Interaction Between Grain Fungi Environmental Factors and Niche Occupation

Fungi seldom occur on grains in isolation, but usually as a mixed consortium of bacteria, yeasts, and filamentous fungi. It is thus inevitable that interspecific and intraspecific interactions will occur depending on the nutritional substrate of the grain. Furthermore, environmental factors may exert a selective pressure influencing community structure and dominance of individual species. It is important to understand the type of interactions that occur between fungi under different environmental regimes in grain to enable better prediction of not just dominance by key spoilage fungi, but also the potential for production of mycotoxins. Wicklow (1988) used the in vitro interactions between hyphae of different fungi based on: (a) intermingling of hyphae, (b) inhibition on contact, (c) inhibition at a distance, and

(d) dominance by one species over another on contact, and

(e) inhibition by one species at a distance with the dominant species continuing to grow. They used these categories to develop an index of antagonism by giving numerical scores to each interaction type. This enabled antagonistic interactions between A. flavus and a range of species to be identified. However, these studies did not examine the dynamics of interacting environmental factors and dominance of species.

Subsequent studies by Magan and Lacey (1984c, 1985) modified this scoring system to give a higher numerical score to fungi able to dominate in vitro than antagonism and developed an index of dominance (ID) to assist with interpreting patterns of colonization and dominance in grain ecosystems. The ID was found to significantly change with aw and temperature and also with nutritional grain substrate. Of the 15 species, the most competitive species in wheat grain in United Kingdom were found to be P. brevicompactum, P. hordei, P. roqueforti, A. fumigatus, and A. nidulans.

Decreasing the aw conditions increased competitiveness of P. brevicompactum. Only F. culmorum could compete with storage molds, at > 0.93-0.95 aw. They also found that rate of growth was not directly related to dominance. Previously, Ayerst (1965) had suggested that speed of germination and growth were key determinants of colonization of nutrient rich matrices such as grain. The ID approach has been adapted over the years for many food-based ecosystems.

More recently, alternative approaches have been utilized to try and understand the relative competitiveness of different species within fungal communities colonizing grain. It was suggested by Wilson and Lindow (1994a,b) that the coexistence of microorganisms particularly on plant surfaces may be mediated by nutritional resource partitioning. Thus in vitro carbon utilization patterns could be used to determine niche overlap indices (NOI) and, thus the level of ecological similarity. Based on the range of similar c-sources utilized and those unique to an individual isolate of species they suggested that NOI values of > 0.9 were indicative of coexistence between species in an ecological niche, while scores of < 0.9 represented occupation of separate niches. This approach was modified by Lee and Magan (1999a,b) and Marin et al. (1998c) to include a multifactorial approach by including water availability and temperature into the system. This approach demonstrated that based on utilization of maize c-sources the NOIs for F. verticillioides and F. proliferatum were > 0.90 at > 0.96 aw at 25 and 30°C, indicative of coexistence with other species such as Penicillium species, A. flavus and A. ochraceus. However, for some species pairing with F. verticillioides resulted in NOI values < 0.80 indicating occupation of different niches. Interestingly, no correlation could be found between ID and NOI methods. The results suggested that niche overlap was in a state of flux and significantly influenced by both temperature and water availability. The nutrient status is very important as Lee and Magan (1999) demonstrate that comparison of c-sources in a standard BIOLOG test plate with only those relevant to maize grain gave very different results in terms of niche size and NOI under different environmental conditions. This approach confirms that interactions and dominance are dynamic and not static and emphasizes the importance of taking account of such fluxes in any integrated approach to control the activity of spoilage molds in the stored seed ecosystem.

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