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

Oedogonium is a genus of filamentous, free-living green algae. It was first discovered in the fresh waters of Poland in 1860 by W. Hilse, and later given its name by German scientist K. E. Hirn.

The morphology of Oedogonium is unique, with an interior and exterior that function differently from one another and change throughout its life cycle. These algae reside in freshwater ecosystems in both hemispheres and are both benthic and planktonic in nature.^{[3][4][5][6][7]} They form algal patches on the water's surface and so interact closely with a multitude of other algae.^[8] These filamentous cells' life cycles include both sexual and asexual reproduction, depending on the life cycle stage.

Although quite common, Oedogonium is difficult to identify since key definitive markers 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

First named Oedogoniaceen (in German), the name Oedogoniales is derived from the Latin oedos (meaning swelling or tumor) and gonos (meaning offspring or seed). This name describes the morphology that Hirn witnessed during Oedogonium sexual and asexual reproduction and later described in his publication, “Monographie und iconographie der Oedogoniaceen."^[12]

Discovery

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] Mrozińska^[3][4][5] was the first to examine the genus in terms of morphology, ecology and distribution and described more than 400 species, mainly from southern Poland.

In 1900, Hirn wrote a monograph concerning his finding of a new taxon, to which he gave the name Oedogoniaceen. This paper was published and translated 60 years later. Hirn discovered Oedogonium in a ditch, appearing from June – October.^[18]

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.^{[citation needed]}

Morphology

Exterior

Cells of the genus Oedogonium are narrow and cylindrical in shape. The algal body consists of green, un-branched, 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 rounded at its tip relative to the other cells of the thallus. The holdfast cell produces elongated growths from both unattached sides which aid in firmly attaching the filament to a 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, all three of which provide rigidity and protection. 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 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 males. 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, oval shaped, and sits in the centre of the cell, usually touching 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 Golgi bodies, mitochondria, and endoplasmic reticulum.^{[citation needed]}

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 displays a wide tolerance to variation in nutrient type and amount in water.^[18] Cells are 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, created by interweaving multiple different algal filaments suspended in a gelatinous matrix.^[18] This matrix is a result of secretions by free floating thalli.

Oedogonium filaments typically appear during the warmer months, appearing at the end of June (northern hemisphere), 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, forming an algal bloom.

Life cycle

Asexual reproduction

Oedogonium can reproduce asexually by fragmentation of their 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. The splitting of fragmentation may or may not be intentional – it can 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 remain dormant under these conditions. With the onset of favourable conditions (such as warm winter months), they can germinate to form a new individual.

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 from 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.

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 to occur. Zygotes (oospores) are initially green but will gradually become an orange-red colour and develop a thick multi-layered 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).

Genome

Oedogonium genome size and organisation remain largely unstudied within its phylum.^[21]

Oedogonium contain chloroplasts with a distinctive genome architecture. This genome is 196,547bp in length, and is the most compact among photosynthetic chlorophytes.^{[citation needed]} It has a nonconforming quadripartite structure, with 17 group I and 4 group II introns – making it intron-rich. It has four long open reading frames (ORFs), containing 99 different conserved genes. Two of these ORFs show high similarities to genes not usually found in cpDNA (chloroplast DNAs). These molecular signatures are evidence for the theory that Oedogonium is the earliest-diverging lineage of the OCC clade.^[21]

Practical relevance

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 occurs in two stages; an initial rapid uptake due to surface adsorption on the three major cell wall components, followed by 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]

Due to their position at the surface of a body of water, 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 their 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