CHAPTER SEVEN

PHYSIOLOGY AND REPRODUCTION

7.1Physiology of Diatoms - Nutrition

Depending on the mode of nutrition they may be photosynthetic autotrophs or photosynthetic symbionts or heterotrophs. Diatoms are mostly phototrophic organism with a few species that are mixotrophic and even fewer that are solely heterotrophic. Apart from nutrients and growth factors, diatoms require all other conditions that eukaryotic organisms need for growth. Above all these they also require greater amounts of silicon, for their frustules. They obtain this silicon from the environment where it is dissolved in the form of silicic acid. The oceans are usually sufficient in silicic acid concentration for diatoms to grow effectively. However, there are patches of areas where iron concentrations are low which leads to limited growth in diatoms. This usually only occurs in the limnetic zone (open waters).

Unlike with land plants the carbon that diatoms fix almost never goes into long term storage. Instead, it ends up being used for cell division and reproduction or they get eaten as a strong base for marine and aquatic ecosystems. Some diatoms are not solely photoautotrophic. The species that are heterotrophic are mostly pennate species that live in the benthic regions. In polar waters they are found in a confusing symbiotic association with sponges. The symbiosis has a mutualism and a parasitism part to it.

The mutualism part of the symbiosis occurs when the diatoms are able to have enough light to produce the energy to cover their metabolic demand; the parasitic part occurs when the diatoms are unable to produce enough energy to cover their metabolic demand and they shift to eating the sponge.

This occurs in conditions of low light to no light conditions at all. During the mutualism part of the symbiosis the sponges provides protection to the diatoms from predation and the diatoms provide energy in the form of extracellular polysaccharides, which the diatoms produce, and they assist as support for the sponge. But during the parasitic part of the symbiosis the diatoms being unable to fulfil their metabolic demand begin to consume the sponge to prevent them from becoming vegetative and allowing them to continue to divide. The diatoms in these situations end up being facultative parasites and facultative mutualism depending on time of year and if the area in which the host sponge is growing has sufficient light to prevent the diatoms from becoming parasitic.

7.2Reproduction

Diatoms like all protists can undergo sexual and asexual reproduction; also, like all protists they predominantly reproduce asexually.

7.2.1Asexual Reproduction or Cell Division

The mechanism they use to reproduce asexually is simple mitosis or binary fission.

Diatoms reproduce asexually by binary fission. The division of diatoms usually occurs at night. The plane of division is usually at right angles to the longitudinal axis, that is, parallel to the valvar surface. During this process, the cell first increases in size and the two halves separate slightly. Mitotic division of the nucleus then occurs, and this is accompanied by fission of the protoplast in a plane parallel to the valve faces. New siliceous valves (hypothecae) are then deposited (inward) on the fresh protoplasmic surfaces. The new wall, which is the inner valve, then grows throughout the respective protoplasts. The two new cells receive a parent cell theca as epitheca, and a new hypotheca is formed for each new cell.

As a result of reproduction, the new cells experience a size reduction. The valves of the parent cell separate as the connecting band develop and the new silica becomes the hypotheca of each daughter cell. The daughter cell with the original hypotheca of the parent is always smaller than the parent cell. After cell division, the original hypotheca of the parent cell becomes the epitheca of the daughter cell. Each division cycle leads to a progressive decrease in the average cell size until the diatom cell becomes too small. When this happens sexual reproduction occurs to form an auxospore which restores the diatom’s normal size.

One of the cells produced gets continuously smaller and smaller. This isn’t constructive for the cells. The solution to this phenomenon is through their method of sexual reproduction. To justify or bring about sexual reproduction, there must be special triggers that cause diatoms to take the sexual reproduction pathway such as unfavourable conditions, predations, time of year, etc. For diatoms, there are several possible triggers. There could be that the cell has become too small or that one of the valves is damaged (often the larger older one), conditions are unfavourable, and the cells must form a vegetative cyst, and sometimes it is just random.

Diatoms are encased in a glass shell of two parts that fit together. Although the way diatom cells are structured is very useful for reproduction, they have a slight problem as discussed below.

The frustules fit together like a Petri dish, a larger valve called the epitheca and the smaller valve called the hypotheca. When they divide each cell ends up taking one of the valves for its epitheca. They then proceed to secret the hypotheca. There are several problems associated with the form of asexual reproduction. They, however, find the solution to this phenomenon through their method of sexual reproduction.

