Algae - building bodies from balls, chains, sheets and tubes

Volvox - ball-shaped colony

The theme of this article is the algae or protoplants. In addition to explaining general and some specific aspects of algal biology, this page concentrates on the theme of building bodies. Algae illustrate many different ways in which single cells can evolve into more complex and larger multicellular forms by certain key modifications. The diversity of algae is astonishing and only a few examples can be covered here.

Building bodies - spheres

Algae demonstrate well some of the different approaches to building multicellular bodies - there are a number of ways to construct a 3D body from cells. Cells can be joined together to form chains (essentially 1D organisms) or more precisely remain joined together when they reproduce by cell division. If all the cells divide in the same plane, then a chain will result. By dividing sometimes in a second plane at right angles to the first, a 2D sheet of cells may be produced (see Ulva and Porphyra below). Volvox is a green alga that consists of a hollow ball of cells, which is essentially a 2D colony or sheet folded around and joined together. Each ball, or coenobium, consists of a single layer of superficial cells - it is a colony of cells. Each cell is surrounded by a thick mucilaginous wall, forming an enclosing cell. In some species, these mucilaginous walls may extend toward the centre of the ball and almost completely fill it. Each cell sits on the surface of the sphere and bears two flagella which protrude into the surrounding water and beat to propel the whole colony through the water. Each cell has a red eyespot (stigma) which forms part of a photosensor. Volvox, like other algae, is photosynthetic and so it has to keep itself illuminated and so it will swim toward the light (or away from very bright lights that may damage its chlorophyll). This immediately poses a problem - if each cell beats its flagella independently of the others then the colony will move nowhere, rather they must coordinate themselves so as to beat in unison. To achieve this coordination, all the cells are connected to their nearest neighbors by protoplasmic bridges, such that all the cell cytoplasms are continuous with one another through these bridges. This allows waves of electric charge to travel throughout the colony, triggering flagella motion in an organized and controlled manner.

The number of cells present in Volvox and related genera (which form a group called the Volvocales); Gonium forms flat plates with 4 or 16 cells, Pandorina forms a sphere of 16 cells. The number of cells in the larger Volvox is always a power of two, and may number several thousand, a result of coordinated cell division with all cells dividing and doubling in unison. The Volvox ball has a preferred front-end and cells in this part of the sphere have larger eyespots than the rest. Chlamydomonas is an example of an alga
comprising a single cell that resembles a cell from a Volvox colony.

Reproduction in Volvox

Volvox can reproduce asexually by forming daughter colonies. Daughter colonies form as hollow balls of cells inside the parental colony. The daughter colonies form from enlarged cells in the surface of the parent colony, called gonidia, at the posterior end of the colony. Each gonidium divides repeatedly in a plane at right-angles to the surface of the parent coenobium, forming a cup-shaped plate of cells. Divisions continue and the plate forms a small spherical daughter colony inside the parent colony and suspended from its surface. The daughter colonies are originally formed inside-out, with their flagella pointing inwards, and invaginate or invert, often at around the time that they escape from the parent colony which ruptures and dies, releasing the daughters which are miniature versions of the parent and grow by cell division. Daughter colonies may contain small granddaughter colonies upon hatching.

Some species are monoecious (hermaphroditic) whilst others are dioecious (with two separate sexes). In dioecious forms, sexual reproduction begins when some male colonies appear and secrete pheromones that induce the gonidia of other colonies to undergo sexual rather than asexual reproduction. The gonidia of colonies of female clones (descended or cloned by asexual reproduction from a mother colony) are stimulated to develop into female daughter colonies at the posterior of the parent colony. In colonies of male lineages (descended or cloned by asexual reproduction from a father colony) the gonidia are stimulated to develop into male colonies. Female colonies produce specialised egg cells - enlarged superficial cells at the posterior of the colony and which lack flagella and remain attached to neighbouring cells by the protoplasmic bridges. Male colonies produce packets of spermatozoids, also at the posterior of the colony. These form much like daughter colonies in asexual reproduction - a superficial cell divides to produce a hollow ball of cells hanging down from the wall into the interior of the coenobium. These cells are naked (lack cell walls) and flagellated and are originally inside-out but invert so that their flagella point outwards. These cells detach from the parent colony and swim towards egg cells, which are immotile and remain bound to the female colonies.  This is oogamy - large immotile egg cells are formed, toward which the sperm swim. The gametes are produced by mitosis, so the parent coenobia are haploid. When a spermatozoid fertilises an egg cell, a thick-walled hypnozygote is formed. A hypnozygote is a diploid cell formed by fertilisation (zygotes) which enters a mandatory dormant stage. It has a thick spiny wall to protect the zygote within.

