Oedogonium

Oedogonium
Oedogonium sp., showing an oogonium (swollen cell) and antheridia (short stacked cells)
Scientific classification
(unranked):	Viridiplantae
Phylum:	Chlorophyta
Class:	Chlorophyceae
Order:	Oedogoniales
Family:	Oedogoniaceae
Genus:	Oedogonium Link ex Hirn, 1900[1][2]
Type species
Kützing ex Hirn
Species
see text

Oedogonium is a genus of filamentous, free-living green algae, first discovered in the fresh waters of Poland 1860 by W. Hilse and later named by German scientist K. E. Hirn. The morphology of Oedogonium is unique, with an interior and exterior that function very differently from one another and change throughout its life cycle. These protists reside in freshwater ecosystems in both hemispheres and are both benthic and planktonic in nature.^{[3][4][5][6][7]} Forming algal patches on water's surface, they interact closely with a multitude of other algae.^[8] These filamentous cell's life cycles include both sexual and asexual reproduction, depending on life cycle stage. Although quite common, Oedogonium is difficult to identify, since key ID factors are only present during reproduction, which is an uncommon life stage among this genus.^[9] Oedogonium has been found to be important in the fixation of heavy metals in freshwater ecosystems.^[10][11]

Etymology[]

Although K.E. Hirn was the first to publish concerning Odeogoniales, it is not clear as to whether he was the first to discover this new genus. First named Oedogoniaceen (German), Hirn used his knowledge of the Latin language to describe and name the green algal genus; oedos meaning swelling/tumor, and gonos meaning offspring/seed. This name was meant to describe the morphology during sexual and asexual reproduction which he saw and described within his publication, “Monographie und iconographie der Oedogoniaceen”.^[12]

History of Knowledge[]

Oedogonium species were first reported in the late 19th century by Hilse (1860),^[13] Gołowin (1964),^[14] Kirchner (1878),^[15] Kozłowski (1895)^[16] and Gutwiński (1897).^[17] Hilse was a Polish phycologist who studied freshwater systems in hopes of learning more about microorganisms and how they interacted with their environment. Along with Oedogonium, Hilse is also credited with the discovery and classification of many diatoms. Mrozińska^[3][4][5] was the first to exam this group in terms of morphology, ecology and distribution and in his time described more than 400 species – mainly from southern Poland.

In 1900, German scientist K.E. Hirn wrote a monograph concerning his finding of a new taxon, to which he promptly gave the name Oedogoniaceen – now Oedogonium. This paper was published and translated 60 years later. Hirn discovered Oedogonium in a ditch, appearing from June – October, but not much else is known as this was his only published contribution and he died in 1907 (7 years following his discovery).^[18] Since this 1900 monograph, this taxon has been vital in ongoing studies regarding biosorption of heavy metals – particularly lead – from fresh water ecosystems.^[10][11] Identification of species within Oedogonium is extremely difficult since I.D. factors are mainly based on reproductive characters, and very rarely are species in this genus discovered in their reproductive state. For the most part they exist in a filamentous form.

In 1991 a paper by Mrozińska presented a new taxonomic classification of the genus Oedogonium and a proposed division into two sections: I. Monospermatozoideae and II. Dispermatozoideae. These sections were based on the different number of spermatozoids the antheridial (male sex organ containing) cell expresses.^[7] This classification is not widely accepted, as it still requires support through further studies.

Morphology[]

Exterior[]

Cells of the genus Oedogonium are narrow and cylindrical in shape. The algal body consists of green, un-branched, and multi-cellular filaments, arranged end to end.^[18] Every cell of the filamentous algal body (called the thallus) is similar in shape apart from the apical cell (the uppermost) and the holdfast cell (the lowermost). The apical cell is wider and always rounded at its tip (having a cap) relative to the other cells of the thallus. The holdfast cell, however, produces elongated growths from both unattached sides which aid in firmly attaching the filament to substrate.^[19] The holdfast is also the only colourless cell of the filament. All other cells in the filament exist as green structures very similar in nature, with only some cells having caps. The number of caps per cell illustrates the number of times that cell has divided. Every cell of the filament has a cell wall consisting of three layers – the innermost is made of cellulose, the middle of pectose, and the outermost is made of chitin. These three layers provide rigidity and protection for these benthic species. Most cells are attached to the substrate by the holdfast and are vegetative cells, although some are free-floating.Species of Oedogonium are divided into two major groups based on distribution of the sex organs: macrandous and nannandrous species.^[19] Macrandous species have a male sex organ (the antheridia) and female sex organ (the oogonia) produced on filaments of normal size. This group is further subdivided into macrandous monoecious and macrandous dioecious. In macrandous monoecious species, the antheridia and oogonia are always found on the same filament. In contrary, in macrandous dioecious species, the antheridia and oogonia are produced on different filaments. Although filaments bearing antheridia and oogonia are morphologically similar, they differ physiologically.^[19] In nannandrous species, filaments producing antheridia and oogonia show morphological distinction. The antheridia, which are much smaller than the oogonia, are called dwarf male. Nannandrous species are always dioecious; i.e. antheridia and oogonia are always produced on different filaments. Small male filaments are likely to be attached to a female filament, near an oogonium.^[19]