Figure 8: Asexual reproduction of diatoms (Culled from Wikipedia)

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Figure 9: Sexual Reproduction: (a) Centric diatom (oogamy), (b) Pennate diatom (morphological isogamy, physiological anisogamy) (Culled from Wikipedia)

7.2.2Sexual Reproduction

The pattern of sexual reproduction differs in both orders - Pennales and Centrales. During this process, auxospore is formed in both groups. During cell division, the cells become reduced in size, can regain their normal size through the formation of auxospore, so it is a “restorative process” rather than multiplication.

The process by which diatoms go through sexual reproduction is rather complex. First of all, one of the cells forms the male gametes, this is often a smaller cell. First the cell divides into several smaller diploid cells that are called spermatogonia and they remain within the frustules until the frustules open and then they undergo meiosis to form haploid sperm. For the female gametes there are three pathways depending on the species:

The first is they form oogonia (protective mucus around diatom eggs) containing two eggs. This way it starts out undergoing equal cytokinesis in the first division of meiosis and then during the second meiosis there is no cytokinesis.

The second involves production of oogonia containing a single egg and a polar body. This method starts by undergoing an unequal cell division after the first cell division; after the second division it does the same as the first form.

The third form is an oogonium containing a single egg. It is formed by meiosis where there is no cytokinesis present and ends up forming one large active nucleus formed by two smaller pycnotic nuclei around it. These are also evolutionary steps, and the third form is the most derived form of how the eggs are developed and the first form is the most primitive form of egg development.

Fertilization takes place by the sperm cell approaching the egg and interning the oogonial thecae (outer coat of the oogonia) through a sperm channel. It then fuses with the egg cell to form a zygote. The zygote of a diploid cell is called an auxospore and it is virtually a diatom without the silica shell. During this time the cell grows rapidly but doesn’t divide. It undergoes an incomplete mitosis lacking cytokinesis forming the epitheca on the inside of the auxospore. Then one of the nuclei is removed or broken down by the cell. The cell repeats the process again to produce the hypotheca. This process ends with a complete internalized diatom cell with everything including frustule. Then the auxospore lyses open releasing the newly formed full size diatom cell. There are variations of this between the different divisions, classes, and orders. Auxospores function in growth, sexual reproduction and dormancy (resting spores).

Figure 10: The Diatom Life Cycle—The sexual phase - Courtesy Royal Botanic Garden Edinburgh

7.2.3Auxospore Formation in Pennales

It takes place through gametic union, autogamy and parthenogenesis. These are of the following types:

Production of one auxospore by two conjugating cells.

In this process two uniting cells come close to each other and become covered by a mucilaginous sheath. The diploid nucleus of each cell undergoes meiosis. Out of four nuclei, three degenerate and only one survives. The surviving nucleus behaves as gamete (n). The gametes come out from the parent frustules and unite, to form a zygote (2n). After a short period of rest the zygote increases considerably and functions as an auxospore. The auxospore projects out from the parent frustules along with mucilage and elongates in a plane parallel to the long axis of the parent diatom.

The auxospore is enclosed in a pectic membrane, the perizonium. The auxospore then develops new frustules inside the perizonium. Thus, a new diatom cell is formed which regains the normal size. This is found in Cocconis placentula, Surirella saxonica, etc.

Production of Two Auxospores by Two Conjugating Cells:

This is a very common process of auxospore formation. In this process the conjugating cells come close to each other and get enclosed by mucilage. The nucleus (2n) of each cell undergoes meiotic division and forms four nuclei. Out of four nuclei, two degenerate, the rest two survive. The cytoplasm then divides either equally or unequally and along with one nucleus they behave as gametes. Thus, two gametes are formed in each cell. The pattern of union between the gametes varies from species to species. Both the gametes of a cell may be active and fuse with the gametes of other cell, thus two zygotes are produced in a single cell or out of two, one becomes active and fertilises with the opposite one and thus one zygote is produced in each cell. The zygotes elongate and function as auxospores. The auxospores develop the perizonium around themselves and both develop new frustules on their outer sides, that is, inside the perizonium, forming two diatom cells of normal size. It is found in Cymbella lanceolata, Gomphomema parvulum, etc.

