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Biogeography of Aspergillus species in soil and litter

     USDA, ARS, Southern Regional Research Center, PO Box 19687, New Orleans, Louisiana 70179

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 

Based on counts of Aspergillus species reported in over 250 studies of microfungi from soils and litter, chi-square analyses were conducted on species occurrence in five biomes and five latitude ranges to determine variations from expected distributions. There was no overall trend in distribution of the members of the entire genus by biome, however, individual sections of the genus appeared to have distinct distribution patterns. Most members of sections Aspergillus, Nidulantes, Flavipedes and Circumdati occurred at greater than expected frequencies in desert soils. There was no distinct pattern of species occurrence for forest, wetland, or cultivated soils, although members of section Nidulantes were quite rare in cultivated soils. Most species occurred at or below expected frequencies in grassland soils. Members of the genus tended to occur at greater than expected frequencies at latitudes in the subtropical/warm temperate zone between 26 and 35 degrees. Most species occurred at expected frequencies in the lower latitudes, and below expected frequencies in latitudes greater than 35 degrees.

Key words: Aspergillus, biome, Emericella, Eurotium, latitude, Neosartorya, species occurrence

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Members of the genus Aspergillus have been of interest for centuries because of their positive impact as fermentation agents and because of their negative impact as degraders of agricultural products, their toxicity, and their pathogenicity. Recent scientific interest in biogeography and biodiversity, as well as the increasing importance of aspergilli as causal agents of mammalian toxicity and disease, have led to an increased need to understand where these fungi occur in nature. This study compiles and analyzes data that address the question of where the more common Aspergillus species occur in the soil and litter environment.

Most published work on the global distribution of aspergilli in soils has been descriptive, based on the knowledge and experience of the individual authors (e.g., Domsch et al 1980 , Raper and Fennell 1965 ). Christensen and Tuthill (1985) , however, compiled species diversity data from 62 soil surveys and concluded that the relative percentage of Aspergillus species was greatest in the studies from the tropical and subtropical latitudes, and that many rare species of Aspergillus have been reported only from tropical and subtropical soils.

Mycologists conducting soil microfungal studies are not evenly distributed worldwide. Most published studies are from subtropic to temperate zones (latitudes of 26–45 degrees north or south), and in forest and cultivated soil biomes (Fig. 1 ). To assess whether or not the fungi were evenly distributed, the uneven distribution of studies conducted in the various categories had to be taken into account. This was accomplished by converting all of the data to percentages and conducting a chi-square analysis.

View larger version (41K): [in this window] [in a new window]    FIG. 1. Reported occurrence of Aspergillus species based on data from 270 studies of biome and 243 studies of latitude. A bold number with a plus sign indicates the species occurred at greater than expected frequency in the indicated category and a greyed number with a minus sign indicates the species occurred at less than expected freqeuncy in a chi-square test comparing expected occurrence (% of published reports in a category) with the percent of published reports of each species in each category. No statistical analyses were conducted on the unspecified category. Teleomorph name abbreviations: Eu = Eurotium, Neo = Neosartorya, Em = Emericella.

View larger version (47K): [in this window] [in a new window]    FIG. 1. Continued

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

A table of reported occurrence of aspergilli was compiled by Klich et al (1992) in which data from 327 studies from soils and other substrates were used to create a table of reported occurrence of aspergilli, but no statistical analyses were conducted. In that study, 194 reports were from soil and litter, and from these, latitude could be determined for 183 studies. For the present study, soil and litter data from Klich et al 1992 , as well as an additional 60 reports (these are denoted by an asterisk in the Literature Cited) were used. A few of these contained data from more than one biome or latitude range, so for the statistical analysis, some of the studies were ‘counted’ more than once. In the analysis, there were a total of 270 studies of biome and 243 of latitude.

Biomes and latitude ranges – Most of the studies fell into one of the broad biome categories listed in Fig. 1 . In some cases, the authors did not indicate the biome from which their samples came, or the samples were from unusual soils that did not match one of the main categories. These were placed into the category ‘unspecified.’ Since analysis of data in this category would yield no useful information on the biogeography of the species, these data were not analyzed. Relatively few studies of soil microfungi have been conducted in low latitudes. Therefore, the first latitude category in the analysis includes 0–15 degrees north and south. The other latitude ranges are in 10 degree intervals up to 45 degrees. Forty six degrees and above are reported as one category because relatively few aspergilli were reported from these latitudes.

