4.2: Water Molds

Water molds belong to the phylum Oomycota in the Stramenopile supergroup. Oomycetes are also fungus-like organisms with cell walls made of cellulose. Similar to myxomycetes, they have motile spores with 2 flagella. However, one of these flagella is "normal"-looking (called a whiplash flagellum) and the other is ornamented. This strange characteristic puts organisms into a group called the heterokonts (meaning "different flagella").

Many oomycetes, such as Saprolegnia, are important decomposers in aquatic ecosystems, while others -- namely those in the genus Phytophthora -- have adapted to life on land and are some of the most destructive plant pathogens.

Figure \(\PageIndex{1}\): A water mold from the genus Phytopythium growing on a sesame seed (first image). The fine fuzz surrounding the seeds is the filamentous thallus. The other two images show a zoosporangium, with the zoospores released in a single mass (last image). All photos by Tom Bruns (pogon) CC-BY-NC.

Saprolegnia

This organism reproduces asexually by producing zoospores (zoospores are spores that swim, zoo- meaning ‘to live’) 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. 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.

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 \(\PageIndex{5}\)) and P. infestans (causal agent of late blight of potato and the Irish potato famine, see Figure \(\PageIndex{6}\)).

Figure \(\PageIndex{5}\): 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 \(\PageIndex{6}\): The image on the left shows two lemon-shaped sporangia, each at the end of a hyphal filament. These sporangia will release zoospores when conditions are favorable. The image on the right 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.