lichens, protists and green algae - cnx.org · lichens grow very slowly, expanding a few...

OpenStax-CNX module: m47381 1

Lichens, Protists and Green Algae*

Robert Bear

David Rintoul

Based on Groups of Protists� by

OpenStax

This work is produced by OpenStax-CNX and licensed under the

Creative Commons Attribution License 4.0�

Introduction

A process which led from the amoeba to man appeared to the philosophers to be obviously a

progress�though whether the amoeba would agree with this opinion is not known.

Bertrand Russell, from "Current Tendencies", delivered as the �rst of a series of Lowell Lectures in Boston(Mar 1914).

1 Lichens

Lichens display a range of colors and textures (Figure 1) and can survive in the most unusual and hostilehabitats. They cover rocks, gravestones, tree bark, and the ground in the tundra where plant roots cannotpenetrate. Lichens can survive extended periods of drought, when they become completely desiccated, andthen rapidly become active once water is available again.

*Version 1.13: Apr 6, 2016 2:57 pm +0000�http://cnx.org/content/m44617/1.7/�http://creativecommons.org/licenses/by/4.0/

http://cnx.org/content/m47381/1.13/


Figure 1: Lichens have many forms. They may be (a) crust-like, (b) hair-like, or (c) leaf-like. (credit a:modi�cation of work by Jo Naylor; credit b: modi�cation of work by "djpmapleferryman"/Flickr; creditc: modi�cation of work by Cory Zanker)

Lichens are an example of a mutualism, in which a fungus (usually a member of the Ascomycota orBasidiomycota phyla) lives in close contact with a photosynthetic organism (a eukaryotic alga or a prokaryoticcyanobacterium) (Figure 2). Generally, neither the fungus nor the photosynthetic organism can survive aloneoutside of the symbiotic relationship. The body of a lichen, referred to as a thallus, is formed of hyphaewrapped around the photosynthetic partner. The photosynthetic organism provides carbon and energy inthe form of carbohydrates. Some cyanobacteria �x nitrogen from the atmosphere, contributing nitrogenouscompounds to the association. In return, the fungus supplies minerals and protection from dryness andexcessive light by encasing the algae in its mycelium. The fungus also attaches the symbiotic organism tothe substrate.



Figure 2: This cross-section of a lichen thallus shows the (a) upper cortex of fungal hyphae, whichprovides protection; the (b) algal zone where photosynthesis occurs, the (c) medulla of fungal hyphae,and the (d) lower cortex, which also provides protection and may have (e) rhizines to anchor the thallusto the substrate.



Lichens grow very slowly, expanding a few millimeters per year. Both the fungus and the alga participatein the formation of dispersal units for reproduction. Lichens produce soredia, clusters of algal cells surroundedby mycelia. Soredia are dispersed by wind and water and form new lichens.

Lichens are extremely sensitive to air pollution, especially to abnormal levels of nitrogen and sulfur. TheU.S. Forest Service and National Park Service can monitor air quality by measuring the relative abundanceand health of the lichen population in an area. Lichens ful�ll many ecological roles. Lichens are often earlycolonizers of bare rock. Caribou and reindeer eat lichens, and they provide cover for small invertebrates thathide in the mycelium. In the production of textiles, weavers used lichens to dye wool for many centuriesuntil the advent of synthetic dyes.

2 Protists

Amoebae are just one of the creatures that are lumped into the Kingdom Protista, and amoebae andphilosophers do share a common ancestor, as Russell points out. In the span of the last several decades,the Kingdom Protista has been disassembled and rearranged, as DNA sequence analyses have revealed newgenetic (and therefore evolutionary) relationships among these eukaryotes. Moreover, protists species thatexhibit similar morphological features may not be closely related, but may have evolved analogous structuresbecause of similar selective pressures � rather than because of recent common ancestry. This phenomenon,called convergent evolution, is one reason why protist classi�cation is so challenging. The emergingclassi�cation scheme groups the entire domain Eukarya into six �supergroups� that contain all of the protistsas well as animals, plants, and fungi that evolved from a common ancestor (Figure 3). The supergroups arehypothesized to be monophyletic, meaning that all organisms within each supergroup are hypothesized tohave evolved from a single common ancestor, and thus all members are more closely related to each otherthan to organisms outside that group. There is still evidence lacking for the monophyly of some groups.



Figure 3: This diagram shows a proposed classi�cation of the domain Eukara. Currently, the domainEukarya is divided into six supergroups. Within each supergroup are multiple kingdoms. Dotted linesindicate suggested evolutionary relationships that remain under debate.



The classi�cation of eukaryotes is still in �ux, and the six supergroups may be modi�ed or replaced by amore appropriate hierarchy as genetic, morphological, and ecological data accumulate. Keep in mind that theclassi�cation scheme presented here is just one of several hypotheses, and the true evolutionary relationshipsare still to be determined. For this module, we are focusing on the Archaeplastida which contain the greenalgae - the group of organisms most closely related to plants.

3 Archaeplastida (Red Algae, Green Algae and Plants)

Red algae and green algae are included in the supergroup Archaeplastida. It was from a common ancestorof these organisms that the land plants evolved, since their closest relatives are found in this group. Molec-ular evidence supports that all Archaeplastida are descendents of an endosymbiotic relationship between aheterotrophic protist and a cyanobacterium. The red and green algae include unicellular, multicellular, andcolonial forms.

