Discussion of Lichen Biology (Nash 2008)
Chapter 2: Photobionts (T. Friedl and B. Budel)
A photobiont is the green algae or the cyanobacteria that live inside the lichen and perform the duty of transforming light into sugar. There are approximately 40 genera of algae and cyanobacteria compared to over 280 genera of fungi that are involved in the lichen symbiosis. Most frequent include Trebouxia, Trentepohlia, and Nostoc.
The two types of photobionts:
1. Phycobionts: Green algae. These are eukaryotic organisms, meaning there are membranes around the cell organelles, including the nucleus. The green algae is in phylum Chlorophyta, which share many features with land plants including cell structures and pigmentation including the presence of chlorophylls a and b. There are also two genera of eukaryotic photobionts that contain chlorophylls a and c that are thus far reported, these are Heterococcus from the Xanthophyceae family and Petroderma from the Phaeophyceae family.
2. Cyanobionts: Cyanobacteria. Cyanobacteria are prokaryotic and do not have chlorplasts (or a mitochondria nor nucleus) since prokaryotes do not have any membrane bound organelles. So how do they photosynthesize sugars? Well, they have thylakoids roaming around freely in their cytoplasm; the thylakoids are usually found closer to the periphery of the cell due to the increased amount of light found there. And they have circular DNA, unlike the eukaryotes which have histone proteins in their DNA which pulls the DNA into tight coils, and plays a part in which genes are exposed (this is how epigentics influences phenotypes), Nostoc is one of the primary cyanobionts.
The types of sugar created by the different photobionts:
The photobiont creates sugars and shares it with the mycobiont, but the sugar created by the phycobiont and cyanobiont is different. The Phycobiont, or green algae creates sugar alcohols while the cyanobacteria creates glucose.
Photosynthesis and water:
The physiology of the two different photobionts is remarkably different, and this effects the requirement of water to perform photosynthesis. The eukaryotic green algae can produce net-gain photosynthesis by taking up water vapor alone, while in cyanobacteria there needs to be a much high water content to activate photosynthesis. This is why cyanolichens such as many Nephroma spp. are usually found in old growth forests because so much water is dripping from the dense canopies. and the green algae lichen are often found in drier environments.
A deeper look at the Cyanobionts:
Identification and taxonomy of Cyanobionts
Cyanobacteria are basically in the Kingdom Protista, and they are very difficult to identify even outside of the lichen, and inside the lichen the situation gets even more tricky. Morphological changes of cyanobiont due to symbiosis are extensive, making it difficult to identify the cyanobacteria to genus without 1) culturing the lichen, or 2) using 16S DNA primers (Lohtander etc al 2003l O’Brien et al. 2005). These morphological changes in cyanobionts are various: 1) most filamentous cyanobacteria loose their regular form within the lichen thallus, and are thus deformed to a point where their characteristic filamentous form can’t be recognized (exceptions include the unbranched filamentous Nostoc, and the extensively branched Stigonemas; 2) characteristic stages of most cyanobacteria development are not seen when lichenized; 3) cyanobacteria with heterocysts including Nostoc have the frequency of heterocyst occurence increased by up to 5 times that found in the free living state; and 4) cell size of the cyanobacteria is increased when in the lichen thallus, but when the cyanobacteria is isolated from the lichen and cultured, its cell size decreases back to normal. Friedl and Budel note that the vegetative trichrome cells of Scytonema in the lichen Dictyonema sericeum are a good example of this change in size of the cyanobacteria from photobiont to independent state.
When culturing cyanobacteria, Friedl and Budel suggest using the system suggested by Anagnostidis and Komerk (1985,1988,1990) and the bacteriological approach of Castenholz and Waterbury (1989). Friedl and Budel note that currently the taxonomy of cyanobacteria at the species level is “in a state of flux.” They state that identifying isolated unicellular cyanobacteria is “almost impossible” due to the current species concepts in use being defined by ecological features (see Geitler 1932 and Komarek and Anagnostidis 1998,2005) and/or morphological features (which they note is an issue because morphological can change dramatically due to environmental factors).
Friedl and Budel discuss that there has been some headway recently with the examination of the genes found on a subunit (16S) of the ribosmal RNA; and this research is revising the systematics and phylogeny of cyanobacteria – see Wilmotte and Golube 1991, Turner et al 2001, Fewer et al 2002, Gugger and Hoffmann 2004, Henson et al 2004, Svenning et al 2005, Tomitani et al 2006. This genetic data supports using some morphological criteria as diagnostic criteria, including mode and planes of division, cell differentiation and the cells morphological complexity, and differences in developmental stages in the life cycle.
The genetics of Cyanobacteria:
Lets look at the advances made by connecting genetics and these particular morphological features just mentioned, cause some of the findings are interesting. First off, the phylogenetic tree (basically a tree of life showing the relation of organisms) created by comparing the genes of cyanobacteria shows that cyanobacteria with one cell type and binary division form a polyphyletic group. A polyphyletic group is a group of organisms that are assumed to have a close common ancestor due to the characteristics of the species, but after genetic testing is performed it is found that some of the species in the group are not genetically similar enough to the others to be part of the same genus, or perhaps even the same family, even though they share many similar character traits.
As the complexity of the cyanobacteria in question increases, so does the stability of certain areas of DNA and RNA. Stability of genes helps geneticists to find a certain section of DNA or RNA to use to judge genetic relatedness between species. For instance, to find ancestral relatedness in humans using a gene section that codes for hair color wouldn’t be as smart as using mitochondrial DNA, since mitochondrial DNA is passed from mother to child with little to no alterations.
