Abstract
The discovery of new natural products from fungi isolated from the marine environment has increased dramatically over the last few decades, leading to the identification of over 1000 new metabolites. However, most of the reported marine-derived species appear to be terrestrial in origin yet at the same time, facultatively halo- or osmotolerant. An unanswered question regarding the apparent chemical productivity of marine-derived fungi is whether the common practice of fermenting strains in seawater contributes to enhanced secondary metabolism? To answer this question, a terrestrial isolate of Aspergillus aculeatus was fermented in osmotic and saline stress conditions in parallel across multiple sites. The ex-type strain of A. aculeatus was obtained from three different culture collections. Site-to-site variations in metabolite expression were observed, suggesting that subculturing of the same strain and subtle variations in experimental protocols can have pronounced effects upon metabolite expression. Replicated experiments at individual sites indicated that secondary metabolite production was divergent between osmotic and saline treatments. Titers of some metabolites increased or decreased in response to increasing osmolite (salt or glycerol) concentrations. Furthermore, in some cases, the expression of some secondary metabolites in relation to osmotic and saline stress was attributed to specific sources of the ex-type strains.
Highlights
Genome sequencing confirms that fungi possess many more secondary metabolite gene clusters than are expressed in fermentations employing conventional methods and feedstocks
The ex-type strain of A. aculeatus was either obtained from three different culture collections (NRRL 20623, ATCC 16872, BCRC 32190) or distributed among the seven participating laboratories. This strain was chosen because it was accessible in public collections, and the draft genome sequence of this strain was available at the Joint Genome Institute [17], and a bioinformatic analysis of its genome revealed an extraordinarily rich secondary metabolism [18]
Robust growth was observed for each of the three A. aculeatus ex-type strains when exposed to increasing concentrations of glycerol and seawater salinity
Summary
Genome sequencing confirms that fungi possess many more secondary metabolite gene clusters than are expressed in fermentations employing conventional methods and feedstocks. The impact of fermentation conditions and nutritional composition on secondary metabolite expression has been well documented and even minor variations in medium composition, dissolved oxygen, or temperature may modulate secondary metabolite biosynthesis [1,2]. Gene regulation studies involving halophilic fungi indicate that, individual fungi exhibit different strategies for osmoregulation [5], differentially expressed genes encoding ion and metabolite transporters were found to be upregulated as a shared response due to salt stress (halotolerance) [6,7,8]. A link between secondary metabolite production as a response to salt stress is evident from comparative studies where differential expression in secondary metabolite production was observed from a strain of Aspergillus terreus (isolated from a saltern) and a marine-derived strain of Mariannaea elegans (as Spicaria elegans) due to increasing salinity of the cultivation medium compared to water controls [9,10]
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