Normally our index is one based upon the seven properties of life: cellular structure, homeostasis, growth, movement, reproduction, response, and evolution. For Chromista, you are challenged to find these properties among the descriptions for several example members of Kingdom Chromista.
| Clickable Index of Chromista | ||
|---|---|---|
| Ectocarpus | ||
| Fucus | ||
| Diatoms | ||
| Water Molds | ||
| Chytrids | ||
What are Chromista?
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 Chromista (aka Kingdom Stramenopiles), shown above as a yellow branch on the tree of life.
The diagram also shows that one distinguishing feature of Chromista is that, in spite of the endosymbises that produced the first eukaryotic organisms, chromists have obtained their chloroplast secondarily (as we observed in euglenoids). In this group the chloroplast arrived apparently from another eukaryotic source... perhaps an ancient green alga. The evidence for this is that the chromist chloroplast has three bounding membranes. The outermost membrane gives the strong appearance of rough endoplasmic reticulum. The two interior membranes are perhaps more typical of green algal chloroplasts. Between the outermost membrane and the two interior membranes lies a nucleomorph. This is the remains of the green algal nucleus with a bit of its cytoplasm. This three-layered organelle is more specifically called the phaeoplast as it is more than just a chloroplast.
Within Kingdom Chromista are not only the brown algae and the yellow-brown algae, but also the egg fungi and chytrid fungi. The latter two do not have chloroplasts or phaeoplasts! So how did they get into this kingdom? These are either chromists that either never obtained phaeoplasts or that lost them later in evolution. The feature that brings them into Chromista is instead their flagella. The members of Chromista have "tinsel type" flagella meaning that the flagellum is "decorated" with brush-like extensions all around the main body of the flagellum. It looks like a bottle brush or some holiday tinsel. One interpretation of this is that chromists also received a unique spirochete endosymbiont that evolved into their unique flagellum. This feature, of course, is not the only feature that includes them.
The bodies of most of the chromists are actually multicellular, in fact some are quite large. Certain brown algae get to be 45 meters tall (!) of course supported by marine waters and attached to the seabed by a complicated holdfast organ. The organisms often have leaf-like appendages and gas-containing floats. Other species lack holdfasts and are pelagic. The Sargasso sea is a vast region of the North Atlantic ocean in which the ocean currents surround floating mats of Sargassum weed...a chromist.
For the photosynthetic members of Chromista, the photosynthetic pigments include chlorophyll a, chlorophyll c, and fucoxanthin. The result of this combination is that the plants are generally brownish to khaki in color. The photosynthetic products are converted to a range of polysaccharides. The storage polysaccharides include laminarin. The cell wall polysaccharides are cellulose and alginic acid.
The chromists also have reproductive structures. Zoosporangia produce flagellated zoospores by mitosis for asexual reproduction. Antheridia produce male gametes and oogonia produce female gametes by mitosis in haploid bodies. The gametes join in syngamy. The diploid bodies have sporocytes that divide by meiosis to make meiospores (often zoospores). So these organisms are capable of sexual reproducation as well. Motile gametes and zoospores (haploid and diploid) have the tinsel-type flagella.
The gametes found among chromists include isogametes (both male and female are the same size and both are motile), anisogametes (female gamete is larger than the male gamete but both are motile), and oogametes (the large female gamete is sessile and the small male gamete is motile). In the latter case, we sometimes call the one gamete an egg and the other a sperm.
These features are all found within the major phylum of chromists known as Phaeophyta (the brown algae). This phylum includes 1500 species. We will now look in more detail at some specific examples.
| Ectocarpus siliculosis |
|
Ectocarpus siliculosus is a marine filamentous brown alga. This means that macroscopically it looks like brown fur growing on marine surfaces. The body (called a thallus) exists in three forms that look very similar...what we call isomorphic thalli. The bodies show true branching of the thallus.