The possession of dormant stages is particularly important to freshwater organisms that live in ephemeral ponds. If a pond dries or freezes, then the dormant stages can survive until better conditions for growth return. After dormancy, the Volvox hypnozygote undergoes meiosis and germinates to produce a haploid zoid which undergoes mitosis to form a new coenobium. The zygote is the only diploid stage, making Volvox an example of what is called a haplont.

Above: Chlamydomonas with a detailed version on the right and simpler versions thumbnailed on the left. A, mitochondrion; B, cell wall; C, thylakoid membrane of chloroplast; D, cell-surface membrane; E, flagellum x 2; F, vesicle/vacuole or storage granule; G, nucleus; H, Golgi apparatus; I, (cup-shaped) chloroplast, and J, starch grains. One of the morphologically simplest single-celled algae is Chlorella, which grows on walls and tree bark.

Click for a most simplified version of the Chlamydomonas diagram

Click for the simplified version of the Chlamydomonas diagram

Building bodies - filaments

Many algae are filaments - chains of cells that form when the plane of cell division is fixed resulting in a '1D' colony. Many bacterial and fungal forms are filamentous and one of the advantages is that it allows organisms to reach above the substratum (surface) and through the boundary layer (a layer of almost motionless fluid that forms around all solid objects in a fluid) and obtain a good supply of nutrients in the flowing water and to shed spores into the faster moving air or water. Some filamentous cyanobacteria (photosynthetic bacteria that are sometimes called the blue-green algae) live inside slime tubes that they secrete for themselves and they can glide up an down inside this tube, positioning themselves higher in the water column when needed. See the pdf book chapter for more advantages of being filamentous to bacteria. An advantage of being multicellular is that different cells can specialize to perform different functions.

See Cyanobacteria for more details on the prokaryotic blue-green algae.

Bacteria - study more about bacteria, including cyanobacteria. Filamentous bacteria are covered in more
detail here.

Spirogyra

Spirogyra, illustrated above in a 3D mathematical computer model (Pov-Ray) is a green alga which forms filamentous chains of cells, which float freely in ponds and other bodies of still water. Each cell has a distinctive and characteristic spiral green chloroplast. Many cells will typically be joined in long filamentous chains. Spirogyra lives in freshwater ponds and wet drainage ditches and the like.

Spirogyra, like other algae, needs light for photosynthesis. Simply relying on passive means of reaching the light (like flotation) can be problematic and many algae can also move toward the light. Spirogyra has no flagella and cannot swim quickly as can Chlamydomonas, but it is nevertheless capable of limited movement. Blue light will trigger positive phototaxis or movement toward the light.  First the filaments align, pointing toward the light source and then they bind together into bundles (this bundling is essential for the following movements). First the anterior filaments in the bundles, nearest the light, curve toward the light and then those at the rear also roll up toward the light forming open hoops and then the filaments stretch out again. Repeated rolling and stretching allows the mat of filaments to move toward the light in mass, albeit quite slowly at about 1 millimetre per minute. Filaments are also capable of slowly gliding along, possibly as a result of mucilage secretion, and will move up the sides of glass containers, generally moving faster in the dark. It really is amazing teh tyricks that apparently simple organisms have evolved to enhance their survival!

