Saprolegnia and Phytophthora: Oomycetes or Water Molds
The Oomycetes or Water Molds
Oomycetes or water molds are interesting organisms that share some features with fungi. They often grow in aquatic and damp environments but are also found in drier areas. Saprolegnia and Phytophthora are important examples of the group. Saprolegnia is a common cause of the so-called fungal infections experienced by freshwater fish. Phytophthora was responsible for the devastating Irish potato famine of the nineteenth century and is also a pathogen of other plants.
Oomycetes (pronounced oh-oh-my-see-tees) were once classified as fungi because their body and behaviour have similarities to these organisms. They grow as branching filaments known as hyphae, as fungi do. They also absorb nutrients through the walls of the hyphae and reproduce by spores. Biologists have discovered that there are some important differences between oomycetes and fungi, however.
Though biologists agree that oomycetes shouldn't be classified in the same group as fungi, there isn't yet a consensus about how they should be classified taxonomically. They are thought to be related to the Chromista, a group that contains several types of algae. These algae share certain features with oomycetes that fungi lack.
Saprolegnia Hyphae and Nutrition
The body of Saprolegnia consists of branching hyphae that extend through its food source. The walls of the hyphae are made of cellulose. The hyphae generally lack cross-walls, except at the base of the reproductive structures, and contain multiple nuclei.
Fungi very often (but not always) have cross-walls known as septa in their hyphae. These divide the hyphae into cells, each with its own nucleus or nuclei. The walls of fungal hyphae are made primarily of chitin and don't contain cellulose.
The various species of Saprolegnia are either saprophytes or parasites. Saprophytes feed on dead bodies or decaying material that was once living. Saprolegnia hyphae release digestive enzymes into their environment in order to convert dead or decaying material into a suitable form for absorption.
Parasitic forms of Saprolegnia are found in living organisms. They obtain their food by digesting materials, cells, and tissues in their environment and then absorbing the products. They are sometimes classified as necrotrophs because they kill living cells and extract nutrients from them.
The Saprolegnia photographs in this article show real organisms or structures made by them as viewed under a light microscope. The colour of the photos has been digitally adjusted to make the parts of the organisms easier to see, however. The photo above shows a hypha (item C) and reproductive structures.
Some of the hyphal branches of Saprolegnia develop a zoosporangium at their tip, as shown in item B in the photo above. Item D is the upper wall of the zoosporangium, or the septum. The zoosporangium produces spores by asexual reproduction. Each spore is known as a zoospore and is motile. When a zoospore is released from the zoosporangium and germinates, it produces the first hypha of a new individual.
Each zoospore has two flagella, which are of different types. Flagella are long and thin extensions that are often found on motile cells. As flagella move, they propel a cell through a liquid. One of Saprolegnia's two flagella is known as a whiplash flagellum and the other as a tinsel flagellum. Each points in a different direction. Hair-like extensions surround the tinsel flagellum.
The two types of flagella possessed by a zoospore can be seen in the illustration of the Phytophthora infestans life cycle shown below. The nature of the flagella supports a link to the Chromista. Fungal flagella are of the whiplash type while the flagella of the Chromista are the same as those of oomycetes. The video below shows zoospores being released from a Saprolegnia zoosporangium and then swimming away. Their thin flagella can't be seen, however.
Sexual Reproduction in Saprolegnia
Saprolegnia also reproduces sexually. The female organ is called an oogonium and is item F in the photo above. The oogonium produces large oospheres or eggs. These are said to be haploid (n) because their nucleus has half the number of chromosomes present in the nuclei of the hyphae. The hyphal nuclei have double the number of chromosomes found in the oospheres—or a double set—and are said to be diploid (2n). The situation is somewhat similar to a woman's eggs (haploid) having half the number of chromosomes as her body cells (diploid).
Unlike the case in oomycetes, the hyphae of fungi contain haploid nuclei. This is yet another significant difference between fungi and oomycetes.
The male organ is known as the antheridium. It's smaller than the oogonium. The antheridium of some organisms contains sperm cells, each containing a haploid nucleus. In Saprolegnia, the haploid nuclei are present in the antheridium, but sperm cells aren't made.
The stalk bearing the antheridium grows, causing the antheridium to come into contact with the side of the oogonium. The antheridium then creates a short tube that pierces the oogonium. A male nucleus travels along the tube and fuses with the female nucleus in an oosphere. The resulting diploid structure is known as an oospore or a zygote (item A in the illustration above). The oospore is released into the environment and produces a new Saprolegnia.
Items A and B in the photo above are immature oogonia of Saprolegnia. Item C is a mature oogonium and item D is an oosphere or egg.
Saprolegniasis in Fish
Saprolegnia can cause disease in freshwater fish and their eggs. It can also infect amphibians and their eggs as well as crustaceans. Saprolegnia parasitica is the chief species that affects fish. It causes an infection known as saprolegniasis.