Two different but macroscopically indistinguishable fuzzy thalli produce plurilocular gametangia that release hundreds isogametes. These two bodies (one of each mating type) are what we call the gametophytes. They are haploid in genetic constitution and produce the gametes by mitosis.
http://www.biology.lsa.umich.edu/courses/bio458/Ectocarpus.jpg
The third kind of Ectocarpus thallus is, again indestinguishable macroscopically, looking just like more brown fuzz. Microscopically it is different. This filament is made of cells that are diploid rather than haploid. This thallus produces unilocular sporangia with diploid cells undergoing meiosis to produce unicellular haploid zoospores. This thallus is therefore called the sporophyte. The meiotically-produced zoospores swim away, settle down on a marine surface, and grow by mitosis into gametophytes...about half of the zoospores will make gametophytes of one mating type, the rest will be the opposite mating type.
http://www.biology.lsa.umich.edu/courses/bio458/Ectocarpus--2.jpg
The sporophyte of Ectocarpus can also produce plurilocular sporangia that are morphologically indestinguishable from those of the gametophyte and contain cells produced by mitosis. But the motile cells that swim away from these plurilocular sporangia are, of course, diploid zoospores rather than gametes. They swim away, settle down on a marine surface, and grow by mitosis into a new sporophyte, thus cloning the sporophyte. This would be an example of asexual reproduction.
A view of a haploid gamete of ectocarpus is also instructive:
http://www.biologie.uni-erlangen.de/botanik1/photobiologie/images/kap9/abb9-22.JPG
This gamete shows the flagellum (FS) with the eukaryotic arrangement of microtubules (none pairs surrounding two pairs) inside the flagellum. The phaeoplast (Ch) is shown with the stacked thylakoids and dense fucoxanthin granules indicated by two arrows. The nucleus (Nu) is centrally located. There are a large number of mitochondria (M) in the cell for respiration. There are several unmarked vacuoles. The cell wall is very thin and the cell membrane is tightly appressed to it.
http://hjem.get2net.dk/bojensen/EssentialOilsEng/EssentialOils05/ectocarpene.gif
What is interesting about these gametes is that the "female" gamete is positively thigmotactic. This means if it touches down on a surface, it settles down and attaches to the surface. It then secretes ectocarpene a chemical that diffuses into the surrounding water. The "male" gamete has positive chemotaxis: it swims toward the source of ectocarpene. Should water current or other movement start to move the cell away from the source, the male gamete shows inverse negative chemotaxis: it changes swimming direction when the decreasing concentration of ectocarpene is detected.
Here then is a diagram of the life history of Ectocarpus.
| Fucus spiralis |
|
Our next example brown alga is the common rockweed, Fucus spiralis. This brown alga lives on the rocks along the marine coasts of New England (among many other places). This organism has a holdfast for attachment to the rocks at the shore. The ribbon-like thallus is dichotomously branched and has air bladders for floatation when under water. There are also much smaller swollen spots called conceptacles for reproduction. Because Fucus lives in the tidal zone, at low tide its thallus lies exposed on the rocks as shown here, but at high tide the air bladders lift the thallus into a more upright position in the water.
http://www.theseashore.org.uk/theseashore/Resources%20for%20seashoreweb/Copy%20of%20Fucus%20spiralis.jpg
The thallus that you see on the rocks is diploid. It is therefore the sporophyte. The conceptacles of the thallus produce a 2N sporangium. Meiosis occurs to make haploid cells. These become either antheridium or oogonium. In these 1N gametangia, gametes are produced by mitosis. Fucus shows oogamy...so oogonia release eggs into the water...the antheridia release sperm into the water. The sperm is chemotactic toward the egg. Upon contact, the gametes join in syngamy to make a diploid zygote. This cell divides asymmetrically into a small holdfast cell and a larger prothallial cell. This thallus settles down on a substrate and the holdfast attaches to the rocks. The prothallial cell grows into a new thallus. This kind of life history is sometimes called gametic.
Here is the life history of Fucus.
| Diatoms |
|
An old taxon, Chrysophyta, includes the diatoms. These are quite important in terms of the earth's primary productivity as their numbers are very large in aquatic environments world-wide.
