THE WORLD OF LIFE with Douglas Drenkow

Brown algae are almost entire marine, typically living in cold coastal waters and able to withstand periods of drying out, in "intertidal" zones (between high and low tides). Kelp may create their own environments, which are important sources of food and shelter for other organisms: In the western North Atlantic, the "Sargasso Sea" (of Sargassum species) floats freely; and anchored off the Pacific Coast of North America are vast kelp "forests" (of Macrocystis species, the fastest growing organisms on the planet, able to grow up to three feet a day).

Golden-green algae typically live in the water, mostly freshwater, but occasionally live in damp environments on land.

Golden algae live in saltwater and freshwater.

OVERALL STRUCTURE

The body of oomycota is like that of fungi, with which they have been traditionally classified; however, the cell walls are not composed of "chitin", as in fungi, but rather of compounds related to "cellulose", as in plants. Although they may be unicellular (each cell with a wall), most oomycota are multicellular, in the form of an irregular "mycelium", a mass of multicellular threads ("hyphae"), which are usually "nonseptate" (without cross-walls) and "coenocytic" (with numerous nuclei, from the conjoined cells).

Diatoms are unicellular, although colonies may form filaments. Diatoms may be "planktonic" (drifting) or "sessile" (attached, to the bottom or even to other animals, up to and including whales). The cell wall of a diatom is impregnated with the stony, white mineral silica, giving the cell an often elaborately sculptured or lacey appearance; this wall is divided into two halves, fitting together like the top and bottom of a Petri dish! Deposits of prehistoric diatoms are mined as "diatomaceous earth", as used in swimming pool filters.

Brown algae are entirely multicellular, varying from small, branching filaments to large seaweeds, with mock roots ("holdfasts"), mock stems ("stipes"), and mock leaves ("laminas") with floats ("bladders", holding the laminas up in the light) -- see the section on Internal Transport, below, for the anatomical similarities and differences between these "mock" structures and their counterparts in truly "vascular" plants.

Most golden-green algae are unicellular, with propeller-like flagella; many are colonial (as living within a gelatin); and some are multicellular and filamentous. A cell wall gives each cell shape, although the wall is not composed of "cellulose" (as in higher plants) or of "chitin" (as in fungi); and cross walls may be absent within a filament, each cell thus coenocytic (having many nuclei), like oomycota (above) or fungi (which are probably descended from similar organisms).

Many golden algae are unicellular, a cell wall giving the cell shape; and many golden algae swim, by means of a propeller-like flagellum. Other golden algae are colonial, some forming filaments or spheres. Some are "amoeboid" (like amoebas), during at least part of their life cycle.

ENERGY CAPTURE

Water molds and similar oomycota are "saprophytes" (feeding on dead or waste matter); but many other oomycetes are pathogens (disease-causing organisms) infecting (and, thus, obtaining nourishment from) plants, invertebrates, or vertebrates.

Diatoms capture light-energy, for photosynthesis, by means of pigments (green chlorophylls a and c, aided by yellowish "carotenes") in their chloroplasts (These colors are tinged by brownish "fucoxanthin"). As "phytoplankton", they are a fundamental part of the food chain in the sea.

Brown algae capture light-energy, for photosynthesis, by means of pigments (green chlorophylls a and c, aided by yellowish carotenes) in their chloroplasts (These colors are tinged by brownish fucoxanthin).

Golden-green algae capture light-energy, for photosynthesis, by means of pigments (green chlorophyll a and perhaps c) in their chloroplasts (This color is tinged by yellow "xanthophylls" but not, as in the other photosynthetic stramenopiles, by brownish fucoxanthin).

Golden algae typically capture light-energy, for photosynthesis, by means of pigments (green chlorophylls a and c) in their chloroplasts (This color is tinged by brownish fucoxanthin). However, some golden algae are colorless; and others that are photosynthetic may become "heterotrophic" (not producing their own food molecules) if sunlight is insufficient or if food substances or prey organisms (such as bacteria or diatoms) are readily available to consume.

EXCHANGE OF MATERIALS WITH THE ENVIRONMENT

Water, dissolved gases, and other materials are typically exchanged through a cell membrane via simple diffusion or via "passive" or "active" transport (both forms of transport employing proteins embedded within the membrane, the active form also requiring the biochemical expenditure of energy).

INTERNAL TRANSPORT

Materials are typically moved within eukaryotic cells by the active process of "cytoplasmic streaming". In multicellular diatoms or golden algae, materials simply diffuse or are passively or actively transported between cells.

Within oomycota, materials can move directly from cell to cell, as the cytoplasm is continuous, with no cross-walls in the hyphae.

