Common "Mold" Spores: Ascospores

Common “Mold” Spores: Ascospores

If you’ve ever sent in an air sampling cassette to evaluate your air for mold spores, you’ve seen the term ‘ascospores’ on the report. In the most technical sense, ascospores are spores that are produced from a fruiting body (called an ascomata) and are initially formed within a sac-like structure called an ascus. Each ascus always contains 8 identical ascospores. But what does this really mean? Today, we are taking a brief dive into the world of classifying fungi and going to do an overview of the Ascomycetes fungi, the largest and broadest group of fungi with over 64,000 species.

A lesson in Fungal Reproduction

The first thing to understand is that fungi can have two main forms of reproduction – sexual and asexual – and the fungi can look really different when its growing in each form, even though they are genetically identical. The two stages can look so different that they have been historically categorized as different fungi! Only the usage of DNA technology has allowed scientists to reunite the separated sexual and asexual stages for some fungi. For one example: Aspergillus is the asexual stage of reproduction, but for various species of Aspergillus, the sexual stage has been classified into at least nine unique genera (some sources say “approximately ten,” highlighting the difficulty scientists face in sorting out mold families) including Fennellia, Emericella, Neosartorya, and Eurotium (Ref 1). And within the field, there is considerable disagreement about the “correct” name for each fungus, which is a whole different can of worms to dissect.

Ascospores are produced during the sexual stage of fungi (Fig 1, Ref 2). Many fungi spend most of their lives in the asexual stage, and the sexual stage is relatively minor although very important for introducing genetic diversity into the species. Asexual reproduction essentially ‘clones’ the fungus by creating a genetic duplicate. Some molds are only known by their asexual stage (called the anamorph), although scientists believe that every fungi has a sexual stage somewhere, even if it occurs exceedingly rarely. Spores produced from asexual reproduction are called conidia or condiospores.

Fig. 1, Life cycle of Ascomycetes; Ref 2

Other fungi live a significant part of their life cycle as the sexual stage (called the teleomorph) and the sexual fruiting bodies are typical characteristics of the mold species. The spores produced by the sexual stage are called ascospores and are normally contained in fruiting bodies that have different names based on their shape/function. Different sexual fruiting bodies are known as: cleistothecium, perithecium, and apothecium. Some species will also produce ascospores in sacs just along the hyphae and these are known as naked asci.

Common fungi that belong in the Ascomycete group and regularly produce ascospores include morels, truffles, and cup fungi. Indoors, the most important fungi that produces ascospores is Chaetomium. Chaetomium is an important mold because it indicates active water damage from a flood or leak and is very capable of producing toxic compunds called chaetoglobosins. For this reason, the spores from Chaetomium are not classified as general “ascospores” on mold reports but are instead given their own unique category.

Ascospores on your Spore Trap Report

Figure 2. Various type of asci and ascospores, Ref 3.

In terms of air quality mold reports, ascospores are generally considered harmless outdoor spores, produced by fungi that typically live outside and do not frequently thrive indoors. Even though there are hundreds of different outdoor ascospore-producing fungi, they all have the same biological/ecological actions, so we feel safe grouping them as one group. In addition, the spores can be very small, are often pale or completely colorless (called hyaline), and look very, very similar, making visual spore identification nearly impossible in most cases (Fig 2, Ref 3).

Outdoor ascospores often come from fungi growing on plants and from dead and decaying plant material. The numbers of ascospores in outdoor air can vary greatly due to time of year, time of day, and current weather conditions. Often, ascospore levels are the highest during warm, humid, or even rainy conditions – rain droplets can actually splash the sac-like ascus and burst it, launching the spores into the air. Other times, the ascus walls degrade, releasing spores.

Most of the time, the ascospores we report are from outdoor-living molds. Figure 3 shows a huge array of ascospores we saw from an outside sample that was taken during rainy summer weather in the Midwest. A few clumps of spores are present – these are the 8 ascospores that all emerged from one ascus, still unseparated. Ascospores are fairly small and lightweight and are designed to be easily spread by air. Unsurprisingly, the indoor air samples that accompanied this outdoor sample had elevated ascospore levels compared to “normal” but were still far below the ascospore levels that were observed in the outside sample. Findings like these are common – mold spores outside can frequently and easily make their ways indoors. In the case of ascospores, there is rarely a good indoor environment for them to start growing on and they fall inactive to the ground, mixed in with the dust. This is true for all molds, even the “bad” ones. Many different spores can be accidentally blown into a house, but without the proper growing conditions (often wet or humid places in the house), the spores do nothing and are cleaned up the next time you dust or vacuum.

Figure 3. Outdoor air sample our lab received from a client in Ohio, where the weather was warm and rainy.

Peziza domiciliana

Only in rare cases will outdoor ascospore-producing fungi actually take up residence in your home, and these fungi tend to be very noticeable. The most common indoor ascomycete (well, one that isn’t Chaetomium) is Peziza, a cup fungus (Figure 4). This fungus grows on wet wood and on wood chips, causes wood rot, can grow on wet carpet and drywall, and also in basements or cellars. Its so commonly found indoors that one species is named Peziza domiciliana, the ‘domicile cup fungus.’ It forms a brown-tan fleshy mushroom that slightly curves to form a cup-shape and can easily reach 2-3” in diameter. While this mushroom’s presence inside indicates heavy water damage, its spores are not considered dangerous or toxic, but they can be allergenic. Most ascospores are the same – they are generally not dangerous but can cause allergy-like reactions in sensitive people.

Figure 4a. Fleshy mushroom sample sent in by a client, likely a Peziza species

Figure 4b. the microscopic view of the sample, showing the many sac-like asci containing 8 ascospores each.

To sum it up, ascospores are very common outdoor molds and will at times make their way into indoor air, especially through open windows and doors. Having an outside air sample is critical for comparison in these cases – if ascospore spore levels are high outside, then it is expected to see some inside. However, high ascospore levels inside can indicate serious water damage and should be evaluated to identify and remediate the problem.