7.2.2 Algae: The Plagt Hike Protists
Algae are mostly aquatic and found in marine and freshwater ponds, lakes, moist soil, streams, hot springs, polar ice, moist rocks, and trees. Their plant body may be unicellular or multicellular, some are filamentous. The filaments are either unicellular or multicellular. The multinucleated filaments lack cross walls (ie. coenocytic) or distinct cells.
Many multicellular algae possess leaf-like or branched bodies called thallus. Chlorophyll a, carotenoids, xanthophylls, and phycoerythrin are their photosynthetic pigments. The life cycle of algae is extremely varied, many have isomorphic alteration of generation.
All algae except rod algae exhibit flagella at some stage of their life cycle. How do algae differ from plants? Their sex organs are mostly unicellular. There is no embryo formation in algae and having a simple unorganized body.
Major groups of algae:
1. Euglenoids are unicellular freshwater organisms. Their body is covered by protein pellicles e.g., Euglena.
2. Dinoflagellates are also unicellular and vary in color from yellow, and green to brown.
3. Diatoms: Usually unicellular, found in water and marine, and are the most numerous algae found in oceans. They are called pastures of the sea because they are an important source of food in marine food webs. The body shape is like a box because the cell wall possesses two halves, the bigger half acts as a lid for the smaller half.
4. Brown algae: Multicellular plant-like algae live in colder marine water. They are the largest of all alga, ranging from a small form with simple filaments to giant "kelps" (up to 75 meters in length).
Red algae: Mostly multicellular some unicellular prefer warm sea water. In size they may be up to one meter long and attached to the rock or other submerged objects by a root-like baal hold fest.
6. Green algae: May be unicellular (such as Chlorella and Chlamydomonas), Colonial (Volvox), or multicellular and filamentous (Spirogyra and Ulva). Mostly marine, some fresh water, and a few terrestrial. Mostly green, some may be orange, red, or rust color.
7.2.3 Fungi-like protists: Myxomycota and Oomycota
Myxomycota: Slime Molds:
The slime molds take many forms, during their life cycle and resemble other types of protists. They have a mobile amoeba-like feeding stage and a stationary filamentous, saprotrophic fungi-like reproductive stage that produces spores. The acellular slime mold consists of a mass of cytoplasm (Plasmodium) containing thousands of diploid nuclei that are multinucleated and covered by a slime sheath. Therefore, also called plasmodial slime mold.
Sporangia are their reproductive structures which during unfavourable conditions produce spores by meiosis. In plasmodial slime mold, spores release a haploid flagellated cell or an amoeboid cell. Eventually, two cells fuse to form a diploid zygote that feeds and grows, producing a multinucleated Plasmodium once again by mitosis.
Slime molds differ from fungi due to the presence of a motile stage in their life cycle e.g., Physarum polycephalum procephalic is an example of a plasmodial slime mold.
Oomycota: (The water molds):
Oomycetes are either parasites or saprotrophs which feed on humus. Their cell wall unlike fungi contains cellulose instead of chitin while fungi possess filamentous structures called hyphae. The hyphae are aseptate (coenocyte) that is without intercellular septum or cell wall between nuclei.
They asexually produce zoospores in their sporangium, which are biflagellates and motile. All oomycotes also exhibit sexual reproduction. There are two types of gametangia. The female gametangia is called oogonium while the male gametangia is called an antheridium. The gametangia produce gametes by meiosis, thus showing gametic meiosis.
The male gametes flow from antheridium to oogonium leading to the fusion of one or more pairs of male nuclei with eggs to form zygotes (resulting in a diploid phase). The zygotes are soon covered by a thick cell wall known as oospores. Due to this special kind of thick cell wall on oospores, their phylum is named oomycete, e.g., water molds, white rust, and downy mildew. and downy mildew.
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