Interior[]

The protoplasm of Oedogonium is contained by a plasma membrane, and consists of a single nucleus, reticulate chloroplasts, cytoplasm and a central vacuole. Cell sap (contained by the central vacuole) is made up of inorganic compounds, excretions and secretions. Between the innermost cell wall and the central vacuole is a thin layer known as the protoplast. The single nucleus is large and oval shaped and sits in the centre of the cell – usually along the membrane and internal to the chloroplast. This large nucleus contains 1-2 nucleoli and elongated chromosomes. The reticulate, parietal chloroplast extends over the whole interior of the cell, enveloping the protoplast. Whether these networked strands are narrow or broad varies between species, but with most species these reticula are parallel to the long axis of the cell. At the strand junctions are pyrenoids, covered in starch plates.^[19]

Cells of Oedogonium also contain very typical Golgi bodies, mitochondria, and endoplasmic reticulum.

Habitat and Ecology[]

Oedogonium resides in freshwater ecosystems and prefers stagnant waters, such as small ponds, pools, roadside ditches, marshes, lakes, and reservoirs.^[9] It grows over a large pH range (7.3-9.6) and presents a wide tolerance to variation in nutrient type and amount in water.^[18] Cells exist either fastened to substrate at the bottom of the water system or free-floating within. When free-floating they form polyalgal patches (mats) on the water's surface to establish a relatively static habitat. Mats are created by interweaving multiple different algal filaments that are suspended in a gelatinous matrix.^[18] This matrix is a result of secretions by free floating thalli. Benthic cells are most often juvenile filaments and once matured they tend to let go, float to the top and form the mats. Oedogonium filaments typically appear during the warmer months, appearing at the end of June (north of the equator), and throughout July and August are found prevalent in polyalgal mats.^[20][8] Mats formed by Oedogonium are multi-species, associated with Spirogyra, Rhizoclonium, and Cladophora.^[8][20] Together these species use holdfast cells to grip one another in order to photosynthesize. These mats/patches are also known as algal blooms.

Life Cycles[]

Asexual Reproduction[]

Oedogonium can reproduce asexually by fragmentation of the filaments, germination of aplanospores and akinetes, and through zoospores.^[19] In fragmentation, the filament splits apart and each fragment reproduces to form a fully functioning thallus. Splitting can occur more than once at the same position of the filament, explaining why some cells have more than one cap. The splitting of fragmentation may or may not be intentional – it could occur due to natural damage by the environment or predators.

Asexual reproduction via zoospore is also very common and occurs in vegetative (benthic) cells.^[19] Vegetative cells produce zoosporangia – the enclosure in which spores are formed – which give rise to the zoospores. Each zoospore has a small hyaline anterior region, and at the base of this region is a ring of flagella (~150). Once emerged from the zoosporangium, a zoospore is still enveloped by a fragile vesicle, from which it is soon discharged. Following dispersal, the zoospore experiences a short period of motility in which it searches for a substrate. When attached to a substrate, the ring of flagella is lost, and the zoospore begins dividing to form a new filament.^[19]

Germination of aplanospores and akinetes is uncommon but possible.^[19] An aplanospore is non- motile and formed within a vegetative cell, the wall of which is distinct from that of the parent cell. Under certain unfavourable conditions, aplanospores will secrete thick walls around them and store abundant food reserves. An akinete spore is large, non-motile, and thick walled, the wall of which is fused to that of the parent cell. Akinetes thick cell walls are enriched in food materials. Both aplanospores and akinetes are able to withstand unfavourable habitual conditions (cold, winter months or nutrient poor waters) and within these conditions remain dormant. With the onset of favourable conditions (such as warm winter months), they can germinate to form a new individual.