Statistical analysis – The Pearson chi-square statistic (Conover 1980 ) was used to compare observed occurrence to expected occurrence for each category within each species. Only the 52 species reported in at least 4 publications were included in the analysis. The expected occurrence was the percentage of studies in the given category. For example, 7.4% (18 of 243) of the studies were conducted in latitudes from 0–15 degrees north or south. If the Aspergillus species were evenly distributed in the world, one would expect 7.4% of the reported occurrences of each species to be in this category. The observed occurrence was the percentage of studies in which a species was reported, out of all of the published reports of that species in that category. For example, Aspergillus giganteus was reported in 10 publications. Six of these, or 60%, were conducted in the 0–15 degree latitude range. The 60% observed was significantly greater that the expected 7.4% in the chi square test, and is marked in bold in the figure along with a plus sign. Throughout the figure, bold with a plus sign indicates occurrence at greater than the expected frequency and grey italics with a minus sign indicates species occurring at less than expected frequency in the chi-square test at P = 0.05. The figure contains the actual number of studies in which each species was reported in each biome, rather than the percentages used in the statistical analysis, in order to give the reader an idea of how frequently the species have actually been reported. Percentages may be derived from the data given in the figure. Only species reported in at least four studies are included in the figure. As noted above, no analysis was conducted on the ‘unspecified’ biome group.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
The occurrence data and the results of the statistical analysis are presented in Fig. 1 for the 52 species for which there were more than four reports. Five species were reported in over 100 studies, A. fumigatus, A. versicolor, A. terreus, A. flavus, and A. niger var. niger. With one exception, these five species occurred at the expected frequencies in all of the biomes; A. terreus occurred at greater than expected frequencies in cultivated soils and less than expected frequencies in forest soils. However, only two of these five species occurred at expected frequencies in all of the latitude ranges: A. fumigatus and A. versicolor. Aspergillus terreus and A. flavus were reported at greater than expected frequencies in the 26–35 degree range and at less than expected frequencies in one of the higher latitude ranges. Aspergillus niger var. niger occurred at less than expected frequency in latitudes above 45°.

Of the 52 species included in Fig. 1 , 22 species occurred with greater than expected frequencies in desert soils, compared to 6 in forest, 2 in grassland, 11 in wetland, 8 in cultivated soils. The biomes with the lowest number of species occurring at less than expected frequencies were the desert and wetland soils (10 and 6, respectively), in contrast to 14–17 species in the other three biomes.

In the tropical latitudes, 18 species occurred at greater than expected frequencies and 11 at less than expected frequencies in the 0–15° latitude range, whereas 11 species occurred in each of those two categories in the 16–15° latitude range. In the subtropical to warm-temperate latitudes between 26° and 35°, 30 species occurred at greater than expected frequencies and only 3 at less than expected frequencies. At higher latitudes, the majority of species occurred at less than expected frequencies; 31 and 41 for the 36–45° and 46°+ ranges, respectively. Only 7 species occurred at greater than expected frequencies in the 36–45° latitude range and only one species in the 46°+ latitude range.

Not all subgeneric sections of the genus followed the overall trends. Although there was no strong overall trend for occurrence of all of the species in any one biome, there were trends for some of the sections within the genus. The xerophilic species in subgenus Aspergillus and many species in sections Nidulantes and Circumdati occurred with greater than expected frequency in desert soils. Unlike most species which occurred at greater than expected frequencies in the 26–35° latitude range, most members of sections Cervini and Ornati occurred with greater than expected frequencies above 35 degrees, and most species in sections Flavi and Nigri occurred with greater than expected frequencies in the 0–15° latitude range.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
The results of the current study generally support those from previous reports (e.g., Domsch et al 1980 , Christensen and Tuthill 1985 ). However, these results provide more precise undertanding of the occurrence of species of Aspergillus in soils and litter. For instance, Domsch et al (1980) describe many of the species as tropical to subtropical in distribution. The present study indicates that most of these species occur at expected frequencies in the tropical latitudes below 25 degrees north and south; with greater than expected frequencies in the subtropical to warm temperate zone at latitudes between 26 and 35 degrees; and at less than expected frequencies in higher latitudes. The results of the current study, based on species occurrence, concur with those of Christensen and Tuthill (1985) who concluded that species abundance peaked in the subtropics.

Why do most aspergilli occur most frequently in the 25–35 degree latitudes? This phenomenon is probably the result of a number of interacting factors. The same general biomes occur in other latitude ranges, and the data herein indicate that most aspergilli do not occur at greater than expected frequencies in any one biome, so biome per se is probably not a major overall factor. Temperature may be a factor. Most aspergilli grow optimally at temperatures between 25 and 40 C and have minimum growth temperatures around 10 C (Klich et al 1992 ). In the subtropical/warm temperate zone, temperatures probably stay within the optimal ranges for greater periods of time than in the other latitudinal zones. Microhabitat factors such water and nutrient availability, as well as competition and herbivory, may also play a role.

	ACKNOWLEDGMENTS

 
I would like to thank Shannon Brennan for technical assistance, and Debbie Boykin for statistical advice.

	FOOTNOTES

 
¹ Corresponding author, Email: mklich{at}nola.srrc.usda.gov

Accepted for publication May 22, 2001.

	LITERATURE CITED

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
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