Normally our index is one based upon the seven properties of life: cellular structure, homeostasis, growth, movement, reproduction, response, and evolution. As for Chromista, you are challenged to find these properties among the descriptions for several example members of Kingdom Rhodophyta.
|Clickable Index of Rhodophyta|
What are Rhodophyta?
We have learned that Biology can be illustrated as a tree of life:
We have learned that the former Kingdom Protista was formulated as a catch-all holding any unicellular aquatic organisms that could not certainly be included in other kingdoms. Kingdom Protista was obviously polyphyletic (unnaturally putting unrelated groups together). The dissection of Protista has been underway for many years now and the kingdom has been divided into several more-natural kingdoms. One of these is the new Kingdom Rhodophyta (aka Kingdom Biliphyta), shown above as a yellow branch on the tree of life.
The Rhodophytes are photosynthetic and mostly marine red algae. Their bodies are multicellular, supported by water, and are therefore a simple thallus. The benthic forms are attached to substrates by a holdfast but pelagic forms are also known (though rare). The chloroplasts have three membranes, indicating their orgin as a secondary endosymbiosis. The photosynthetic pigments include chlorophyll a and phycoerythrin and phycocyanin. This combination of major pigments means that the thallus can be red, brown, purple, blue, green, and even blackish. After photosynthesis, excess carbohydrate is stored as floridean starch (an α-1,4-glycan) in the cytosol. The structural polysaccharides in the walls include cellulose, agar, and carrageenan. Some of the polymers are very useful in microbiology media and a range of dairy products.
The Rhodophytes never have flagella and so the cells also have no centrioles. The movement of reproductive cells is accomplished solely by water currents. Cells that contact each other's surfaces must be adhesive.
The Rhodophyte thalllus produces gametangia. The male gametangium is called a spermatangium; the female gametangium is called the carpogonium. The non-motile male gamete is called a spermatium.
The 4000 or so species of Rhodophyta include many that accumulate calcium carbonate that encrust surfaces, cement objects, and contribute to tropical and other reef systems.
Below are a group of four photos of commonly-available Rhodophytes that you may observe in laboratory.
Porphyridium is a marine red alga that is unicellular. These are adhesive onto surfaces and to each other, often forming encrusting sheets of cells in the environment. In the photo below, the pinkish area has been artificially colored to show the interesting and unique chloroplasts of the Rhodophytes.
Polysiphonia nigrescens is a benthic (attached) filamentous Rhodophyte that is common in the marine communitites along the New England rocky coast. The thallus looks like some pale pinkish fuzz covering marine surfaces as a sort of turf alga.
Looking at this thallus a bit closer, we can see that it is multi-seriate (the filaments are more than one cell wide). We can also observe the true branching we would expect of higher filamentous organisms. If we measure the amount of DNA in the nucleus of each cell, we discover that this is a diploid (2N) sporophyte. Inside the filaments we observe tetrasporangia. These contained a 2N tetrasporocyte that divides by meiosis to produce four (tetra!) tetraspores. the brown objects in the filament below, then, are tetraspores (meiospores).
The haploid tetraspore is released from this filament and is carried by water currents and tides to new locations. They settle and adhere to marine surfaces and then germinate into filamentous isomorphic homothallic gametophyte thalli. This means that the gametophytes are similar (isomorphic) pinkish turf algae and they have both genders of reproductive structures on the same body (homothallic).
Microscopic examination of the gametophyte thallus above under the right conditions would reveal that male branches near the tops of the thalli produce spermatangia. Female branches near the base of the thalli produce carpogonia. Below is the tip of a male branch showing several spermatangia. Inside each spermatangium are many hundreds of developing spermatia. These will soon be relased into the water currents to be carried to female branches.
The female branches produce carpogonia as shown below. This structure has a long, projecting trichogyne that attaches to any spermatium that comes into contact with it. The trichogyne below shows about three spermatia attached to it, the most obvious near the tip.
As the spermatium and the egg in the carpogonium unite in syngamy, the diploid zygote develops within the base of the carpogonium (now called the cystocarp) attached to the haploid thallus.
The cystocarp expands and matures into a carposporangium. Inside this carposporangium is a diploid carposporocyte. This cell divides mitotically to make a diploid carpospore.
The carposporangia release the carpospores to the marine currents and tides. They settle down on a surface and germinate by mitosis and grow into the diploid tetrasporophyte that we met at the start of this life cycle discussion.
You might notice how the tetrasporangium has four (tetra) tetraspores packed into each tetrasporangium. These meiotic products are in a tetrahedral arrangement, so viewed from one side, you can see three of the four (the fourth is on the back side of the group of three that you see). Each spore has a circular contact area with the other three spores in the tetrahedron, and the spore will have a tri-radiate ridge marking in this circular contact area.
To summarize this life history, I show you this composite diagram...the references for the photos in this composite are included above with each of the individual photographs.
This page © Ross E. Koning 1994.
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