Diatoms have cell walls that are fortified with silicon dioxide making them very strong and quite resistant to change over time. The so called frustules of diatoms persist in the fossil record, and deep deposits of diatom frustules are mined as "diatomaceous earth". These almost glass-like porous deposits can be used for filtration of wine or other liquids; they are used in powdered or other forms in the filters for swimming pools. Diatomaceous earth can be poured on wet paint lines on roads to make them reflect light for driver safety.
http://www.ucmp.berkeley.edu/chromista/diatoms/diatomdiverse.jpg
The frustules have ridges, valleys, and pits in the silicates that are species-specific. In the 1800s, the patterns on diatom frustules were used to sell microscopes by demonstrating the outstanding resolution of the optics. A 19th century pastime was creating microscopic works of art using diatom frustules on glass slides for observation in the microscope. In a living diatom, the frustules fit together much like the two halves of a Petri dish. One shell is larger in diameter, the other smaller in diameter.
http://www.ucmp.berkeley.edu/chromista/diatoms/diatomthecae.gif
The diatom cytoplasm has one or more chloroplasts, and just as in brown algae, the photosynthetic pigments include chlorophyll a, chlorophyll c, and xanthophylls. The products of photosynthesis are generally stored as oils in oleosomes.
http://otl.stanford.edu/about/brainstorm/archives/summer03/Images/Diatoms.jpg
Asexual reproduction in diatoms is achieved by mitosis. Each half-cell produces a smaller internal frustule, so each generation gets smaller than the previous.
Sexual reproduction occurs when the cell size reaches a certain minimum. The male diatom produces motile sperm by meiosis. Meiosis in a female diatom produces a sessile egg. This gamete system is an example of which of these descriptions: isogamous, anisogamous, oogamous? Syngamy produces a zygote in the female frustule. The zygote leaves its frustule behind, grows to maximum size for the species, and then secretes new frustules to complete the life cycle and to achieve the full cell size for the species that may have been lost to rounds of mitosis.
Synedra, shown below, shows the chloroplasts, oleosomes, and the ridges in the two frustules. The cell is a fusiform (cigar) shape.
http://forest.mtu.edu/students/ebwright/algaefiles/ew_lastream1_4020_Synedra.jpg
Navicula in the image below shows two chloroplasts, oleosomes, the nucleus in the middle of the view, and the long groove in the frustule between the two half-cells.
http://www.systematyka.republika.pl/pictures/eukaryota/chrysophyta/navicula.jpg
| Water Molds |
|
In addition to brown and yellow-brown algae, described in a few examples above, Kingdom Chromista also includes some non-photosynthetic species. These chemoheterotrophs have some characteristics that, at first, confused scientists thinking they were fungi. But these similarities are clearly merely superficial, and deeper study has shown clearly that these organisms are instead Chromists! The phylum Oomycota includes the water molds.
Water molds include the species Ichthyophthirius multifiliis which is a parasite of fish. The disease this mold causes is commonly called "ich." The symptoms of this disease are white patches of mycelium growing on the fish's surfaces.
http://aquaworld.netfirms.com/Disease/SCHIMMELICK.JPG
A closer look reveals what is growing at the tips of the fuzzy mycelia. The photo below shows a small portion of a dead host (maybe an insect in the lower-left corner). Along the left edge is the diploid hypha. The cylinder pointing to the upper-right is a zoosporangium. The cells inside have been produced by mitosis, so are diploid. When the tiny operculum (cap) on the end of the zoosposrangium opens, hundreds of motile zoospores will be released into the water to infect other fish in the environment. These diploid zoospores have the tinsel-type flagellum. All cell walls are polysaccharide of chromista rather than the chitin of fungi. So indeed water molds are chromists and not fungi.
http://www.biology.lsa.umich.edu/courses/bio255/Achlya.jpg
When the resources being taken from the host or saprobic food sources are running low, the water mold responds by moving into sexual reproduction. Here we see in the left panel the antheridia and oogonium developing. Inside the oogonium the oospheres (eggs) have formed.
http://protist.i.hosei.ac.jp/PDB/Images/Eumycota/Achlya/Achlya.jpg
In the right panel above, the eggs and sperm have joined together to form zygotes. These rest as hypnospores through difficult environmental conditions, and crack open under good conditions to release diploid zoospores that colonize more hosts.
