19.4: Oomycota

Learning Objectives

• Compare and contrast oomycetes with plants and fungi.
• Explain some of the roles oomycetes have in both terrestrial and aquatic ecosystems.
• Identify structures in the Saprolegnia life cycle and know their ploidy.

The water molds, phylum Oomycota (“egg fungus”), were so-named based on their filamentous morphology and their use of glycogen as a storage carbohydrate. However, molecular data have shown that the water molds are not closely related to fungi. As diploid spores, many oomycetes have two flagella (one ornamented and one smooth) for locomotion, placing them in the Heterokonts (hetero- different, kont- flagella). The oomycetes are heterotrophic eukaryotes characterized by a cellulose-based cell wall and an extensive network of filaments that allow for nutrient uptake. This group of organisms has a diplontic life cycle. Most oomycetes are aquatic and are important decomposers in these ecosystems, but some have evolved to parasitize terrestrial plants (though these still rely on water). One particularly famous plant pathogen is Phytophthora infestans, the causal agent of late blight of potatoes, which caused the nineteenth century Irish potato famine.

Some notable water molds:

• Some species (e.g., Saprolegnia and Achlya) are parasites of certain fish and can be a serious problem in fish hatcheries.
• Downy mildews (Peronosporaceae) damage grapes and other crops.
• Phytophthora infestans, the cause of the "late blight" of potatoes. In 1845 and again in 1846, it was responsible for the almost total destruction of the potato crop in Ireland. This led to the great Irish famine of 1845–1860. During this period, approximately 1 million people starved to death and many more emigrated to the New World. By the end of the period, death and emigration had reduced the population of Ireland from 9 million to 4 million.
• Phytophthora ramorum, which is currently killing tanoaks and several species of true oaks in California. This pathogen is capable of infecting hundreds of species of plants and was likely introduced to California from ornamental Rhododendron.

Figure \(\PageIndex{1}\): A saprobic oomycete engulfs a dead insect. (credit: modification of work by Thomas Bresson)

Saprolegnia

Saprolegnia is a genus of primarily saprotrophic water molds. This genus is often studied for the life cycle features of oomycetes. This organism reproduces asexually by producing zoospores (zoospores are spores that swim, zoo- meaning ‘to live’ refers to its motility) inside of an elongated sac called a zoosporangium (-angium meaning vessel, so a zoosporangium is what zoospores are produced inside of). These zoospores grow by mitosis into a diploid thallus, an undifferentiated body.

Figure \(\PageIndex{2}\): A zoosporangium of Saprolegnia with mature zoospores. The elongated, sac-like zoosporangium is partially visible in this image, located at the end of a filament of the thallus. Inside the zoosporangium are many small, round zoospores. Each of these would have two flagella: one whiplash and one tinsel. Photo by Maria Morrow, CC-BY-NC.

Saprolegnia's sexual reproducing structures include the globose oogonium and smaller, pad-like antheridia (singular, antheridium) that attach to the oogonium. Because these structures produce gametes--much like spores are produced in sporangia--the oogonia and antheridia are also referred to as gametangia (gametangium singular). The oogonium produces haploid eggs via meiosis. These eggs are fertilized by the haploid male nuclei produced by meiosis within the antheridium, creating a diploid, thick-walled zygote called an oospore.

Figure \(\PageIndex{3}\): Multiple antheridia can be seen appressed to the outside of the globose oogonium. In the center of the image is a spherical structure (oogonium) filled with smaller brown spherical structures (eggs or oospores). On the outside, there are empty-looking filaments with swollen ends (antheridia) appressed to it. Photo by Tom Bruns (pogon), CC-BY-NC.

Figure \(\PageIndex{4}\): An oogonium of Saprolegnia with fertilized oospores. This large, globose oogonium would be located at the end of a filament on the thallus. Within the oogonium, there are many diploid oospores, which you can distinguish from unfertilized eggs by the thick wall that encloses them. On the outside of the oogonium, there are several antheridial pads apressed to the surface. The haploid nuclei within these antheridia have already been released into the oogonium. Photo by Maria Morrow, CC-BY-NC.

The oospore will be released and grow by mitosis to create a new multicellular thallus, completing the diplontic life cycle (Figure \(\PageIndex{5}\)).

