Chapter 2. Eukaryotic cells and the Algae

 

Eukaryotic cells, with nuclei, cytoskeletons and dynamic, interacting internal organelles, make up the large, multicellular forms life we are most familiar with:  plants, animals, and fungi, as well as  a vast assortment of simple organisms traditionally referred to as protists, which include photosynthetic forms known informally as algae.

1. Eukaryotic cells arose as a result of endosymbiosis between an early amoeboid cell and prokaryotes which were not digested but instead were  "domesticated" to become mitochondria and chloroplasts.

2. All higher plants are photosynthetic only by virtue of their possession of chloroplasts descended from cyanobacteria, and so one could say that cyanobacteria are the only true plants. 

3. Protists constitute a vast polyphyletic array of simple, animal--like, fungus-like, and plant-like organisms inhabiting primarily aquatic or marine environments.

4. Plant-like protists, i.e. the different kinds of algae, have either red, green, or brown chloroplasts,a basis for their traditional classification.

5. These varied algae arose multiple times, through primary, secondary, or even tertiary endosymbiosis in some cases.

 

 

Figure 2.1. The "photosynthetic" sea slug, Elysia clarkii, is one of a handful of animals that 

feed upon  green algae, but do not digest their chloroplasts, a process sometimes dubbed as 

"kleptoplasty." The chloroplasts are maintained  internally for a number of months and the 

animal benefits from the photosynthetic product. This process is analogous to how the first 

photosynthetic eukaryotes formed. 

Photographed in the laboratory of Sydney K. Pierce, University of South Florida. 

 

Figure 2.2. As a second example of mutualistic symbiosis among higher organisms, 

a lichen is an intimate association between a fungus and algal cells. The algae provide 

photosynthetic product to the fungus, which, in turn, provides water and minerals, 

as well as protection, for the algae. Drawing from Brown 1935.  

Figure 2.3. Amoeboid cells change shape by extending portions of the internal cytoskeleton 

and the adjacent  flexible cell membrane. This allows the cell to move in a particular direction, 

as well as to form pockets around food items. A prokaryotic organism that evolved this ability

 was the ancestor of the first eukaryotes. Redrawn from Hartog 1906.

Figure 2.4. The first eukaryotic cells evolved from amoeboid prokaryotes with cytoskeletons. 

Parts of the cell membrane were internalized to form not only food vacuoles and other internal 

structures, but also a double membrane around  the mass of DNA strands, forming a true nucleus. 

The cytoskeleton allowed cells to become much larger, as it provided a mechanism to move 

nutrients from the outside inward and around the interior of the cell. Aerobic bacteria were 

captured and domesticated, becoming mitochondria. Later, cyanobacteria were likewise 

captured and became chloroplasts. These events are considered primary endosymbiosis.

 

Figure 2. 5. Flagella are essentially extensions of the cytoskeleton extruding from 

eukaryotic cells, providing a means of mobility.  Internally, they consist of paired microtubules 

that can bend in a coordinated manner, creating a whip-like swimming motion. The nearly 

identical internal structure of the flagella among protists and the sperm cells of both plants 

and animals suggests that they were present in the common ancestors of all eukaryotic life. On 

the left is a drawing of the single-celled green alga, Chlamydomonas, after Haupt, 1953. 

 

Figure 2.6. The red algae are a natural and highly diverse group named for their possession of

 distinctive reddish chromosomes. Many are multicellular and plant-like as in  

Batrachospermum moniliforme (A, B) and Delessaria sanguinea (C). Red algae are 

peculiar in that not a single species possesses flagella, likely due to adaptation by 

their common ancestors adaptation for dispersal of their sperm cells by ocean currents, for 

which the production of flagella would have been an unnecessary expense.

Drawings from Oltmanns, 1905, attributed to Sirodot. 

Figure 2.7. Kelp (Macrocystis pyrifera) exemplifies the complex plant-like bodies and 

distinct brownish chloroplasts (due to presence of fucoxanthin) of the marine Brown Algae..

Drawing from Oltmanns 1905.

Figure 2.8. Diatoms are distinctive for their glass shells, and have brownish chloroplasts 

like the Brown Algae, but acquired independently through secondary endosymbiosis. 

Drawing modified from Kerner and Oliver 1895. 

Figure 2.9. Paramecium is an animal-like protist that acquired chloroplasts through 

secondary endosymbiosis, or kleptoplasty with chloroplasts from a green alga. B. Ceratium is 

one of the diverse grop known as Dinoflagellates, which have acquired chloroplasts through 

secondary or tertiary endosymbiosis with mostly red and brown algae, though their bodies are 

related to those of ciliates like Paramecium. It is the occasional population explosion of a

 dinoflagellate that causes red tide, which results in large-scale fish kills due to a toxic 

byproduct of dinoflagellate metabolism. Drawings from Oltmanns 1905.

Figure 2.10. A family tree of eukaryotic organisms. The spiky green symbols mark

groups in which symbiosis with chloroplasts has occurred at least once. it is believed

that the original endosymbiosis with a cyanobacterium occurred in the ancestors of Red Algae, 

Green Algae, and terrestrial plants which constitute the supergroup Archaeplastida..The 

chloroplasts of the ancestors of the Red Algae then underwent a pigment shift, developing 

their distinctive red coloration, while terrestrial plants evolved from green algal ancestors 

and kept the green colored chloroplasts. 