To help understand the life history of water molds, below is the life cycle of another example, Saprolegnia.
One water mold, Phytophthora infestans, was an important contributor to human history...perhaps even that of your own family. After Europeans found North America and over-ran the native Americans and took their lands, they also stole the germplasm (seeds and other crops) from these peoples. One of these crop plants was the potato (Solanum tuberosum). This plant produces large underground stems called tubers. These are edible and the plants could out-produce grain crops in limited farm spaces. Moreover, a single tuber could be cut into pieces and planted like seeds to establish a short row of potato plants in the field. One potato variety from South America was imported to the island nation of Ireland. Irish farmers grew the potatoes and shared them with others. Before too long, Irish farms were producing bumper crops of potatoes, and farmers were abandoning their traditional wheat, oats, and barley crops.
In the late 1840s, Ireland was hit with the disease known now as "late potato blight." The potatoes that the Irish farmers were growing were all one genotype, cloned by cutting up tubers...and they were susceptible to this disease. The Irish potato crops failed several years in a row. Irish people were starving by the millions. Because Irish farmers did not own their farms, they were merely tenant farmers required to pay rent to English landlords. Without any income from crops, the Irish could not pay their rent, and were evicted from the land. Those who had no money left became homeless and many starved to death. Those who had any money hopped onto ships and emigrated to the US and elsewhere in the world. If your family emigrated to the US in the 1850s or so, you may thank, in part, the water mold, Phythphthora infestans for your American freedom!
| Chytrids |
|
As our last example of Kingdom Chromista, we are looking at parasitic organisms that also often attack in moist environments, the chitrids. Our example chytrid here is Allomyces arbuscula.
http://ocid.nacse.org/classroom/fungi/bot461/images/Chytridiomycota/Allomyces/allomycesgametangia2.jpg
As you can observe above, the haploid thalllus consists of filamentous cells that have been called mycelium. This name is an artifact of their former status in Kingdom Fungi in previous decades. Near the tips of the hyphae (filament) you can see swollen gametangia. The gametangia develop one or more papillae (bumps) that open to release motile gametes. The smaller orange gametangia release smaller anisogametes and so can be considered antheridia. The male gametes are orange. The larger gametangia are colorless and release colorless but larger female gametes and can be considered oogonia. The two motile gametes swim with tinsel-type flagella (typical of Chromista), are attracted to each other chemotactically, and unite in syngamy in the water. The zygote is a motile zoospore (with tinsel-type flagellum) that settles down in the environment to grow into a diploid thallus.
http://www.ucmp.berkeley.edu/fungi/allomyces.jpg
The diploid sporophyte thallus above shows the filamentous structure as well as sporangia. The diploid sporangium has a thick wall for enduring less than ideal conditions as a hypnospore. You can see them breaking away from the mycelium in the lower right corner of the micrograph above. Once better conditions arrive, the sporocyte cell inside the thick wall divides by meiosis, and releases four meiospores that are motile (therefore zoospores!). These zoospores swim away to produce new gametophytes elsewhere in the environment.
Below is a sketch of the life history of Allomyces arbuscula.
The chytrids are very similar, then, to the water molds, but one difference from them is the use of chitin (N-acetylglucosamine) rather than cellulose (β-1,4 glucan) in the cell wall. Fungi have chitinous walls, so should chytrids be classified as fungi on the basis of this one trait? Members of Kingdom Fungi never have motile gametes and zoospores, so should that exclude chytrids from Kingdom Fungi? If we are not sure, should we keep chytrids in Kingdom Protista? Well, macromolecule sequences (DNA and protein) seem to indicate that these are either Fungi or Chromists. So we will decide for this course to leave them in Chromista for now, remembering that in future years, further study may reveal that they belong somewhere else in the tree of life!
Summary
Chromista
This page © Ross E. Koning 1994.
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