also a component of chlorophyll), such as beta carotenoids and haematochorome. These pigments gave the algae their charateristic bright yellowish orange or red colouration. Trentepohlia sp. is well distributed across the humid tropic region.
Since the algae has chlorophyll pigments hence the ability to photosynthesize and the host plants do not show any signs of malnutrition or being parasited, the interactions of these green algae with the tree most probably are commensalism. While for non-living substrata, I was thinking that it too, merely serve as a host. However, these green algae actually post problems to substratum like the concrete wall. Pigments produced by Trentepholia sp. caused local erosion like pitting and discoloration of wall when present as biofilm (Gaylarde P. et.al., 2006).
Okay, doubts cleared. Hopefully next time when you stumble across some trees with strage orangy trunks, you will know that it is actually algae growings instead of rust !
Hugh, T.W.Tan et.al.,2007, The Natural Heritage of Singapore 2nd Edition. Prentice Hall, Pearson Education, South Asia Pte. Ltd. Singapore.
Corlett, R. and Chin W.Y., 1986, The City and The Forest: Plant Life in Urban Singapore. Singapore University Press, National University of Singapore.
Rindi F. et.al., 2009, An Overview of The Biodiversity and Biogeography of the Terrestrial Green Algae, Nova Science Publishers, Inc. http://bama.ua.edu/~dwlam/index_files/Rindi_Biodiversity%20hospots_PC.pdf (retrieved 10th April 2010)
Gaylarde P. et.al., 2006, Lichen-like colonies of pure Trentepohlia on limestone monuments, Elsevier Ltd. http://www.sciencedirect.com.libproxy1.nus.edu.sg/science (retrieved 10th April 2010)
Trentepohlia, Wikipedia.http://en.wikipedia.org/wiki/Trentepohlia (retrieved 14th April 2010)