Figure \(\PageIndex{5}\): Saprolegnia life cycle. This life cycle is diplontic; the "multicellular" stage is the diploid thallus, though the filaments are coenocytic. The diploid thallus can reproduce by forming elongate zoosporangia where diploid zoospores are formed by mitosis. These zoospores have a whiplash flagellum and one decorated (hairy) flagellum (AKA zoospores with heterokont flagella). Zoospores can germinate and grow into a new diploid thallus, gentically identical to the parent. To sexually reproduce, the thallus can produce globose oogonia and/or filamentous antheridia. Within these structures, meiosis occurs to produce haploid eggs within the oogonium and haploid "male" nuclei within the antheridium. The antheridia adhere to the oogonium and deposit their nuclei inside. Some of these nuclei fuse with egg cells (completing fertilization) to form thick-walled, diploid oospores. When conditions are right, oospores may germinate to grow into a new thallus, genetically distinct from the parent(s). Diagram by Nikki Harris, CC BY-NC, with labels added by Maria Morrow.

Phytophthora

Phytophthora is a genus of water molds that parasitize plants. They have specialized zoosporangia that detach, allowing zoospores to be transported terrestrially and await germination until moisture is present. Some notable Phytophthoras are P. ramorum (causal agent of sudden oak death, see Figure \(\PageIndex{6}\)) and P. infestans (causal agent of late blight of potato and the Irish potato famine, see Figure \(\PageIndex{8}\)).

Figure \(\PageIndex{6}\): Phytophthora ramorum can infect hundreds of different plant species, sometimes causing a leaf blight (as seen in the first photo), sometimes causing root disease. When it infects certain species of oak (Quercus spp.) and tanoak (Notholithocarpus densiflorus), it causes a lethal stem canker that oozes and stains a dark reddish color (second photo). First photo by Kerry Wininger, CC-BY-NC. Second photo by Chris Shuck, CC-BY-NC.

Figure \(\PageIndex{7}\): The first image shows two lemon-shaped sporangia, each at the end of a hyphal filament. These sporangia will release zoospores when conditions are favorable. The second image shows the leaf where these sporangia were produced, seen here as a discolored region with some white fuzz. Photos by 大肚魚, CC-BY-NC.

Phytophthora infestans is an oomycete responsible for potato late blight, which causes potato stalks and stems to decay into black slime (Figure \(\PageIndex{8}\)). Widespread potato blight caused by P. infestans precipitated the well-known Irish potato famine in the nineteenth century that claimed the lives of approximately 1 million people and led to the emigration of at least 1 million more from Ireland. Late blight continues to plague potato crops in certain parts of the United States and Russia, wiping out as much as 70 percent of crops when no pesticides are applied.

Figure \(\PageIndex{8}\): These unappetizing remnants result from an infection with P. infestans, the causative agent of potato late blight. (credit: USDA)

Secondary Endosymbiosis

At some point in evolutionary history, a heterotrophic heterokont engulfed a red alga. This secondary endosymbiotic event resulted in several lineages of photosynthetic heterokonts, including the brown algae and diatoms. It is possible that the Oomycota also descended from this event, as there are apparent algal and cyanobacterial genes present in the nucleus of oomycetes. However, it is possible that these genes were acquired through horizontal gene transfer or evolved from homologs. As our understanding of genomes improves, so too will our interpretation of these events. Regardless, oomycetes currently live a chloroplast-free lifestyle and lack the vestigial plastids present in many lineages that have acquired and subsequently lost photosynthesis.

Figure \(\PageIndex{9}\): In the diagram above, we see a unicellular photosynthetic eukaryote with a 2-membrane chloroplast. In step one, this organism is engulfed by a heterotrophic eukaryote. In step two, we see the photosynthetic organism inside the heterotrophic organism. In step three, the original photosynthetic organism within the heterotroph has been reduced to a chloroplast with 4 membranes. The result is a new type of photosynthetic organism. Artwork by Nikki Harris CC BY-NC with added labels by Maria Morrow.

Summary

Oomycota, also called the water molds, is a group of fungus-like organisms with a history of living in aquatic ecosystems. These organisms have swimming spores and at least one stage in their life cycle has heterokont flagella: one whiplash flagellum and one decorated (hairy) flagellum. They have important roles as decomposers and parasites. Some have evolved to live terrestrially and are infamous parasites of plants, namely those in the genus Phytophthora. Among these is Phytophthora infestans the causal agent of late blight of potato and the Irish potato famine that resulted from an infestation during paricularly harsh, synergistic conditions in Ireland.

This group is closely related to the diatoms and brown algae. These photosynthetic lineages of heterokonts acquired their 4-membraned chloroplasts through secondary endosymbiosis of a red alga.

Members of this group share the following characteristics:

• Heterotrophic by absorption
• Morphology: Filamentous
• Cell wall composition: Cellulose
• Storage carbohydrate: Glycogen
• Life cycle: Diplontic

Attributions

Curated and authored by Maria Morrow, CC BY-NC, using the following sources:

• 19.1.2 Protists from Biology by John. W. Kimball (licensed CC-BY)
• 23.3 Groups of Protists from Biology 2e by OpenStax (licensed CC-BY). Access for free at openstax.org.

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