Cladosporium sphaerospermum - Molds on the ISS
It is well known that molds are among the most important organisms on earth, as they play a role in almost all material cycles. But today we would like to use the example of Cladosporium sphaerospermum to look at two very special areas of application for molds.
Bioremediation - mold as an environmental protector
Research projects on the use of Cladosporium sphaerospermum in bioremediation are currently underway worldwide. This involves the use of organisms and microorganisms to detoxify habitats. So far, it has been proven that Cladosporium sphaerospermum is able to degrade petroleum hydrocarbons (PAHs) (Hamad et al. 2021). Other research groups have been able to demonstrate the degradation of polyethylene by Cladosporium sphaerospermum (Sathiyabama et al. 2024).
Space mission on the ISS
But what does this have to do with the ISS (International Space Station) mentioned in the title?
Now we come to the really wacky properties of Cladosporium sphaerospermum. The dark conidiospores shown in Figures 3 and 4 are dark-colored due to the storage of melanin. This melanin is often found as a response to UV radiation from the sun and fulfills the same purpose in the case of Cladosporium sphaerospermum. A study from 2022 has now shown that surfaces covered with Cladosporium sphaerospermum transmit ionizing radiation in space less well than untreated surfaces. This will lead to new scientific experiments in which the possible use of Cladosporium sphaerospermum and other microorganisms to protect humans and technology from ionizing radiation in space will be investigated.
But that's not all: compared to the control group on Earth, the colony studied in space showed a 21% increase in growth rate. This is currently explained by so-called radiotrophy. What looks absolutely utopian in sci-fi movies like Godzilla becomes reality here. A few moulds seem to be able to use ionizing radiation as an energy source and thus increase their growth. This is very reminiscent of the plant process of photosynthesis, although different wavelengths of light are used here.
It therefore seems that molds will also accompany us in our plans to explore the solar system.
Picture 3) Light microscope image of Cladosporium sphaerospermum at 400x magnification (plus subsequent digital magnification). Shown is a mycelial strand subdivided by several septa which produces a spore carrier in the middle. The spore carrier has primary and secondary ramoconidia. The latter form the typical spherical spores at their ends. The preparation of such filigree structures for the light microscope requires a modified sample preparation. The adhesive preparation is carefully removed from the colony and placed on a microscope slide with the sticky side facing upwards. Cotton blue is then added for staining and superfluous spores are rinsed off. Finally, the preparation is covered with a cover slip.
Picture 4) Light microscope image of Cladosporium sp. at 1000x magnification (plus subsequent digital magnification). The clearly cylindrical conidiospores chains at the end of the respective ramoconidia can be recognized.
Taxonomy:
The mold species Cladosporium sphaerospermum was first described and published by O. Penzig in Michelia 2 in 1882. Unlike many other molds, this species has only been described once more over the decades under the genus Torula (Höhn 1927). However, Cladosporium sphaerospermum has been established as the basionym (original name) and simultaneously valid name to this day (see mykobank.org as of 04.2025). As already described for some other molds, the species epithet in this case is also a direct allusion to one of the most characteristic morphological features. The Latin “sphaero” means sphere and “spermum” refers to the germ cells. This results in the description of the very round and spherical conidiospores of Cladosporium sphaerospermum (see picture 3).
Routine analysis:
In routine analysis, the genus Cladosporium often plays a rather ambivalent role. As these are considered saprophytic molds from a physiological point of view, the genus Cladosporium is very strongly represented in the outdoor air, especially in the warmer seasons. In fact, it is often so strongly present in the outdoor air that other molds can be significantly under-represented. A strong Cladosporium finding indoors can therefore indicate an outdoor air influence and is generally not resolved at the species level due to its limited relevance for indoor diagnostics. Especially since the differentiation of Cladosporium species is disproportionately time-consuming in routine analysis due to cryptic species, species complexes and not always clear morphological characteristics (reliable identification is based on the sequencing of the beta-tubulin sequence; Atlas of Clinical Fungi (4th Ed. 2020)). However, there are very important exceptions. These exceptions include the species Cladosporium sphaerospermum and the species Cladosporium halotolerans. To be precise, both are species complexes, so there are several species that are subsumed within the complexes. These two exceptions are of increased relevance because they have an increased water requirement (aW value of 0.86 - 0.88) and are therefore among the recognized indoor humidity indicators (mould guideline 04.2024). Fortunately, the two species / species complexes can be distinguished from the other Cladosporium species by their predominantly round conidiospores (Food and Indoor Fungi Sec. Ed. 2019). Therefore, in routine analysis, it is imperative to use microscopic preparations to rule out the presence of round / spherical spores ( pict. 3) and thus humidity indicators or cylindrical / ellipsoidal spores (pict. 4) and thus the influence of outside air when there is a high number of Cladosporium spores indoors. Cladosporium sphaerospermum is a species that is resistant to both cold and dry conditions. Regular evidence therefore comes from moldy window reveals or from refrigerators.
From a biological point of view, the genus Cladosporium can be used to explain the so-called Ramoconidia. What are they and what is special about them? Ramoconidia are fertile, apical branches of spore carriers that produce conidiospores and, after decay, function as a germinable conidia themselves (see definition Food and Indoor Fungi Sec. Ed. 2019). The degree of branching, the coloration and also the complexity (primary and secondary ramoconidia) are species-specific characteristics. Ramoconidia generally produce long spore chains with conidiospores, whereby an interesting phenomenon occurs. The basal conidiospores are often somewhat more elongated / ellipsoidal than the terminal ones, which are usually round / spherical. The spore chains and ramoconidia are generally very unstable and disintegrate comparatively quickly. Detection of intact chains or spore carriers can therefore always be an indication of an infestation in the vicinity of the sampling point.
Front of a pure culture on MEA agar and on DG18 agar
Picture 1) Front of a Cladosporium sphaerospermum pure culture on MEA agar. Incubated for seven days at 25 °C. Both the typical olive green coloration of the colony and the furrows within the colony are relevant for morphological identification. These furrows are a common artifact of the artificial incubation of molds, especially in the genus Cladosporium. The furrows occur when the mold draws a lot of water from the underlying medium. The underside of Cladosporium colonies (not shown here) is usually dark brown to black and also has distinct furrows.
Figure 2) Front of a Cladosporium sphaerospermum pure culture incubated for seven days on DG18 agar. The olive-green coloration caused by the conidiospores is identical to the MEA plates. Only the furrows described are somewhat less prominent, presumably because DG18 agar has a lower water availability than MEA agar.
Scanning electron microscope image
Picture 5) Scanning electron micrograph of Cladosporium sphaerospermum sputtered with gold at about 2902x magnification. Scale bar and on the right of the image indicates 20µm. Both primary and secondary ramoconidia can be seen. The latter with long spherical conidiospore chains. Neither the conidiospores nor the spore carrier show the usual indentations and dents caused by the negative pressure of the vacuum. This may indicate a higher internal pressure of the cells.
Want to learn more about mold and our mold analysis?
Contact us at: moenchengladbach@gba-group.de
