Overview of Protists,Protist Morpholog,Encystment and Excystment,Protist Reproductive Cells and Structures,Protist Taxonomy

Most eukaryotes are microbes. 

 It is therefore not surprising that the vast diversity of protists is a function of their capacity to thrive in a wide variety of habitats.

 Their one common require­ ment is moisture because all are susceptible to desiccation.

 Most protists are free living and inhabit freshwater or marine environ­ ments.

 Many terrestrial chemoorganotrophic forms can be found in decaying organic matter and soil. 

Whether terrestrial or aquatic, protists play an important role in nutrient cycling.

Protozoa, or chemoorganoheterotrophic protists, may be saprophytes, securing nutrients from dead organic material by releasing degradative enzymes into the environment. 

 They then absorb the soluble products-a process sometimes called osmotrophy.

 Other protozoa employ holozoic nutrition, in which solid nutrients are acquired by phagocytosis.

 Photo­ autotrophic protists are strict aerobes and, like cyanobacteria, use photosystems I and II to perform oxygenic photosynthesis.

 It is difficult to classify the nutritional strategies of some protists be­ cause they simultaneously use both reduced organic molecules and C02 as carbon sources. 

This strategy is called mixotrophy.

Protist Morphology

Despite their diversity, protists share many common features. 

 In many respects, their morphology and physiology are the same as the cells of multicellular plants and animals.

 However, because many protists are unicellular, all of life's various functions must be performed within a single cell.

 Those that are multicellular lack highly differentiated tissues. Therefore the structural complexity observed in protists arises at the level of specialized organelles.

The protist cell membrane is called the plasmalemma and is identical to that of multicellular organisms.

 In some protists, the cytoplasm immediately under the plasmalemma is divided into an outer gelatinous region called the ectoplasm and an  inner  fluid  region,  the  endoplasm.  

The  ectoplasm  imparts rigidity to the cell body.

 Many protists also have a supportive mechanism called the pellicle.

 The pellicle consists of the plasma­ lemma and a relatively rigid layer just beneath it. The pellicle may

be simple in structure. 

For example, Euglena spp. are protists with a series of overlapping strips with a ridge at the edge of each strip fitting into a groove on the adjacent one, much like how the "tongue-and-groove" boards of a hardwood floor fit together.

 In contrast, the pellicles of ciliate protists are exceptionally complex with two membranes and a variety of associated structures. 

Al­ though pellicles are not as strong and rigid as cell walls, for those that possess them, pellicles impart the characteristic shape asso­ ciated with that particular species.

One or more vacuoles are usually present in the cytoplasm of protozoa.

 These are differentiated into contractile, secretory, and food or phagocytic vacuoles.

 Contractile vacuoles function as osmoregulatory organelles in those protists that live in hypotonic environments, such as freshwater lakes. 

Osmotic balance is maintained by continuous water expulsion.

 Phagocytic vacuoles are conspicuous in protists that ingest food by phagocytosis ( ho­ lozoic protists) and in parasitic species. Phagocytic vacuoles are the sites of food digestion.

 In some organisms, they may occur anywhere on the cell surface, while others have a specialized structure for phagocytosis called the cytostome (cell mouth). 

When digestion commences, the phagocytic vacuole is acidic, and as digestion proceeds, the vacuole membrane forms small blebs.

 These pinch off and carry nutrients throughout the cyto­plasm. 

The undigested contents of the original phagocytic vacu­ ole are expelled from the cell either at a random site on the cell membrane or at a designated position called the cytoproct.

Several energy-conserving organelles are observed in protists.

 Most aerobic chemoorganotrophic protists have mitochondria, while photosynthetic forms have chloroplasts.

 A dense protein­ aceous area, the pyrenoid, which is associated with the synthesis and storage of starch, may be present in chloroplasts. 

The majority of anaerobic chemoorganotrophic protists lack mitochondria; some of these organisms have hydrogenosomes.