In cross-section, the stipe of a higher brown alga (kelp) is somewhat similar to the stem of a truly vascular plant (Similar adaptations often arise independently in two, only distantly related groups of organisms, in a process termed "convergent evolution" -- natural selection demands that the most efficient "form follows function"). The "meristoderm" (the outermost layer of cells in such a stipe) functions not only as a protective epidermis but also as a "meristem" (in which cell-division, for growth, takes place). To the inside of this layer there is a thick "cortex", composed of thin-walled, relatively undifferentiated cells, which presumably function in photosynthesis and food-storage (like the similarly structured "parenchyma" cells within the cortex in a stem of a truly vascular plant). The cortex also contains canals of cells that secrete slimy "mucilage", which bathes the "medulla" (the central cylinder of the stipe, analogous to the "stele", or central vascular cylinder, of vascular plants). Within the medulla, there are supportive cells and also food-conducting "sieve cells" (very similar to the "sieve tube members" in the "phloem" tissue of truly vascular plants). Somewhat similar to the woody "secondary" growth in the stems of truly vascular plants, the meristematic region of many kelps is inactive over winter: Like in a tree trunk, "annual rings" form in the large, perennial stipes of such kelps.

DEVELOPMENTAL CONTROL

The growth and development of oomycota, diatoms, golden-green algae, or golden algae are under genetic and perhaps (almost certainly for multicellular species) hormonal control.

As in green plants, cells within the multicellular body of a brown alga divide, grow, and develop in response to such environmental stimuli as light, temperature, gravity, and acidity/alkalinity -- hormones typically communicate the appropriate message from the site of reception to the site of reaction, which (like everything else operating within living cells) is ultimately under genetic control. In addition, the sperms of at least some brown algae locate the eggs of the species by swimming towards "pheromones" (hormones released into the environment).

ASEXUAL REPRODUCTION

In oomycota, the mycelium typically grows until the food supply is exhausted; then, cross-walls typically form towards the tips of the hyphae, forming "sporangia", in each of which numerous spores develop. Typically, these spores are flagellated "zoospores", which emerge, swim about, and ultimately settle down and grow into a new mycelium. For oomycota parasites growing within plants, ultra-lightweight sporangia may be pinched off from hyphae that grow- or burst-out of leaf tissues. These "conidiosporangia" are then carried by air currents to the foliage of other plants. The zoospores that emerge swim out into an often thin film of water (Such diseases are favored by high humidity) and ultimately grow hyphae into and thus infect their new "host" plants.

Diatoms reproduce asexually by dividing in two in an unique fashion: In effect, the top and the bottom of the "Petri dish" each produce the other part; however, the offspring that receives the smaller half of the cell wall forms a yet smaller half, to fit inside, and thus cannot re-grow to the size of the parent -- eventually, the generations along these lines get too small to divide any further (compare with sexual reproduction, below).

Brown algae reproduce asexually from body parts or spores.

Golden-green and golden algae reproduce asexually like most other algae, typically by fission (simple division) but sometimes by fragmentation of body parts or the production of spores (flagellated or not).

SEXUAL REPRODUCTION

In oomycota, the nuclei in the mycelium are "diploid" (each nucleus with both sets of chromosomes), not "haploid" (with just one set) as in true fungi (with which the oomycota were traditionally grouped). Typically in oomycota, cross-walls form at the ends of certain hyphae, some developing into large "oogonia" -- in which (haploid) eggs are produced -- others developing into slender "antheridia" -- in which the (haploid) male gametes are produced (Because these male gametes are unflagellated, some do not consider them true sperms). Note that the diploid cells of the oogonia and antheridia produce the haploid gametes by means of "meoisis" (cell division that cuts the number of chromosomes in half). By the growth of the hyphae, one or more antheridia will come into contact with an oogonium; and short hyphae ("fertilization tubes") will grow from the antheridia into the oogonium, allowing fertilization. The (diploid) fertilized egg develops a heavy wall, thus becoming an "oospore", which will typically resist unfavorable environmental conditions and not germinate, into new (diploid) hyphae, until conditions become more favorable.

In diatoms, the diploid stage of the life cycle (with both sets of chromosomes) is dominant; the haploid stage (with just one set of chromosomes) is reduced to a single-celled gamete, which forms within an old cell wall -- typically, there is only one survivor amongst the four haploid nuclei formed within the cell, by meiosis. These gametes drift about. If two come together, they will fuse, outside of their small silica walls, and create a new diploid cell, which will form a new, full-sized cell wall.

In brown algae, the "sporophyte" (the diploid stage of the life cycle, with both sets of chromosomes) produces (via meiosis) "meiospores". Meiospores develop into the "gametophyte" (the haploid stage of the life cycle, with just one set of chromosomes), which is often very small and may not even be independent of the thus dominant sporophyte. The gametophyte produces the gametes, some of which eventually reach one another through the watery environment: In some species of brown algae, the male and female gametes are similar ("isogametes") and use flagella to swim together; in other species of brown algae, the male and female gametes are different ("oogametes"), the male a true sperm, swimming to the egg, which is either attached to the female gametophyte or released into the water.

Some golden-green algae reproduce sexually, typically with flagellated sperms swimming to non-flagellated eggs (although in some species the gametes are similar to one another).

Golden algae reproduce sexually like most other algae: See green algae.

Eukaryotes (Domain Eukaryota)

Doug@DouglasDrenkow.com

Photo of Cells: H.D.A. Lindquist, US EPA

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