Production of One Auxospore by One Cell:

This process of auxospore formation is called Paedogamy (Pedogamy). In this process the diploid nuclei of a vegetative cell undergo meiosis and form four haploid nuclei. Out of the four nuclei two partially degenerate. Each of the rest two along with the cytoplasm and one partially degenerated nucleus, behaves as gamete. Later, the union between the two sister gametes takes place and forms the zygote. The zygote comes out from the parent frustules and behaves as an auxospore. The auxospore then gets covered by perizonium and develops wall inside the perizonium giving rise to one diatom cell of normal.

Production of One Auxospore by Autogamy

In this process the diploid nucleus undergoes first meiotic division producing two haploid nuclei. The two haploid nuclei in the protoplast come side by side, fuse together and form a diploid (2n) nucleus. This is called autogamous pairing. The protoplast along with diploid (2n) nucleus comes out from the parent frustules and behaves as an auxospore. The auxospores are then covered by perizonium. New wall develops on the auxospore inner to the perizonium. Thus, a new individual of normal size is developed. This is found in Amphora normani.

Production of Auxospore by Parthenogenesis:

The diatom cells come together and are covered by a common mucilage envelope. The diploid nucleus undergoes two sequential mitotic divisions. Meiotic division does not take place here. One nucleus in each mitotic division degenerates. Thus, only one diploid (2n) nucleus along with protoplast remains and comes out from the mother cell and behaves as an auxospore. The auxospore is then covered by perizonium and secretes new wall around itself forming a normal size cell.

Production of Auxospore by Oogamy:

In this process the nucleus (2n) of female cell which behaves as oogonium, undergoes meiosis and forms four nuclei. The protoplast is also divided into two unequal parts, each containing two nuclei. The lower half is larger and behaves as functional ovum and the upper smaller one as non-functional ovum. The functional ovum contains one functional nucleus and one non- functional nucleus, which gradually degenerates at maturity. The male cell (2n) behaves as an antheridium, undergoes meiosis and forms four nuclei. The protoplast also divides into two parts forming two microgametes. Each microgamete contains two nuclei, of which one is functional and the other, non-functional. The microgametes are naked, globular and non- flagellate. After emerging, the male gamete fertilizes the egg and forms the zygote (2n). Later it functions as an auxospore and forms new individual of normal size. It is found in Rhabdonema adriaticum.

7.2.4Auxospore Formation in Centrales

It takes place by autogamy and oogamy

7.2.4.1Auxospore Formation by Autogamy

The protoplast of the vegetative cell secretes mucilage which separates both the thecae. The nucleus (2n) then undergoes meiosis and forms four nuclei. Of the four nuclei two degenerate and the other two undergo fusion to form diploid (2n) nucleus again. This is called autogamy. The protoplast with 2n nucleus functions as an auxospore. The auxospore forms fresh frustule inside the perizonium covering and forms cell of normal size. This is found in Melosira nummuloides.

7.2.4.2Auxospore Formation by Oogamy

Oogamy takes place by the fusion of egg and sperm developed inside the oogonium and antheridium respectively.

Oogonium

Single vegetative cell behaves as an oogonium. The protoplast of oogonium undergoes meiotic division and forms four nuclei. Of the four nuclei three degenerate and the remaining one functions as an egg.

Antheridium:

The pattern of development of sperms varies in different species. In species like Melosira varians the protoplast undergoes meiotic division and forms four haploid nuclei. Each haploid nucleus with some protoplast metamorphoses into a uniflagellate (tinsel type) sperm. In others the number of sperms may go up to 8 or even 128.

Fertilisation:

After coming out of the antheridium only one sperm enters inside the oogonium and fertilises the egg. The resultant zygote undergoes mitotic division but one nucleus degenerates in each division. The remaining nucleus with its protoplast behaves as an auxospore. The auxospore then develops new wall inside the perizonium covering and forms new cell of normal size like the mother. It is also called firstling cell.

From the above processes of sexual reproduction in both pennales and centrales, it becomes clear that the sexual process in diatom does not lead to multiplication but is to regain the normal size.

7.2.4.3Resting Spores

These spores are formed during unfavourable conditions. Some members reproduce by the formation of thick-walled resting spores, called the cysts or statospores. They are formed in Melosira.