Saprolegniasis may be a special problem in fish farms. The Fluffy Fish organization carries out scientific research into the disease in the UK. According to the organization's website, the Scottish salmon farming industry experiences a financial loss of five million pounds a year due to saprolegniasis. Wild and aquarium fish may also be infected by a species of Saprolegnia. Chemical treatments for the disease exist. These may be helpful, but this isn't always the case.
The parasite starts by infecting the outer layer of a fish. A mass of fine white threads may appear on the scales of the body and the fins. The mass may resemble cotton wool. The hyphae of the parasite may extend into the gills or muscles of the fish and also enter its blood vessels, which can cause serious effects.
Cysts are produced in some stages of the reproductive cycle of oomycetes. A cyst is a thick-walled and dormant structure that protects its internal contents from harmful environmental conditions. Researchers have found that a primary cyst in Saprolegnia parasitica has hook-like projections on its surface. These may help it to attach to fish that are passing by.
Phytophthora: The Plant Destroyer
The various species of Phytophthora can cause serious problems for plant growers. They can infect many different kinds of plants. Economic losses caused by the genus may be severe. The name "Phytophthora" is derived from two Greek words: phyto, meaning plant, and phthora, meaning destroyer.
As in Saprolegnia, the body of Phytophthora consists of branching hyphae. The hyphae have similar features to those of Saprolegnia and obtain nutrients in the same way. The life cycle of Phytophthora resembles that of Saprolegnia but has some slightly different features.
Reproduction in Phytophthora infestans
Like Saprolegnia, Phytophthora reproduces asexually by producing a zoosporangium containing zoospores. Also like Saprolegnia, the zoospores have a whiplash flagellum and a tinsel one. The zoosporangium or its precursor may produce a new organism directly instead of producing zoospores that do this job, however, as shown in the illustration above. In this case, the sporangium may be called a conidium.
An immature Phytophthora oogonium contains multiple nuclei, but at maturity only one oosphere containing a single nucleus is present. Similarly, an immature antheridium contains multiple nuclei, but by the time it reaches maturity it has only one nucleus. During sexual reproduction, an oogonium grows into and through an antheridium, enabling the female nucleus and the male nucleus to meet.
Another difference between sexual reproduction in Saprolegnia and Phytophthora exists. In order for sexual reproduction to occur in Phytophthora infestans, two mating types must meet. These are known as A1 and A2 organisms.
Late Blight in Potatoes
Phytophthora infestans causes the disease known as late blight or potato blight. The organism infects the stems and leaves of the potato plant, producing dark lesions. White threads may be seen on the undersurface of the leaves. The infection may kill the plant.
The pathogen sometimes reaches the tubers of the potato plant, which are the part that we eat. The inside of the potatoes turns brown. The brown colour appears on the outer layer of a potato first and gradually moves inward, making the tuber inedible. The pathogen can spread through the potato even after it's gathered from the field. An additional problem is that the pathogen makes the potato plant susceptible to infection by other organisms. These may cause further damage to the tubers while they are in the field or while they are in storage.
Late blight was given its name because it appeared later in the year than early blight. Early blight is caused by a fungus and can also destroy potatoes. Despite their names, early and late blight may occur at the same time of year.
In one week during the summer of 1846, this disease wiped out almost the entire potato crop of Ireland, a crop which was the primary food of the poor at that time. Nearly a million Irish died in the Great Famine.— University of California Museum of Paleontology (with reference to late blight of potato)
The Irish Potato Famine
Phytophthora infestans can be a serious problem for both potato and tomato plants today. Nevertheless, it no longer produces devastation resembling that of the Irish potato famine of the mid nineteenth-century. The huge number of deaths (around a million) and the massive emigration (around one-and-a-half-million people) that occurred as a result of the famine affected both Ireland and the world.
As a result of studying herbarium specimens that were collected in the past, researchers have discovered that the famine was caused by a strain of Phytophthora infestans that (apparently) no longer exists. New strains appear to have arisen as new varieties of potatoes appeared and the famine strain seems to have disappeared at around the same time.
Researchers who have examined the genome of the famine strain say that it doesn't appear to be inherently more virulent than today's strains. They suspect that the two major conditions that caused the famine were the great importance of potatoes in the diet at the time and the fact that the potatoes grown then were very similar genetically. This low genetic diversity meant that the existence of a potato with genetic resistance to the pathogen was unlikely.
Today Saprolegnia and Phytophthora are significant pathogens that can produce major effects in the environment. They are interesting organisms, despite the harm that they can cause. I think that studying them is a worthy pursuit. Preventing or treating fish disease and enabling crops to survive are important goals. Exploring the nature and behaviour of oomycetes may help scientists to understand the living world better and might be beneficial for us in multiple ways.
Introduction to the Oomycota from the University of California Museum of Paleontology (Oomycota is another name for the oomycetes.)
Introduction to Oomycetes from the American Phytopathological Society or APS
More facts about oomycetes from the APS
Saprolegnia in fish from fuffyfish.org
Late blight in potato from North Dakota State University
Information about late blight in potatoes and tomatoes from the British Columbia Ministry of Agriculture
Information about the strain of Phytophthora infestans that caused the Irish potato famine from the phys.org news site
© 2018 Linda Crampton