Many protists feature cilia or flagella at some point in their life cycle. Their formation is associated with a basal bodylike or­ ganelle called the kinetosome. In addition to aiding in motility, these organelles may be used to generate water currents for feed­ ing and respiration.

Encystment and Excystment

Many protists are capable of encystment. During encystment, the organism becomes simpler in morphology and develops into a resting stage called a cyst. The cyst is a dormant form marked by the presence of a cell wall and very low metabolic activity. Cyst formation is particularly common among aquatic, free-living protists and parasitic forms. Cysts serve three major functions:

(1) they protect against adverse changes in the environment, such as nutrient deficiency, desiccation, adverse pH, and low levels of 02

 (2) they are sites for nuclear reorganization and cell division (reproductive cysts) and

 (3) they serve as a means of transfer between hosts in parasitic species (i.e., they are the infective stage).

 Protists escape from cysts by a process called excystment . Although the exact stimulus for excystment is un­ known for most protists, it is generally triggered by a return to favorable environmental conditions. For example, cysts of para­ sitic species excyst after ingestion by the host.

Most protists have both asexual and sexual reproductive phases in their life cycles. The most common method of asexual repro­ duction is binary fission. During this process, the nucleus first undergoes mitosis and then the cytoplasm divides by cytokinesis to form two identical individuals . Multiple fission is also common, as is budding. Some filamentous, photosynthetic protists undergo fragmentation so that each piece of the broken filament grows independently.

Sexual reproduction involves the formation of gametes. Protist cells that produce gametes are termed gamonts. The fusion of haploid gametes is called syngamy. Among protists, syngamy can involve the fusion of two morphologically simi­ lar gametes (isogamy) or two morphologically different types . Meiosis may occur before the formation and union of gametes, as in most animals, or just after fertilization, as is the case with lower plants. Furthermore, the exchange of nuclear material may occur in the familiar fashion-between two different individuals (conjugation)-or by the develop­ ment of a genetically distinct nucleus within a single individ­ ual (autogamy).

With this level of reproductive complexity, perhaps it is not surprising that the nuclei among protists show considerable di­ versity. Most commonly, a vesicular nucleus is present. This is 1 to  10  1-1m in diameter,  spherical,  and has a distinct nucleolus and uncondensed chromosomes . Ovular nuclei are up to 10 times this size and possess many peripheral nucleoli. Still others have chromosomal nuclei, in which the chromo­ somes remain condensed throughout the cell cycle. Finally, many ciliated forms have two types of nuclei: a large macronu­ cleus with distinct nucleoli and condensed chromatin, and a smaller, diploid micronucleus with dispersed chromatin but lacking nucleoli .

Protist Reproductive Cells and Structures

Most protists have both asexual and sexual reproductive phases in their life cycles. The most common method of asexual repro­ duction is binary fission. During this process, the nucleus first undergoes mitosis and then the cytoplasm divides by cytokinesis to form two identical individuals . Multiple fission is also common, as is budding. Some filamentous, photosynthetic protists undergo fragmentation so that each piece of the broken filament grows independently.

Protist Taxonomy

Ever since Antony van Leeuwenhoek described the first proto­ zoan "animalcule" in 1674, the taxonomic classification of the protists has remained in flux. 

 During the twentieth century, classification schemes were based on morphology rather than evolutionary relationships.

 Protists were often classified into four major groups based on their means of locomotion: flagel­ lates (Mastigophora), ciliates (Infusoria or Ciliophora), amoebae (Sarcodina), and stationary forms (Sporozoa).

 Although these terms may still be encountered, they are without evolutionary context and should be avoided.

 While it is now agreed that the old classification system is best abandoned, little agreement exists on what should take its place. Here we use the higher-level classification system for the eukaryotes based on morphologi­ cal, biochemical, and phylogenetic analyses proposed by the International Society of Protistologists in 2005. This scheme does not use formal hierarchical rank designations such as class and order, reflecting the fact that protist taxonomy remains an area of active research. 

The Classification of the Protists  as proposed by the International Society of Protistologists is presented in a table on the text website.