FILAMENTOUS FUNGI: STRUCTURES, LIFE CYCLE AND NUTRITION - BIOLOGY - 2024

The filamentous fungi, commonly known as molds, are multicellular organisms consist of structures called hyphae. These have branching capacity and are collectively called mycelia. Morphologically, the cells are elongated, vary in length, and have a diameter of 3 to 15 µm.

They are classified into two groups: superior and inferior. The upper ones have delicate and fine hyphae, with cells separated by porous partitions that allow the exchange of material between nearby cells. In lower fungi, the hyphae are thicker and do not have partitions, so they form a multinucleated set.

Source: James Lindsey at Ecology of Commanster, via Wikimedia Commons

When a filamentous fungus develops, a portion is responsible for the absorption of nutrients (the vegetative mycelium), while the area that is projected to the outside is responsible for reproduction.

They are characterized by forming colonies with a cottony or powdery appearance, which allows the distinction of yeast colonies. There are some groups of filamentous fungi that are pathogenic for humans. Among the most relevant phyla are the Zygomycota and Ascomycota.

Structures

In filamentous fungi it is possible to differentiate a fleshy stem consisting of a series of long filaments made up of cells. These structures are called hyphae and their capacity for growth is remarkable, achieving exaggerated lengths. There are reports of hyphae 5,600 meters long.

Hyphae grow by elongation in their terminal portions. Each part is capable of growing and when a fragment detaches, it can form a new hypha. This property of fungi is used in the laboratory to grow them from the stem.

The hypha contains a vegetative portion, whose job is to obtain nutrients. Similarly, the reproductive hypha is projected onto the surface where the fungus is developing.

Under suitable or beneficial environmental conditions for the body, the hyphae grow and form a mass called mycelium, which can be observed with the naked eye.

Types of hyphae

There are two types of hyphae, classified according to the presence or absence of structures called septa:

Septate hyphae

In most cases these hyphae are divided by partitions, they form cell units with a single nucleus. This arrangement is known as "septate hyphae." The partitions may have certain openings that allow connections between nearby cells.

Coenocytic hyphae

In other cases, these partitions do not exist, so the cells that compose them have multiple nuclei embedded in a continuous cytoplasm. These hyphae are called coenocytic.

In biology, a coenocyte is a cell with more than one nucleus product of nuclear divisions where cytokinesis did not occur. A similar term is syncytium, where cells aggregate and the membranes dissolve, obtaining - as in the previous case - a cytoplasm with numerous nuclei.

Lifecycle

Filamentous fungi can give rise to new individuals by asexual or sexual reproduction. The first occurs due to the phenomenon of fragmentation, where a portion can give rise to an individual.

Spores

The presence of spores occurs in both types of reproduction and is a quality of taxonomic interest.

The spores of fungi are not similar to the endospores of bacteria, whose function is to ensure bacterial survival in adverse conditions.

In bacteria the process does not increase the number of individuals, so it is not considered a way of reproduction. In fungi, the spore separates from the individual that originated it and gives rise to a second organism.

Asexual spores

The aerial hyphae are responsible for producing asexual spores. This process varies widely depending on the species of study.

Asexual spores are classified into two types. The conidiospore or conidia, a spore that is not surrounded by a sac and are produced by structures called conidiophores. The well-known genus Aspergillus is a producer of conidia.

In turn, there are different types of conidia, such as atroconidia formed by fragments of hyphae, blastoconidia, formed by shoots that separate from the cell that originated them, and chlamydioconidia.

The other type of asexual spore is called a sporangiospore. It is produced inside the sporangium, in the terminal portion of the hypha, called the sporangiophore. When asexual spores germinate it becomes an identical individual as the fungus that originated it.

Sex spores

Sex spores occur by a nucleus fusion process between strains of opposite sexes. The latter are less frequent than asexual ones.

The production of sex spores occurs in three phases: plasmogamy, where a nucleus with a genetic load enters the cytoplasm of another cell; Karyogamy, where the fusion of these nuclei occurs and meiosis where the new nucleus - now diploid - originates new haploid nuclei.

Individuals that develop from sexual strains will share certain characteristics with both of their parents.

Growth and nutrition

Fungi are chemoheterotrophs, which indicates that they must absorb their nutrients. They do not have photosynthetic capacity like plants, since they do not have chlorophyll, nor the enzymatic machinery necessary for an autotrophic life.

In general, filamentous fungi are of the aerobic type. In contrast to yeasts which are facultative anaerobes.

Fungi generally adapt easily to harsh environments. Filamentous fungi are capable of growing in areas with relatively low humidity, at high osmotic pressures, and at fairly low pH levels.

These properties explain why in most cases fungi colonize our fruits and cereals, and why they are capable of growing in places that are not suitable to the naked eye, such as the bathroom wall or the soles of shoes.

If you want to prevent the growth of these fungi in edible products, such as cheese and beverages, add sorbic acid, potassium sorbate or sodium benzoate.

In the case of bread, the fungistatic calcium propionate is usually added as a preservative. These organic acids interfere with the metabolic pathways of molds.

Human pathogens

There are some filamentous fungi that cause infections in humans, predominantly of the pulmonary type.

Among the genera with clinical importance, the following stand out: Acremonium, which causes infections in the skin and nails; Aspergillus fumigatus, the cause of allergic bronchopulmonary infection; Bipolaris ssp., Cause of sinusitis and other pathologies related to the brain.

References

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