The end cells on each filament of Spirogyra are also capable of adhesion. Blue or UV light will rapidly stimulate these anchor cells to adhere reversibly to glass, probably by secretion of cementing mucoproteins. An increase in temperature and shaking the culture in the dark can also trigger this rapid and reversible adhesion. Red light will also trigger a slow adhesion (takes about 1 hour to develop) which is apparently irreversible.

Filaments of Spirogyra grow by division (binary fission) of cells throughout the filament (except perhaps the end cells) and asexual reproduction comes about when filaments fragment and each sub-filament continues growing. This fragmentation is not necessarily passive - there are changes in the joining walls that weaken the filament at certain breakage points and then one cell may swell more than its neighbour as it takes up water and stretches under turgor pressure pushing against the weakened join and breaking it.

Sexual reproduction occurs by conjugation (not to be confused with conjugation in bacteria which is a different process that serves a very different function). The parent (vegetative) filaments are haploid (n). Two filaments will line up side-by-side (solitary filaments are apparently non-motile) and then projections will grow from the sides of the cells in one filament toward its neighbour. The neighbouring filament responds in like fashion and the protuberances of each filament contact one-another (and push the filaments apart slightly as they develop) and an open bridge is formed. The protoplast of the male cell will round up and crawl across the bridge into the female cell. The two protoplasts fuse and a thick-walled diploid (2n) zygospore is formed. The zygospore can survive harsh conditions, such as the pond drying-up but will eventually divide by meiosis to produce 4 haploid cells(n) only one of which will survive to produce a new vegetative filament on germination.

Above: micrograph of Spirogyra conjugating (fixed permanent preparation). Far right: A Spirogyra cell (fixed permanent prep.) - click to enlarge.

Scenedesmus

Scenedesmus is a beautiful organism to observe under your microscope! The large pyrenoids (for starch synthesis) inside the large green chloroplasts are clearly visible. Flotation helps keep the cells at the surface near the valuable sunlight, and flotation will also help disperse the organism and the spines probably also provide some protection against predators by making the organism larger. Indeed, Scenedesmus often grows as single-cells in culture (each cell often possessing 4 spines) but chemicals released by predators are detected by Scenedesmus (which are too large for many predators) and this induces the formation of longer chains of cells and larger colonies, an effect which has been empirically shown to reduce losses to predation. However, the larger colonies sink more easily and so this defense comes at a cost, which is why in the absence of predators single cells dominate. As a further aid to dispersion, Scenedesmus produces wall-less biflagellate zoospores or gametes (smaller cells, about 5 micrometers long with a chloroplast but no pyrenoid) that swim in the water and if two compatible zoospores meet then they fuse in fertilisation to form a zygote and a new Scenedesmus. In addition to this mode of sexual reproduction, Scenedesmus reproduces asexually by producing a miniature autocolony by division of each protoplast inside the parent cell wall. The wall of the parent cell then splits and the autocolony emerges. In addition to the spines, which are often confined to the end cells, the middle cells may bear long fine bristles. These bristles possibly assist in flotation.

Scenedesmus is a green alga that forms small colonies equipped with spines for flotation. The morphology varies with conditions and species, but this arrangement of 4 cells with 4 spines is quite common. Each cell is about 10 micrometres long. [3D Pov-Ray model].

The spines are flotation devices, keeping the colony floating high in the water column near the source of light needed for photosynthesis. This may seem surprising, but at this microscopic scale water behaves much like treacle (low Reynold's number) and spines are efficient flotation devices.

Sea-Lettuce - Ulva lactuca

Sea-lettuce, Ulva lactuca is a seaweed of rocky shores (upper, middle and lower shores). Its thallus or frond is a thin bright-green translucent sheet of varied shape anchored by a holdfast and sometimes a short stalk (stipe) though in some locations this seaweed survives as a free-floating weed. Size varies from 5 to 5o cm, sometimes over one metre in length. The thallus is only two cells thick.