    

Figure 2.11. These fossils of Bangiomorpha pubescens, 

dating at just over one billion years ago, are the oldest 

known eukaryotes, and strongly resemble

Red Algae. Photo courtesy N. J. Butterfield.  

 

Figure 2.12. An important innovation of eukaryotes was the separation of the genome 

into multiple linear chromosomes, as opposed to the single circular chromosome typical of 

prokaryotes. Prior to cell division, chromosomes have duplicated themselves, with

the identical chromosomes remaining attached together typically in an "X-shaped 

configuration. During cell division the doubled chromosomes line up along the middle of 

the cell, and then separated and pulled to opposite ends of the cell by a complex cytoskeletal

 array. In mature  cells that are not preparing for division, the chromosomes generally 

remain in the undoubled state.

Figiure 2.13. Meiosis is an extension of mitosis in which there is an extra step of separation 

and division in which homologous chromosomes (those that code for the same traits from 

each parent) line up and randomly separate. In this diagram, blue chromosomes from one

 parent and red chromosomes are from the other parent. The beauty of the eukaryotic 

chromosome system becomes evident in this process as it creates the opportunity for 

the mixing of parental genes in  each generation. 

Figure 2.14. In sexual reproduction, fusion of haploid gametes (each contains one

 complete set of  chromosomes) results in a diploid zygote.that contains two complete 

sets of genes. In some simple algae, like this Chlamydomonas, the zygote is the 

only diploid part of the life cycle, as it will undergo meiosis to produce new haploid 

vegetative cells. The zygote has a tough, water-proof covering, enabling it to survive 

harsh conditions before  it undergoes meiosis to form new haploid cells. 

Figure 2.15. In the marine green alga, Ulva, the flagellate zygote settles down and 

undergoes development into a multicellular diploid phase. Zoospores produced by the 

diploid plant through meiosis swim away and can form new haploid plants.

Figure 2.16. Among eukaryotes, some are primarily haploid, with just a diploid 

zygote; some go through multicellular haploid and diploid phases, and others, animals 

in particular, are primarily diploid, with only haploid gametes.  

Figure 2.17. Among the incredibly diverse green algae there are many unique multicellular 

body/colony forms.  In Volvox, an extravagant relative of Chlamydomonas, cells remain 

linked into an intricate sphere (A) as they multiply through mitotic division. Male gametes

 (sperm cells) form through multiple division of specialized cells on the exterior (B), 

while female gametes (eggs) form likewise (B). Fertilization occurs as the motile sperm 

cells swim into the egg chamber creating a diploid resting spore. 

Drawings from Coulter 1910 (A) and Smith 1938 (B).

 

 

 

Figure 2.18. Filamentous algae are essentially one-dimensional colonies, in which cells 

remain attached end-to-end during division. Spirogyra is a green filamentous alga with a 

peculiar, spiral chloroplast (A).  In Spirogyra, non-motile gametes form from the entire 

content of their cells and fuse with cells in an adjacent filament (B), through conjugation, 

to form diploid spores. (C). Drawing from Haupt 1953.

 

Figure 2.19. Unicellular, free-floating (planktonic) green algae are common in fresh water. 

Examples include  Chlamydomonas (A), Protococcus (B), Pediastrum (C),  Scenedesmus (D). 

and desmids (E). Drawings from Haupt 1953 (A, B), Brown 1935 (C), and 

Oltmanns 1905 (D, E)

Figure 2.21 Draparnaldia forms a more complex, plant-like body, with small lateral

 branch systems that simulate leaves. Drawing from Haupt 1905.

Two marine green algae: Caulerpa crassifolia (A) has feathery stalks rising from creeping 

rhizomes. The entire colony is one multinucleate (coenocytic) cell.One mutant variety of 

this species usually confined to tropical waters, in recent decades has invaded the 

Mediterranean, becoming a weed that has crowded out native species. Acetabularia (B) 

is forms unusual satellite-antenna-like shields at the end of long stalks. 

Drawings after Smith 1938 (A) and Oltmanns 1905 (B)

Figure 2.23. Another bizarre architecture is exhibited by the green alga, Hydrodictyon (A). 

Cells are  connected in a hexagonal pattern (B) to form a porous empty sac. Within a 

single cell (C), the cytoplasm may reorganize itself into an entire new colony in miniature, 

which will expand upon its release. 

Drawings from Kerner and Oliver 1895 (A, B) and Haupt 1905 (C).

Figure 2.24. Most plant-like of all green algae, Chara (A) grows from apical buds 

leaving internodes between nodes bearing whorls of leaf-like appendages. The

 terminal bud (B) contains an  apical meristem (body of embryonic cells) that produces the 

shoot structures in sequence. Male and female strobili (C) produce the sperm and egg cells.

Based on Kerner and Oliver 1895 (A, C) and Haupt 1905 (B) 

Figure 2.25. Plant-like in a different way is Coleochaete, a cousin of Chara. The flat body

 architecture is referred to as a thallus, and is similar to structures found in some basal terrestrial

plants. It is a significant example of 2-dimensional tissue development among the green algae.

 The thallus is mostly one cell thick (B) and produces large egg cells (center) and will  be 

fertilized in place by sperm cells (C) arriving from a different plant. Chara and  Coleochaete 

are in fact closely related to the ancestor of green land plants. Drawings from Haupt 1905.