The Good, The Bad and The Friend
The importance of fungi to the ecosystem is immeasurable. Fungi are present in all terrestrial habitats from Antarctica to the hot deserts of Namibia, and in most aquatic environments. Being opportunistic heterotrophs, they have specialized to penetrate solid substrates (rock, bark, dead branches, bare soil or grains), nutritionally exploiting almost any food substrate. Fungi are essential to recycle nutrients. Most of the known taxa are specialized in decomposing complex plant and animal debris.
Several species of fungi namely from genus Penicillium play an important role in industry. For example penicillin, produced by P. chrysogenum (formerly P. notatum), discovered by Alexander Fleming in 1929, was probably the most important discovery in last century and changed the course of medicine. Fungi also play a central role in the food industry (cheese and various meat products) and are becoming increasingly important in biotechnology field, especially on the production of enzymes (e.g. gluconic, citric, and tartaric acids, several pectinases, lipase, amylases, cellulases, and proteases).
Figure 1. Aspergillus-flavus (major producer of AF) colonies morphology on petri dish
Fungi are vital to agriculture and forestry through their global involvement in mycorrhizae (fungus roots). They also established mutualistic symbioses with a wide range of organisms like cyanobacteria (blue-green ‘algae’) and chlorophycota (green algae) in lichens.
Minor fungi taxa can have a parasitic behaviour, and in certain cases can be pathogenic. The ability to penetrate almost any surface can be used to invade host organisms. Fungi attack almost all known taxa of plants and animals, including shrimp (e.g. Fusarium sp in penaeids) and fish (e.g. Saprolegniasis). Focusing on fungi as plant pathogens, they attack all parts and all stages of crop plants (from root hairs to apical buds, grains or fruits). The fungal infections may be restricted to small leaf spots, or may be systemic—killing their host very quickly or remain invisible until it is time to appropriate crucial energy resources, such as those concentrated by the host in anthers, bulbs or seeds.
Figure 2. Penicilium-verrucosum (major producer of OTA) colonies morphology on petri dish
Toxic secondary metabolites
Taking into account the available nutrient content, plant stuffs are desirable targets for fungus. Fungi successfully colonize all plant parts, at all stages of crop/storage and utilize substrates efficiently by growing over their surfaces and penetrating into their matrices. Fungal metabolic processes originate an apparently endless diversity of organic compounds which are not obviously required for normal growth and metabolism: these are called secondary metabolites. Not all secondary metabolites are mycotoxins. Simplistically, we could split them in 3 broad groups, being 1) toxic to bacteria (antibiotics), 2) toxic to plants (phytotoxins) and 3) toxic to animals (mycotoxins).
Mycotoxin contamination in aquatic species is often associated with poor growth and low feed efficiency. The lack of obvious pathological signs make it difficult to identify the source of the problem.
Mycotoxin producing fungi
Aflatoxins (AF), ochratoxin A (OTA), deoxynivalenol (DON), zearalenone (ZEN), fumonisins (FUM) and ergot alkaloids are within the most common mycotoxins found in agriculture commodities and responsible for millions of dollars annually in losses worldwide. These toxins are produced by just a few species from the common genera Aspergillus, Penicillium, Fusarium, and Claviceps. All Aspergillus and Penicillium species either are commensals, growing in crops without obvious signs of pathogenicity, or invade crops after harvest and produce toxins during drying and storage. The most important Aspergillus species, occurring in warmer climates, are A. flavus and A. parasiticus, which produce aflatoxins in maize, groundnuts, tree nuts, and, less frequently, other commodities. Penicillium verrucosum also produces ochratoxin A but occurs only in cool temperate climates, where it infects small grains.
In contrast, the important Fusarium and Claviceps species infect crops before harvest. F. verticillioides is ubiquitous in maize, with an endophytic nature, and produces fumonisins, which are generally more prevalent when crops are under drought stress or suffer excessive insect damage. It has recently been shown that Aspergillus niger also produces fumonisins, and several commodities may be affected. F. graminearum, which is the major producer of deoxynivalenol and zearalenone, is pathogenic on maize, wheat, and barley, and produces these toxins whenever it infects these grains before harvest.
Figure 3. Fusarium verticilioides (major producer of FUM) colonies morphology on petri dish
How mycotoxins reach aquafeeds
Despite efforts to control fungal contamination both on the field and in storage, extensive mycotoxin contamination has been reported in both plants and finished feeds. The type and prevalence of mycotoxin contamination will depend on the type of substrate (plant meal type; finished feed characteristics) as well as geographical area, seasonal and local weather conditions during critical plant growing stages or storage.
Factors contributing to the presence or production of mycotoxins include: environmental (temperature, humidity) and ecological conditions (insect attacks, physical plant damage and general stress)—though these are often times beyond human control.
Incredibly durable, even in processing
The mycotoxins commonly occurring in plant stuffs are not destroyed during most processing operations. On the contrary, processing affects mycotoxins distribution and concentrates mycotoxins into fractions that are commonly used as animal feed (plant by-products; e.g. corn gluten meal, DDGS, etc). The fate of mycotoxins during cereal processing, such as sorting, cleaning, milling and thermal processes has been studied by several authors. However, their level in feedstuffs is variable and affected by several factors such as the type of mycotoxins, the level and extent of fungal contamination, and the complexity of the cereal processing technology.
Not all molds produce mycotoxins and even the ones that have that capacity may be present without producing any toxin. Thus, the confirmation of mold contamination is not the same thing as the demonstration of mycotoxin contamination. As a result, the use of mold inhibitors does not guarantee that feed is free of mycotoxins, as they are also produce in crops and not destroyed during processing. It is recommended that aquafeed and aquaculture producers regularly monitor raw commodity feed ingredients and finished feeds for mycotoxin contamination—either through on-site rapid testing or through an external laboratory that may be equipped with more powerful detection equipment. In cases where feed quality has been compromised by mycotoxins, the use of a mycotoxin deactivator is advised.