Below: a section through an Ulva frond. Two sheets of closely spaced photosynthetic cells make up the frond (Ulva is said to be distromatic). Each cell has a single cup-shaped chloroplast which is parietal (lies in the margin of the cell) and during the day is positioned facing the surface, to intercept the sunlight for photosynthesis. At night the chloroplasts move, typically sitting at one of the long edges of the cell. Some of the cells, especially those in the stalk, give out multinucleate hyphal-like appendages called rhizoids, which strengthen the stalk.

Isogamy is the production of identical male and female gametes, and although Ulva is often described as isogamous, the female gametes are actually considerably larger and Ulva is more correctly described as anisogamous (producing distinctly different male and female gametes).

The zygoge germinates to produce a diploid sporophyte, which is identical in appearance to the gametophytes. The sporophyte, however, produces haploid zoospores by meiosis, each zoospore is quadriflagellate (has 4 flagella) and germinates into a gametophyte thallus.

Both the immature gametophytes and sporophytes develop first into a uniseriate filamentous juvenile stage > with rhizoids (shown above) by repeated mitosis - uniseriate means that the filament is comprised of a single column of cells.

Growth is intercallary (occurring at areas throughout the length of the frond rather than the base or apex only) as any of the cells can divide. The plane of division is always at right angles to the surface of the thallus, so that there are always two sheets of cells.

All the vegetative cells along the margin of the thallus can reproduce by spore formation. There are two genetic forms of Ulva - a haploid generation with only one complete set of chromosomes (n) called the gametophyte and a diploid sporophyte generation with two complete sets of chromosoems (2n). These generations alternate throughout the life-cycle (alternation of generations).

The gametophytes produces haploid wall-less (naked) spores by mitosis, and these spores are the gametes. Female weeds produce female gametes and male weeds male gametes (Ulva is dioecious - the sexes are separate). The gametes are biflagellate (they have two flagella). A male and female gamete fuse in a process of anisogamous fertilisation. The gametes are produced inside the vegatative cell walls and escape into the sea via pores - each vegetative parent cell forming a pore connecting it to the outside surface.

Later on the cells divide in a second plane and a multiseriate filament (made of several columns of cells is produced) followed by a hollow tubular stage. This then expands and flattens to form the sheet-like mature thallus. Enteromorpha is a closely related form that consists of hollow tubes, each made from a single sheet of cells, apparently Enteromorpha passes through a young sheet-like, Ulva-like stage but then the two cell sheets separate and curve around to form cylinders. Ulvaria, another closely related form, passes through an Enteromorpha-like tubular form which splits open at the upper end to form a sheet that is only one cell thick (monostromatic). These species are closely related and hybrid-forms are known. These species all develop from a young filamentous stage, which suggests that they evolved from such forms (often, though not always, the development of an organism resembles the evolutionary stages that it went through as evolutionary progress can occur when genetic changes modify the developmental process, and one such modification is to extend the development by adding additional stages on to the end of it). Thus, we have seen how single-cells that fails to divide completely can evolve into multicellular filaments, like Spirogyra, and how adding in an additional plane of division can produce sheets and cylinders. The most complex seaweeds are types of brown algae (Phaeophyceae) not green algae like Chlamydomonas, Volvox, Spirogyra, Scenedesmus and Ulva.

A myriad of colours

There are many types of algae, but three of the main divisions are the green-algae or Chlorophyta (like Volvox, Chlamydomonas, Spirogyra, Scenedesmus and Ulva), the brown algae and the red algae. All contain a type of chlorophyll called chlorophyll-a to trap sunlight for photosynthesis, but additional pigments give the algae there various colours and expand the range of wavelengths or colours of light that can be used for photosynthesis. Green algae also contain chlorophyll-b and carotenes, especially beta-carotene (whose orange colour is masked by the green chlorophyll). Brown algae contain traces of chlorophyll c and a brown pigment called fucoxanthin. Red algae contain traces of chlorophyll-d, beta-carotene and a red pigment called phcoerythrin.

