If you missed it, you should review the reading from Chapter 25 to get your coverage of blue-green algae (cyanobacteria). These are prokaryotic organisms that are not particularly well-related to the rest of the algae, but the chloroxybacteria are thought to be relicts of the organisms that became endosymbiotic chloroplasts in the original eukaryotic plant cells.
As we focus our attention on eukaryotic algae, we move into Kingdom Protista...that mixed bag of organisms that we need new young scientists like yourselves to help us understand. Found in this mix of algae are descendants of organisms that gave rise to the higher plants (Kingdom Plantae). Some have argued that the algae that have chlorophyll b and chlorophyll a, carotenoids, store starch and have cellulose walls (the green algae or Chlorophyta) BELONG in Kingdom Plantae. They certainly are transitional, having members that compare favorably to the other, more distantly related, algae. This is one diverse collection of organisms with a bewildering array of different life histories! The study of these organisms is called phycology.
The major groups of algae are listed in Table 26.1 on page 635. Such tables help us compare and contrast the various groups. The table is worth some study, and I will be providing you with a similar (but more detailed) table for following my lectures.
The book makes a point that the eukaryotic algae are polyphyletic (belong to very different phyla) and provides you with a cladogram (Figure 24.14, page 588) as evidence. Notice how each group of algae is shown connecting to the base of this cladogram with no interconnections. Thus the algae are not a single branch with many twigs, but separate limbs! While looking there, notice how the cladogram shows the green algae (chlorophyta). Now you understand my comment above. This is further focused on in Box 27.1 (and its cladograms) on page 639...you can see how the green algae vary in their degree of relation to higher plants.
Generalized life cycles finally appear discussed here (kind of late for the Fungi!). The author attempts to distinguish the gametic, zygotic, and sporic life cycles in Figure 27.5 (page 637). By redrawing each of these in different ways, you never see how they are built upon a single basic plan. He also uses these life cycle types in connection with meiosis which is misleading. While crudely meiosis produces spores in sporic life cycles, spores are produced in many other ways. In zygotic life cycles meiosis occurs in a derivative of a zygote. But in gametic life cycles, meiosis does not occur in gametes, so why would you say "gametic meiosis"? It's an oxymoron! I'd much rather you remember the sporic life cycle as the complete life cycle version with two possible short-cuts: gametic and zygotic. It really is more accurate...humans have a gametic life history (spermatids and ootids are the products of meiosis and they mature to sperm and egg...so meiosis does NOT produce gametes directly contrary to the author's description). Many algae exemplify the zygotic short-cut; the adults are haploid, and the zygote matures to become the sporocyte (the zygote does not undergo meiosis, the sporocyte does!). I'll remind you here of how I want you to remember this:
The author also finally shows us the different styles of motile gametes (isogamous, anisogamous, oogamous). It's kind of late for the water molds and lower fungi, but better late than never (Fig. 27.6 page 637).
Now, what on earth should you study among all this diversity? Well, I'd like you to concentrate on just a few simple ideas. First I'd like you to concentrate on Chlorophyta...as these are considered the "fast path" to the higher plants. I'd like you to learn a few example life cycles:
Beyond these three life cycles, I'd like you to read carefully about Chara (page 645, Figure 27.16-17). The change of the gametangium from a unicellular structure (a cell with contained gametes) to a multicellular structure (one with a sterile jacket of cells around the gametangium) is a major departure. It foreshadows what we shall see in all of the higher plants! If you review the cladogram on page 639 you will see why this alga is considered critical to our "connections" between the algae in Kingdom Protista and the plants in Kingdom Plantae.
Having surged through the close relatives of true plants, we'll kind of coast down through the examples of other algae. In the rest of the chapter, you might read lightly but keep the basic concepts of Table 27.1 on page 635 foremost in your mind. We won't bog down in a discussion of these complicated life histories, nor build up a huge backlog of examples.
The brown algal example of Ectocarpus (pg 647) shows you a life history that happens to be very similar that for Ulva. The main difference is isogamy...so don't sweat it.
The brown algal example of Fucus (pg 649), a common rocky shore representative here in New England, happens to have a life history that is essentially gametic...like humans. The rockweeds are diploid sporophytes, their conceptacles produce meiotic products that (after a few mitoses) mature into egg and sperm, and these unite in the ocean to form a zygote that settles on a rock and grows into an adult rockweed. One might argue that the mitoses after meiosis constitute a haploid (gametophyte) generation; if you think that direction then the life history is sporic...right?
The red algae are interesting...review the table...they have the prokaryotic photosynthetic pigments but are eukaryotic. Porphyridium (Fig 27.26) might be among the slime some of you noticed on the soil of your mystery plants...maybe you should check this out with a wet mount! How would you be sure of your identification?
Go ahead and read the story about Polysiphonia (Fig 27.27), but don't read the text so closely as to get a headache. You might notice some interesting parallels between fungal and algal terms here. Don't memorize the life cycle.
The chrysophytes include the diatoms. These useful organisms are one of the examples of plants impregnating cell walls with silicon. Diatom cell walls were used to demonstrate the resolution of microscopes in the early part of this century. In lab, I may have a demonstration slide to show you where diatom species are specially arranged into a beautiful arrangement. The walls of dead diatoms (diatomaceous earth) are used as a filtering agent for swimming pools and wines. They also make paints reflective when the dust is dispersed over the wet white and yellow lines on our highways.
Euglenophyta and Pyrrhophyta are fairly primitive algae, but have wonderfully interesting features. Euglenoids have a protein pellicle instead of a wall, and it is on the INSIDE of the cell membrane. They also store paramylum instead of starch. You may see this in lab. The dinoflagellates (Pyrrhophyta) are responsible for red tides and make many shellfish toxic to humans and kill fish when dinoflagellates are abundant. At night, dinoflagellates are sometimes bioluminescent. A common Bermuda observation in the Tropical Biology course is these critters "flashing" in the bowl during nocturnal urination (the toilets use sea water to conserve limited fresh water). In lab this week I hope we will see bioluminescence in a fungus that grew too slowly for you to observe last week.
Read the rest of the chapter for some light reading on the amazing lives and uses for algae. Also notice the boxed reading on page 652; what fungal parallel does this bring to mind?
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