As these processes are all forms of asexual reproduction, they do not produce genetic diversity in the offspring. Therefore, asexual reproduced Oedogonium are more vulnerable to changing environments.^[19]

Sexual Reproduction[]

Sexual reproduction in Oedogonium is oogamous; and can be monoecious or dioecious.^[19] Species may either be macrandrous (lacking dwarf males) or nannandrous (possessing dwarf males). Dwarf males are small, short, antheridium-producing filaments attached near the oogonia (female sex organ). These dwarf males are derived by repeated cell division of multiflagellate androspores. When an oogonial mother cell divides it forms a swollen oogonium bound by a supporting cell. Oogonial cells may exist in a series along the filament, and so division may also occur in a series; resulting in each oogonium containing a single egg.^[19] Production of an egg causes swelling of the cell wall, responsible for the name given by Hirn in 1900^[12] – oedos (swelling) and gonos (seed/offspring). Antheridia are short and disk-shaped, containing 1 to 2 multi-flagellated sperm cells. Motile male gametes will exit the antheridia and are chemotactically attracted to oogonia. A single sperm cell will pass through a pore opening in the oogonial cell wall, allowing fertilization. Zygotes (oospores) are initially green but will gradually become an orange-red colour and develop a thick multilayered cell wall with species specific surface adornments. Meiosis occurs in the zygote prior to germination, producing four multi-flagellated cells after germination. Once freed from the oogonium, each daughter cell is only motile for a short period of time. All four cells may eventually attach to a substrate and then divide repeatedly to form new Oedogonium filament.^[19]

The life cycle of Oedogonium is haplontic. The egg from the oogonia and the sperm from the antheridia fuse and form a zygote which is diploid (2n). The zygote then undergoes meiosis and reproduces asexually to form the filamentous green alga which is haploid (1n).

Genetics[]

Oedogonium nuclear genomes are rather unexceptional, and genome size and organisation remain largely unstudied within its phylum.^[21]

Oedogonium contain chloroplast with very distinct genome architecture. This genome is 196,547bp in length, and is the most compact among photosynthetic chlorophytes. It has a nonconforming quadripartite structure, with 17 group I and 4 group II introns – making it incredibly intron-rich. It has four long open reading frames (ORFs), containing 99 different conserved genes. Two of these ORFs showed high similarities to genes not usually found in cpDNA (chloroplast DNAs). The chloroplastic genome of Oedogonium reveals character evidence for a close alliance between the OCC clade. Although more data is required to validate these findings, there molecular signatures are strong supports for this dichotomy and for the branching of Oedogonium as the earliest-diverging lineage of the OCC clade.^[21]

Practical Importance[]

Recent studies from 2007 onwards have revealed that Oedogonium cells have a maximum high heavy metal absorption capacity (qe).^[10][11] The major mechanism of the lead–absorption interaction has been found to be ionic interactions and complex formation between metal cations and ligands contained within the structure of Oedogonium filaments. The biosorption of heavy metal ions by the Oedogoniales occur in two stages; an initial rapid uptake due to surface adsorption on the three major cell wall components, and then a subsequent slow uptake due to membrane transport of metal ions to the cytoplasm of the cells. The three cell surfaces of an Oedogonium filamentous cell consist of polysaccharides, proteins and lipids which provide several functional groups capable of binding to heavy metal ions.^[10][11]

Oedogonium are readily available, non-toxic microorganisms which may be cultivated and/or cultured easily. Due to their position at the surface, algal blooms can block out the sunlight from other organisms and deplete oxygen levels in the water during peak summer months. Each alga included in the bloom is short-lived, and this results in a high concentration of dead organic matter. The decay process consumes dissolved oxygen in the water, resulting in hypoxic conditions. Without enough dissolved oxygen in the water, animals and plants may die off in large numbers.^[22] When blooms are in effect, removing these cells has a positive effect on their ecosystem and may be dried and used to effectively absorb harmful heavy metals from other freshwater systems such as industrial wastes.Oedogonium can also significantly clog irrigation canals when growth on concrete surfaces becomes excessive due to high levels of benthic filaments.^[10] Removal of Oedogonium from clogged irrigation canals can also prove to be cost effective as they may once again be dried and used for absorption of heavy metals.^[10][11]

Species List[]

This is a list of all accepted Oedogonium species:^[23]

A[]

B[]

C[]

D[]

E[]

F[]

G[]

H[]

I[]

J[]

K[]

L[]

M[]

N[]

O[]

P[]

Q[]

R[]

S[]

T[]

U[]

V[]

W[]

Y[]

Z[]

References[]