The blue-green algae or cyanobacteria, like Anabaena, have chlorophyll-a, beta-carotene and the blue pigment phycocyanin. there is a group called the golden or golden-brown algae, which contain chlorophyll-a and beta-carotene.

Siphonaceous Algae

So far we have looked at unicellular algae, simple colonial and filamentous algae and sheet-like seaweeds (and more complex brown seaweeds) that are beginning to resemble plants in complexity and design. One major body-type that has not yet been considered are the siphonaceous algae. These consist of single large multinucleate cells, often with tubular form and with peripheral cytoplasm and large central vacuoles. Caulerpa is one of the most plant-like of these forms; it consists of a long rhizome tube from which branch root-like rhizoids and leaf-like, sometimes feather-like, fronds. The whole structure can be several meters long and each frond several centimeters in height and yet it is a single cell. Struts project from the cell wall into the cytoplasm to give it additional mechanical strength and in rougher waters these trabeculae increase in development. This suggests the reason why most complex algae have adopted multicellularity - it gives tissues mechanical strength. Both the formation of an enlarged multinucleate cell by nuclear division without cytoplasmic division and multicellularity result from modifications to cell division. In the former, there is nuclear division without cytoplasmic division and in the latter by a failure of the cells to separate completely after cell division.

The whorl of fertile sac-like gametangia, which may be fused together, forms the disc or umbrella. These are attached to short basal segments, representing lateral branches that bear the gametangia. the basal segments bear small projections that may or may not bear hairs on their upper surface, forming a corona (below). An inferior corona may also be present in some species. One or more whorls of sterile branches also occur about the stem but these are often shed to leave an annular scar around the stem (not currently shown in this model). A corona is present in Acetabularia acetabulum.

Above: A 3D Pov-Ray computer model of the reproductive stage of Acetabularia, without and with terminal hairs (which may or may not be present). Acetabularia is also called the Mermaid's Wineglass or the Mermaid's Cup and is essentially a single-celled alga that can grow to about 5 cm in height. The vegetative stage consists of the stalk and anchoring rhizoids with whorls of branched hairs along the stalk. These hairs generally drop-off by the time the reproductive cap or umbel forms. A single giant nucleus is situated at the base of the stalk and this produces and sends out smaller nuclei that migrate to the developing cap to become incorporated into the cap rays where they develop into spores. There is a large central vacuole throughout the cell. When mature, the cap becomes divided (by a cross wall) from the stem and then constitutes a separate cellular system. Different species can be identified by the number of fleshy rays and the degree to which the rays are free, fused together or fused at their bases only.

In Acetabularia mediterranea (= Acetabularia acetabulum) the thallus reaches maturity after 2 or 3 years or more. Each year the aerial part dies, leaving the rhizoidal base or a perennating organ embedded in the substrate and which consists of a lobed outgrowth from the rhizoid system. The perennating organ which puts out a new stem each year, bearing whorls of sterile branches (hairs). When mature, it puts out a stem with one sterile wall which falls away (it is deciduous) and one fertile whorl that forms the umbrella. Each gamtangium in the umbrella produces multinucleate cysts which are released when the tip of the gametangium disintegrates. The cysts act as propagules and in the spring these cysts liberate gametes (through a lid or operculum which opens at one end). The biflagellate gametes are either isogametes (male and female gametes the same size) or anisogametes (the female gametes being larger) depending on species (they are isogametes in A. mediterranea). They fuse in compatible (male and female) pairs to form a zygote. The zygote grows and develops into a young thallus, with a giant diploid nucleus situated in the base. When mature this giant nucleus reduces in size, undergoes meiosis and then the daughter nuclei undergo repeated mitosis to produce some 20 000 tiny haploid nuclei that are transported up the stalk to the gametangia to be incorporated into the cysts.

Diatoms (Bacillariophyceae

Brown seaweeds - the most complex algae.

Article updated: 22 July 2022