GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

8. FUNGI

8.8. EFFECT OF EXTERNAL FACTORS ON THE GROWTH AND PHYSIOLOGICAL ACTIVITY OF FUNGI

Fungi differ from other microorganisms in their exceptionally high adaptability to diverse living conditions; however, even among them, there are species capable of surviving only within restricted environmental limits.

Temperature. The majority of fungi grow within the temperature range of 18-25 "С. Nevertheless, psychrophilic (from -3 to +10 ºС), mesophilic (10-38 º С), and thermophilic (20-50 ºС) cultures are also found among them. A distinctive feature of certain fungal species is that their optimal temperature amplitude can be significantly broader than that of Bacteria. For instance, in Aspergillus fumigatus, it spans 25-50 ºС (whereas for many aspergilli the limits are 26-28 °С). The temperature optimal for growth is not always optimal for The production of primary or secondary metabolites. Thus, during submerged cultivation of Fusarium sporotrichiella, the temperature optimal for mycelial growth is 18-24 ºС, whereas for glucose oxidase synthesis, it is 25 ºС. Periodic temperature fluctuations between 18 and 24 ºС during the cultivation of this fungus result in maximum toxin production. The cultivation temperature of Sclerotium glucanicum affects the yield of the polysaccharide scleroglucan, with 20 ºС being optimal for its formation. Lowering the temperature leads to The formation of by-products such as oxalic, malic, and fumaric acids.

Light. Direct light inhibits fungal growth. However, alternating light and darkness stimulates the growth and sporulation of many fungi. At the same time, certain adaptation periods are observed during the transition from light to darkness, and vice versa.

Fungal responses to light fall into two categories: phototaxis (movement toward light) and phototropism (growth toward light). Furthermore, phototropism can be positive (growth directed toward light) and negative (growth directed away from light). Phototropic responses are most characteristic of reproductive Organs, whereas hyphal structures primarily exhibit negative phototropism in response to light.

The mechanism by which ionizing radiation affects fungi is not yet fully understood. However, experimental findings indicate high Radioresistance in fungi. Certain fungal species are capable of surviving and growing on radioactive graphite—a phenomenon known as positive radiotropism.

Medium acidity. Minimum, optimum, and maximum pH values are distinguished for fungal growth, sporulation, and physiological activity. Quite often, these values do not coincide. For most fungi, the optimum pH lies within 4.0-5.0. However, Aspergillus clavatus grows and sporulates at pH 13.0. During growth, fungi alter the acidity of the medium by excreting metabolites (e.g., organic acids). For instance, during pullulan synthesis (produced by Aureobasidium pullulans), the medium pH drops from 6.5 to 4.0 due to the accumulation of organic acids.

Depending on the pH, many fungi may exhibit variations in cultural and morphological traits: coloration of the medium and colonies, The Nature of mycelial growth, size and shape of reproductive organs, formation of chlamydospore-like Cells, etc., which in turn can influence the physiological activity of the fungi.

Thus, in batch culture of Aureobasidium pullulans, the number of Yeast-like cells increases as the initial pH rises from 3.5 to 6.0. Concurrently, The amount of synthesized pullulan increases, while the biomass level remains unchanged. Furthermore, when the pH is maintained within 5.0-6.3, cellular melanin synthesis is minimized, which greatly facilitates pullulan purification. As in batch culture, under chemostat cultivation of Aureobasidium pullulans, Cell Morphology is governed by medium pH: low pH values favor the formation of mycelial forms. The optimum pH for pullulan Biosynthesis is 4.5, at which the culture consists of 50% yeast-like and 50% mycelial cells. As the pH increases, the biomass level rises, whereas the pullulan yield decreases.

Relationship of fungi to oxygen and carbon dioxide. Obligate anaerobes are unknown among fungi, but many species can grow and sporulate under reduced oxygen concentrations in the medium. Examples include fungal growth on fruit juices and preserves in sealed jars. Many species of soil phytopathogenic fungi are tolerant to elevated carbon dioxide levels. In natural environments, fungal growth at high carbon dioxide concentrations occurs in associations with active aerobic microorganisms, inside PLANT AND ANIMAL Tissues, and in aquatic habitats. Elevated carbon dioxide content stimulates spore germination in numerous phytopathogenic fungal species.

Other factors. Osmophilic fungi exist that are capable of growing in media with high osmotic pressure, such as concentrated sugar solutions (up to 60-80%). Typically, such fungi develop on preserves, fruit juices, syrups, and the like. Halophilic fungi can grow in media with high sodium chloride concentrations (from 3 to 20%). They are found in saline soils and occur during the conservation (salting) of vegetables.