That being said, as morphological complexity of cyanobacteria groups increases, so does the ability to find an appropriate genetic subunit to use to gauge relatedness. So far, what’s been found is that there is a common ancestor for cyanobacteria that carry out cell divisions in several planes and also produce nanocytes – the Pleurocapsales and the Chroococcales. And there is another related group showing that cyanobacteria that have heterocysts are also from a common ancestor – these include the Nostocales (which the tree shows needs to be broken up into three different groups) and the Stigonematales. And then there are even more cyanobacteria groups that Friedl and Budel don’t attend to in this chapter, but basically its safe to say that genetic mapping coupled with certain morphological distinctions is helping scientists to create a pretty extensive tree of life for the cyanobacteria. Surely, there are spots on the tree that have what are called “non-existent bootstrap frequencies”, and some with neither a bootstrap frequency of a posterior probability but at least we know that the cyanobacteria all come from a common ancestor – and that makes sleeping at night just that much easier!
A deeper look at the Phytobionts
Identification and taxonomy of Phycobionts
Green algae photobionts are way more simple to work with because 1) their taxonomical system is much more developed because they are eukaryotic organisms, and increases in complexity aids in the ease of categorization, and 2) their morphological features do not change so drastically during the lichen symbiosis, so culturing of the green algae is not necessary to define genus level identification – a simple squash preparation on a slide is all that is needed..
Friedl and Budel note that only coccoid, sarcinoid and filamentous forms are known to be part of the lichen symbiosis – flagellates are not known to be involved. (By the way, one part of lichenology that I really appreciate is how lichenologists state clearly when something is not yet known, or has not yet been found, rather than assuming that something doesn’t exist until there is proof otherwise.)
As far as reproduction, green algae do reproduce when they are inside a lichen, though not as frequently and only asexually (the exception is the order Trentepohliales).
Culturing of green algae is generally not needed to determine the genus, but to determine the species it is often required. This is because the green algae life cycle stages are limited or completely absent while living within the lichen, and life cycle stages are one of the characteristics that are used to define and categorize different green algal species.
Green algal systematics
Just as lichenized fungi come from very different branches of the Fungal Kingdom (see Chapter 3 notes), the lichen-forming green algae also come from different branches within the Plant Kingdom. There are over 7000 species of green algae, and the taxonomical classifications are shuffling like a deck of cards as genetic data from the subunit 18S rRNA of the green algae continues to change our understanding of the origins and genetic relationships between different green algal species. Just a note: contrast that green algal systematics use the 18S rRNA while the cyanobacteria systematics use the 16S rRNA – what are the implications of the genetic diversity being located at these different regions?
Friedl and Budel state that the lichenizing green algae are very morphologically diverse, and they point to Tschermak-Woess (1988) for a complete list of all green algal taxa that had been found by that time.
Different species of green algae found in lichens
Friedl and Budel explain that Trebouxia spp. are the most common green algal photobionts in the lichen, these are coccoid (spherical shaped). There are approximately 16 to 25 different species of Trebouxia, and no free living form of Trebouxia has been found in the wild, but scientists are hesitant to recognize it as an obligate symbiont. Friedl and Budel mention that scientists have found a few small colonies living on bark and soil but it was unclear whether they could “have escaped from damaged lichen thalli” (Friedl and Budel in Nash 22). When Trebouxia are grown in the lab apart from lichen, they have motile spores, but within the lichen they do no not. Their chloroplast is also reduced within the lichen. But their chloroplast is one of their defining features because they are massive or star-shaped and wrinkled in different ways and have pyrenoids that have very different structures. Gene sequencing of the subunit 25S rDNA indicate that examination of chloroplast morphology is an accurate way of classifying the different species. Gene sequencing also indicated that Trebouxia is paraphyletic, meaning that it needs to be divided into at least two genera, though there is some debate about all that because it goes in opposition to the concept of Pseudotrebouxia sensu developed by Archibald in 1975. Note: Friedl and Budel do not elaborate on this topic, so if I’ll try to find out more about that and fill you all in.
Myrmecia biatorellae and Dictyochloropsis spp. are also coccoid and are similar in structure to Trebouxia. They are found living as a photobiont within lichen, but also are found free living on soil and bark
Chlorella and Chlorella-like green algae (including Coccmyxa spp.)are coccoid and do not form motile spores (non-motile spores are called autospores). But Friedl and Budel note that the taxonomical system for autosporic green algae is still very unclear due to their small size and lack of easily studied characters
Coccobotrys spp. and Desmococcus spp. have sarcinoid growth habits. Interestingly, these are known from endolithic habitats, i.e. within rocks.
Trentepohlia spp. is another of the most common phytobionts. In its free living state is grows as an epiphyte on moist rocks or bark, and will often form orange or reddish masses on the leaves of plants due to formation of carotenoids within its protoplast. This pigmentation also occurs within the lichen that Trentepohlia associates with, so to check for Trentepohlia, just scratch the outer surface of the lichen (Friedl and Budel in Nash 24).
Pleurastrum terrestre, filamentous green algae found in soil samples and only in the lichen Vezdaea spp. and Thrombium spp.
Dilabifilium spp. is a filamentous epiphytic alga in marine and freshwater habitats and is also found in the primitively organized crustose aquatic lichens Verrucaria spp. and Arthropyrenia spp.
Occurrence within Lichen
Lecanorales – most frequent photobiont is Trebouxia spp.
Arthoniales – Trentepohlia spp.
Gyalectales – Trentepohlia spp.
Sphaeriales – Trentepohlia spp.
Ostropales – both Trebouxia and Trentepohlia are equally frequent.
Caliciales – Chlorella and Chlorella-like algae are common.
Lecanorales – Chlorella and Chlorella-like algae are common
– Nastassja Noell