3.1 Red Algae

Red algae, or rhodophytes, lack �agella and range in size from microscopic unicellular forms to large, mul-ticellular forms grouped into the informal 'seaweed' category. Most red algae are multicellular. The redalgae life cycle is an alternation of generations (explained in next section). Some species of red algae containphycoerythrins, photosynthetic accessory pigments that are red in color and outcompete the green tint ofchlorophyll, making these species appear as varying shades of red. Other species classi�ed as red algae lackphycoerythrins and are parasites. Red algae are common in tropical waters where they have been detectedat depths of 260 meters. Other red algae exist in terrestrial or freshwater environments.

Red Algae are an economically important food source and additive. Have you ever eaten sushi rolls? Ifso, the crispy sheets wrapped around the rice are from the genus Porphyra (Japanese "nori"). The foodstabilizer carrageenan is found in ice cream, yogurt and other food stu�s and is an extract from red algae.Another extract from red algae commonly used as a thickener and as vegetarian substitute for gelatin isagar. Agar is also commonly used by microbiologists as a solid substrate to contain culture media in orderto grow bacteria.

3.2 Green Algae: Chlorophytes and Charophytes

The most abundant group of algae are the green algae. The green algae exhibit similar features to the landplants. The cell walls of green algae and land plants are made of cellulose and the chloroplasts of bothgroups contain chlorophylls a and b. The hypothesis that this group of protists shared a relatively recentcommon ancestor with land plants is well supported. The green algae are subdivided into the chlorophytesand charophytes. The charophytes are the closest living relatives to land plants and resemble them inmorphology and reproductive strategies. Charophytes are common in wet habitats, and their presence oftensignals a healthy ecosystem.

Green algae are as a group of organisms an integral part of a functional ecosystem, and humans havebeen using green algae as a food source and as a medicine for a long time. In aquatic environments, greenalgae are a major primary producer and release a substantial amount of oxygen in the system. So a healthyecosystem is dependent on a healthy population of green algae. As a economic bene�t to humans, greenalgae are used a food source (Asakusa Nori) for humans and are used a food thickening agent. Green algaeare used in the agricultural industry as a food source for cattle, and as fertilizer.

The chlorophytes exhibit great diversity of form and function. Chlorophytes primarily inhabit freshwaterand damp soil, and are a common component of plankton. Chlamydomonas is a simple, unicellular chloro-phyte with a pear-shaped morphology and two opposing, anterior �agella that guide this protist toward lightsensed by its eyespot.

Volvox is an example of multicellularity in the Chlorophytes (Figure 4). Volvox colonies contain 500 to60,000 cells, each with two �agella, contained within a hollow, spherical matrix composed of a gelatinous



glycoprotein secretion. Volvox moves by rolling in a coordinated fashion. The cells forming the sphere onthe outside do not reproduce while the green cells inside do reproduce, demonstrating division of labor.

Figure 4: Volvox aureus is a green alga in the supergroup Archaeplastida. This species exists as acolony, consisting of cells immersed in a gel-like matrix and intertwined with each other via hair-likecytoplasmic extensions. (credit: Dr. Ralf Wagner)

True multicellular organisms, such as the sea lettuce, Ulva, are represented among the chlorophytes. Inaddition, some chlorophytes exist as large, multinucleate, single cells. Species in the genus Caulerpa exhibit�attened fern-like foliage and can reach lengths of 3 meters (Figure 5). Caulerpa species undergo nucleardivision, but their cells do not complete cytokinesis, remaining instead as massive and elaborate single cells.



Figure 5: Caulerpa taxifolia is a chlorophyte consisting of a single cell containing potentially thousandsof nuclei. (credit: NOAA)

Charophytes are the closest related green algae to land plants, this conclusion was drawn from studiesof nuclear and chloroplast genes from many di�erent types of plants and algae. In addition to the similari-ties listed above, there a some additional structural similarities such as cellulose-synthesizing proteins, thestructure of the �agellated sperm and the gamete producing structures that suggest that land plants arosefrom within the charophyta group. This does not mean land plants are descended from living green algae,but it does allow us to explore the algal ancestor to land plants. For example, the exploration of the Charalife cycle Figure 6 allows us visualize the similarities between the life cycle of green algae and land plants.



Figure 6: Simpli�ed life cycle of Chara. (Work by Robbie Bear, Im-ages credited to "Chara antheridia L" by Jon Houseman - Jon Houseman andMatthew Ford. Licensed under CC BY-SA 4.0 via Wikimedia Commons -http://commons.wikimedia.org/wiki/File:Chara_antheridia_L.jpg#/media/File:Chara_antheridia_L.jpg"Chara oogonium" by Jon Houseman - Jon Houseman and MatthewFord. Licensed under CC BY-SA 4.0 via Wikimedia Commons -http://commons.wikimedia.org/wiki/File:Chara_oogonium.jpg#/media/File:Chara_oogonium.jpg"Chara braunii 1" by Show_ryu - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons -http://commons.wikimedia.org/wiki/File:Chara_braunii_1.JPG#/media/File:Chara_braunii_1.JPG)


lichens, protists and green algae - cnx.org · lichens grow very slowly, expanding a few...

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