Trapping Devices of Carnivorous Fungus Arthrobotrys oligospora Can Isolate Heat-Triggered Excess Irons

Abstract

Under low-nutrient conditions, Arthrobotrys oligospora and other NTFs can differentiate their mycelia into specialized trapping devices for capturing prey as their nutritional source. Using energy-dispersive X-ray spectroscopy (EDX) in conjunction with transmission electron microscopy (TEM), we identified that the characteristic electron-dense bodies in trapping devices contained more iron than vacuoles and mitochondria. Meanwhile, fungal mycelial cells used effective desferriferrichromes for iron chelation and storage. Complex bioassays showed that electron-dense bodies represent a novel type of microbial iron storage particle and trapping devices in A. oligospora function as an unprecedented phenotypic system for iron storage. Unexpectedly, all NTFs lack a crucial Ccc1-mediated vacuolar iron detoxification mechanism, which is conserved in most fungi. Inserting the Ccc1 gene cloned from yeast into A. oligospora significantly reduced formation of trapping devices and inhibited nematicidal activity. Notably, Bayesian relaxed molecular clock analysis indicated that the loss of Ccc1-mediated vacuolar iron storage occurred during the Late Paleozoic Ice Age, while the origin of the trapping devices and the acquisition of desferriferrichrome biosynthesis were strongly associated with significantly elevated temperatures. Temperature bioassays demonstrated that the formation of trapping devices is highly temperature-dependent, with free iron content in mycelial cells being inversely proportional to temperature, consistent with that A. oligospora is sensitive to high temperatures and fails to grow above 30{degrees}C. Our findings revealed that global temperature fluctuations are a crucial driver of the genetic evolution of NTFs, as a catalyst for the origin of trapping devices, which are a novel phenotypic indicator of eukaryotic iron overload. Author summaryWe found that a unique group of carnivorous fungi has evolved specialized trapping devices to sequester excess iron, compensating for the absence of the crucial Ccc1-mediated vacuolar iron detoxification pathway, which is conserved in most fungi. Furthermore, we report for the first time that elevated temperature is a non-trophic factor that induces iron overload in eukaryotes, as iron content and composition in fungal mycelia are negatively correlated with temperature. Our findings suggest that nematode-trapping fungi could serve as a potential eukaryotic model for investigating the dynamic regulation mechanisms of iron homeostasis, which could contribute to the development of therapies for iron overload-related diseases in humans. In humans, iron overload leads to tissue damage, particularly in the cardiovascular system. It has long been assumed that iron overload occurs when iron intake is increased over an extended period, either through repeated red blood cell transfusions or enhanced absorption from the gastrointestinal tract. Caucasians are particularly susceptible to iron overload and the complications of hemochromatosis due to a higher incidence of mutations in the homeostatic iron regulation gene within this population. The discovery of eukaryotes exhibiting an iron overload phenotype when exposed to heat holds significant implications for developing treatments and strategies for human iron overload disorders.