GREEN ALGAE · DIVISION CHLOROPHYTA Introduction Of the approximately 16,000 species of green algae, 90% are restricted to the freshwater environment: damp soil, rivers, lakes, ponds, puddles, tree bark, and even the hair of polar bears. The marine representatives are limited to relatively few orders and are common in the intertidal and upper subtidal regions. Like vascular plants, green algae have chlorophylls a and b in addition to a variety of carotenes and xanthophylls that act as accessory pigments. Nutrition is autotrophic, with the reserve carbohydrates stored in plastids in the form of starch. Green algae exhibit a wide variety of thallus forms, ranging from single cells to filaments to parenchymatous thalli. In tropical and subtropical waters, many forms may be calcified. Reproduction occurs asexually by fragmentation or by the production of spores that develop directly into new individuals, or sexually by the union of two gametes. While the sexual gametes and asexual spores may look very similar, they can be differentiated by the number of flagella; sexual gametes have two flagella and asexual spores have four flagella. In many species the entire thallus becomes reproductive, a term called “holocarpic.” Systematics A few distinguishing characteristics separating key orders for marine Chlorophyta are below. Order Thallus Chloroplasts Reproduction & Life History Examples (asexual; sexual) Ulvales parenchymatous single, parietal, zoospores; gametes iso/ Ulva several pyrenoids anisogomous. mostly isomorphic stages Cladophorales coenocytic cells parietal, reticulate, fragmentation, zoospores; Cladophora, united end-end few to many biflagellate gametes Chaetomorpha pyrenoids Bryopsidales much branched, numerous, discoid, none; biflagellate gametes Codium, coenocytic, w/o pyrenoids Bryopsis nonseptate Morphology The green algae are well represented in the marine plankton and damp terrestrial environments, with many species occurring as unicellular organisms. These include motile vegetative cells that produce vegetative spores and/or gametes (e.g. Chlamydomonas), non-motile vegetative cells that produce flagellated reproductive structures (spores/gametes) (e.g. Chlorococcum), and lastly non-motile vegetative cells without motile reproductive structures (e.g. Chlorella). Colonial green algae (e.g. Volvox) are not commonly encountered in the marine environment, though they are common residents of soil and standing freshwater. Colonies (synobia) are made up of distinct single cells held together by mucilage. A giant evolutionary step was made in the green algae when the cells that resulted from a mitotic event remained contiguous. Connected cells enabled both multicellularity and the complex functions this required. Filamentous green algae are well represented in the sea either as unbranched, simple uniseriate (single thread) filaments of cells that show little or no thallus differentiation (e.g. Chaetomorpha, Ulothrix), or as branched filaments (e.g. Cladophora). If a single cell divides in only one plane, a filament is formed. However, if that single cell divides and remains adherent in two or three planes, a parenchymatous multicellular thallus is formed (e.g. Ulva). A variation of the filamentous thallus that is tubular and lacks cross walls (except when delimiting reproductive structures) is called siphonous. The siphonous filaments may occur singly (uniaxial, e.g. Bryopsis) or may be aggregated into a pseudoparenchymatous multiaxial system (e.g. Codium). Most of the siphonous marine green algae are tropical, but a few genera are prominent in the flora off the west coast of North America (e.g. Codium). Objectives In today’s lab, you will examine green algae that represent a wide range of morphologies: from colonial, to filamentous, to parenchymatous, to siphonous. Examine representatives from each group and make careful drawings of the key features that separate them. Notebook Requirements (15 drawings) 1) Chlamydomonas-drawing of different life stages (at least 2) 2) Volvox- draw different steps in life cycle (at least 2) 3) Chaetomorpha linum- 2 drawings (thallus & single cell) 4) Cladophoracolumbiana- 2 drawings (thallus & single cell) 5) Ulva taeniata- 2 drawings (thallus and cross section) 6) Codium fragile and Codium setchellii -3 drawings (thallus of both & squash of one) 7) Unknown1- 1 drawing & steps to key out 8) Unknown 2- 1 drawing & steps to key out A. Unicellular Forms Key Organism: Chlamydomonas (Class Chlorophyceae, Order Volvocales) Unicellular forms can be motile or non-motile. Chlamydomonas is a single-celled motile alga commonly found in damp soil, ditches, tide pools and occasionally in salt marshes. The organism is typically egg-shaped and has a large cup-shaped chloroplast containing a proteinacious body—the pyrenoid, which functions in starch polymerization. The nucleus is often difficult to see because of the prominent chloroplast. 