2.5亿年前全球气候变化的背景下一种危害树的侵占性真菌加速了世界范围内森林的消亡，这是根据加利福尼亚州伯克利分校的科学家和她的荷兰和英国同事共同研究获悉的。研究人员不能排除的可能性因素是：土壤致病菌摧毁环境协迫下的森林。
Rhizoctonia solani （左）类似于2.5亿年前的 Reduviasporonites
森林的消亡（主要指针叶林）是全球生物大灭绝的一部分，这一所谓的二叠纪大灭绝很可能是由于西伯利亚火山喷发触发的。大量地气体和灰尘喷射到大气中，并改变全球气候，最终导致95%的海洋生物和70%的陆地生物的灭亡。
科学家声称，螺旋状和丝状的化石微生物广泛存在于二叠纪时期的岩石中，与纹枯病真菌Rhizoctonia具有亲缘关系，这种真菌是对植物有攻击和杀伤作用微生物之一。目前的水稻纹枯病真菌包括一些最普遍的植物病原体，它们引起多种植物的根、茎和叶相关的疾病。根据目前的森林衰亡模型，真菌病很可能是森林稳态打破的必需因素，在二叠期末，它加速了大范围内植物的死亡。
其它的研究人员认为螺旋状地化石微生物是藻类的遗迹，而本研究小组的成员却认为它是一种真菌。他们还认为Reduviasporonites真菌在森林消亡过程中发挥正向作用，改变之前的看法——认为这类真菌对消亡的森林有益处。
尽管被认为是全球生态危机的指示，化石遗迹在生物学和生态学方面仍存在根本的争议。为了解决这一争议，丝核菌Rhizoctonia结构用于研究化石微生物形态相似性，从而提出真菌和藻类的亲缘性关系。从当前森林衰退模型估测，二叠纪末真菌毒害对乔木植被大面积破坏起到重要作用。（生物探索译 Pobee）
生物探索推荐英文原文
Did Past Climate Change Encourage Tree-Killing Fungi?
The demise of the world's forests some 250 million years ago likely was accelerated by aggressive tree-killing fungi triggered by global climate change, according to a new study by a University of California, Berkeley, scientist and her Dutch and British colleagues.
The researchers do not rule out the possibility that today's changing climate could cause a similar increase in pathogenic soil bacteria that could devastate forests already stressed by a warming climate and pollution.
The study, available online Aug. 5, will be published in the September 2011 print edition of the journal Geology of the Geological Society of America.
The death of the forests -- primarily composed of conifers, which are distant relatives of today's pines and firs -- was part of the largest extinction of life on Earth, which occurred when today's continents were part of one supercontinent, Pangaea. The so-called Permian extinction likely was triggered by immense volcanic eruptions in what is now Siberia. The huge amounts of gas and dust thrown into the atmosphere altered global climate, and some 95 percent of marine organisms and 70 percent of land organisms eventually went extinct.
The scientists claim that thread-like or filamentous microfossils commonly preserved in Permian rock are relatives of a group of fungi, Rhizoctonia, that today is known for members that attack and kill plants.
"Modern Rhizoctonia include some of the most ubiquitous plant pathogens, causing root, stem and foliar diseases in a wide variety of plants," said coauthor Cindy Looy, UC Berkeley assistant professor of integrative biology. "Based on patterns of present-day forest decline, it is likely that fungal disease has been an essential accessory in woodland destabilization, accelerating widespread tree mortality during the end-Permian crisis."
The conifer forests, which covered the semi-arid equatorial region of Pangaea, were eventually replaced by lycopods -- four foot-tall relatives of today's diminutive club mosses -- as well as by seed ferns (pteridosperms). The conifers didn't recover for another 4 to 5 million years.
Looy and her colleagues -- Henk Visscher of the Laboratory of Palaeobotany and Palynology at Utrecht University in the Netherlands and Mark Sephton of the Impacts and Astromaterials Research Centre at Imperial College, London -- caution that today's changing climate could also lead to increased activity of pathogenic soil microbes that could accelerate the death of trees already stressed by higher temperatures and drought.
"Pathogenic fungi are important elements of all forest ecosystems," said Visscher. "When an entire forest becomes weakened by environmental stress factors, onslaught of damaging fungal diseases can result in large-scale tissue death and tree mortality."
The researchers dispute the conclusion of other researchers who claim that the thread-like microfossils are the remains of algae. Furthermore, while the researchers previously thought that Reduviasporonites were fungi that took advantage of dying forests, they now believe the fungi actively helped destroy the forests.
"Previously, mass occurrences of Reduviasporonites had been ascribed to wood-rotting fungi living off an excessive abundance of dead wood," said Looy, a paleobotanist who focuses on pollen and spores as keys to understanding past plant communities. "However, the notion that the microfossils represent Rhizoctonia-like resting structures suggest a much more active role for fungi in the ecological crisis:"
The researchers' conclusion comes largely from the fact that they have found living fungi in the genus Rhizoctonia that have a dormant or resting stage during their life cycles in which they look nearly identical to Reduviasporonites.
"One of our problems was that the microfossils didn't resemble the hyphae of known fungi," Looy said. "Buta few years ago, we realized that we were looking in the wrong direction; that we should have been looking at fungal resting structures, not normal hyphae."
Fungi typically spread by means of thread-like hyphae, which can form immense underground networks of mycelia, especially in forests where the fungi live in a symbiotic relationship with tree roots. Each filament is a chain of cells with hard walls made of chitin, the same substance that insects use for their exoskeleton.
When these hyphae branch and intertwine, they may form resting structures known assclerotia. Sclerotia of modern soil-borne fungi such as Rhizoctonia look nearly identical to the disc-shaped structures found among the Reduviasporonites microfossils. Sclerotia are energy storage structures that can help fungi survive extreme conditions.
The team concluded that the loss of trees and the roots that hold soil in place led to severe topsoil erosion, which carried the sclerotia to the sea.
The researchers acknowledge that conifer forests probably suffered from other environmental stresses as a result of the long-term volcanic eruptions, which spewed carbon dioxide and methane into the atmosphere and likely destroyed some of Earth's protective ozone layer. Nevertheless, they wrote in their paper, "… whatever (the) sequence of events that triggered ecosystem destabilization on land, the aggressiveness of soil-borne pathogenic fungi must have been an integral factor involved in Late Permian forest decline worldwide."
The work was funded by Utrecht University, Imperial College London and the University of California, Berkeley.