1. Examine mating strains of Chlamydomonas. Identify the steps in the life cycle. Draw 2 different life stages in detail. Be sure to draw and label chloroplast. 2 B. Colonial Forms Key Organism: Volvox (Class Chlorophyceae, Order Chlamydomonadales) While colonial green algae are not common in the marine environment, they are widespread in soil and standing fresh water. Some are motile flagellated species (e.g. Gonium, Pandorina, Eudorina, Volvox), while others are non-motile colonies that produce flagellated reproductive cells (e.g. Chlorococcum). 1. Identify the different colony types of Volvox. Draw at least 2 different colony types and describe where they fit into the life cycle of Volvox. C. Filamentous Forms Key Organism (simple filament): Chaetomorpha sp. (Class Ulvophyceae, Order Cladophorales) Chaetomorpha is a very common unbranched filamentous alga in the intertidal areas of central California. It is usually attached to pier pilings or to rocks and looks like green hair. Chaetomorpha linum 1. Examine Chaetomorpha under the dissecting (thallus) and compound (single cell) microscopes. Draw the thallus and a single cell. What shape is the chloroplast? 2. See if you can observe holocarpic reproduction. To do this, place a piece of the thallus under a light source and observe under a scope. As the thallus heats up, it will begin releasing all the pigmented material inside. Can you think of a benefit of reproducing this way? Key Organism (branched filament) : Cladophora columbiana (Class Ulvophyceae, Order Cladophorales) Cladophora is a branched filamentous alga, its common name is the “pin cushion algae” because it grows in small clumps on rocks and resembles pin cushions used for sewing. There are freshwater and salt water species, including a free-living, unattached ball forming species. The ball forming Cladophora are sometimes called marimo balls. Cladophora columbiana 3 1. Draw the thallus (include the extensive branching) and a single cell of Cladophora. 2. Observe the chloroplast in living cells of Cladophora. How does this chloroplast shape differ form the chloroplast in Chaetomorpha? 3. Iodine stain reacts with starch to form dark patches. Try staining this alga with iodine stain. What do you see? Was the iodine stain positive or negative? What does the result tell you about the carbon storage product utilized by this alga? C. Parenchymatous Forms Key Organisms: Ulva taeniata (Class Ulvophyceae, Order Ulvales) Ulva spp. are common and widespread throughout the world. 1. Observe and draw the thallus morphology of Ulva taeniata with the naked eye and under the dissecting scope. Where does growth appear to occur? Is it diffuse, or localized to a meristem? 2. Make a thin cross section of Ulva taeniata and draw the internal details. How many cells thick is the blade? What is the shape of the chloroplast? Where is the chloroplast located within the thallus? How many chloroplasts are there per cell? 3. The gametes of Ulva are positively phototactic, whereas the zygotes and zoospores are negatively phototactic. Can you hypothesize a reason for this? For ideas, look over the diagram of Ulva’s life history at the end of this lab. D. Siphonous Forms Key Organisms: Codium fragile and Codium setchellii (Class Ulvophyceae, Order Bryopsidales) Codium fragile Codium setchellii 1. Codium is a siphonous alga which is multinucleate and unicellular; the siphons are intertwined to form a spongy, pseudoparenchymatous thallus. Observe and draw the overall external morphology of Codium (both species). 2. Squash a small piece of Codium sp. on a microscope slide and look at it under the compound scope. Draw what you see. Do you see cross walls? Are there chloroplasts in the siphons? Where are they located? What is their shape? Do you see any reproductive structures? 3. Look over the diagram of the life history and close-up showing utricles. Did you see any utricles in your squash above? What is the function of the utricles? 4 E. Identifying Algae to Species Draw and key out the unknown specimen from the division Chlorophyta. It is important to write down the steps you take as you work through the key. This way, if you are incorrect, you can retrace your steps much more quickly and find where you went wrong. If you desire more practice working with the Chlorophyta dichotomous key you may key out individual samples presented to you as “knowns” from this lab. 5 6 Li f e Cy c l e of Cladophora columbiana Is o m o r p h i c a lt er n a t i o n o f g e n e ra t i o n s m ei o s i s o c cu r s i n apical ce lls 1N zoospore 2 N f i la m en to u s s p o r o p h y t e 1 N f i l a m e n t o u s g a m e to p h yt e s 1N gametes sy n g a m y 2N zygote 7 Li f e C yc l e oof Codium fragile An i m a l- l i k e ( di p l o n t i c fragile) sy n g a m y 1 N g a m e t es m e i o si s 2 N o cc u r s i n z y g o t e u t r i c l e s 2 N a d u l t 8 Li fe Cy c l e of Ulva spp.