Climate Denial Crock of the Week

Loathsome bloodsuckers and slime molds need love, too.

New Scientist (paywall):

SCOOP up a handful of soil and you hold an entire ecosystem in the palm of your hand. That precious clod might not be much to look at with the naked eye, but it is teeming with life. A gram of soil contains around a billion single-celled organisms, including tens of thousands of different species, and if you could tease out the fungal strands, they would stretch for hundreds of kilometres. These are indispensable to life on Earth, including you and me. If they all died, we would soon follow.

They are dying.

For a long time, bacteria, fungi and other microbes were thought to be impervious to the agents of extinction wreaking havoc on larger organisms. They are so abundant and reproduce so quickly, the thinking went, that they couldn’t possibly be threatened. In recent years, however, microbiologists have come to question this assumption – and now they are sounding the alarm that microbe populations are in decline, possibly precipitously.

“We’re starting to see scary signals that there may be this large microbial extinction event under way that we barely noticed,” says Colin Averill, an ecologist at ETH Zurich in Switzerland.

When we think of biodiversity decline, we usually sweat the big stuff: plants, fish, reptiles, birds and mammals. But these are just the tip of the iceberg. All told, there are perhaps 7.7 million species of animal, around 80 per cent of which are insects and other arthropods, including arachnids and crustaceans. But there are at least 6 million species of terrestrial fungus and up to a trillion species of bacterium and archaeon, collectively known as prokaryotes. On top of that, there are about 200,000 species of complex unicellular microorganisms called protists, such as slime moulds. These latter two groups make up the majority of Earth’s biodiversity.

Microorganisms are not only remarkable for their sheer weight of numbers and diversity, but also for what they do. They are the main decomposers of organic matter. They form vital mutually beneficial relationships, or symbioses, with 90 per cent of plant species. And they keep the major life-sustaining cycles turning – carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur. “The Earth microbiome provides an essential life-support system to our planet,” says Averill.

The first inkling that this system might itself be vulnerable came in 2007, when Markus Weinbauerand Fereidoun Rassoulzadegan, then at the Pierre and Marie Curie University in Paris, wrote a paper challenging the cosmopolitan hypothesis, first put forward in 1934, which suggests that when it comes to microbes “everything is everywhere”.

This posits that due to their minute size and vast abundance, microbes are universally distributed worldwide. Any regional variation is caused by environmental constraints, not by physical barriers to distribution of the sort that keep larger life forms confined to home ranges. Elephants, for example, cannot migrate to the Americas because crossing the ocean is impossible. Bacteria can simply blow across on the wind. If so, the idea goes, then there is always a vast reservoir of every species that can repopulate any place, any time.

Testing the cosmopolitan hypothesis has always been difficult, as failure to find a microbial species somewhere doesn’t prove it isn’t there. But there are plenty of reasons to doubt it. We know that the genomes of several species of bacterium vary depending on their geographic location – and that the genetic variation is greater the further away they are from each other. This suggests that these microbes have been evolving in isolation from one another with no genetic exchange.

Ditto fungi, many of which live dual lives as microscopic and macroscopic organisms. According to Lynne Boddy, a mycologist at Cardiff University, UK, a good example is the wood-decaying fungus Hyphoderma setigerum. It was once thought to have a global distribution, but DNA analysis has shown that it actually has nine subspecies with geographically distinct ranges.

The “everything is everywhere” dictum is no longer valid, says Boddy. “It really is not true. Not all microbes are everywhere. They have biogeography. In other words, they are found in certain parts of the globe.” That doesn’t mean that no microbial species are cosmopolitan, says Weinbauer, but it does suggest that biologists have been lulled into a false sense of security about the extinction-proofness of microbes.

Certain species have almost certainly already disappeared. Whenever an animal or plant goes extinct, it usually takes a retinue of microorganisms with it. “All the specific microbes living in the hair of the mammoth or the feathers of the dodo, all the specific microbes associated with the specific lice of these species, all their specific pathogens are extinct,” says Weinbauer. More recently, botanists in Brazil discovered six previously unknown species of fungus growing on the leaves of a tropical shrub, Coussapoa floccosa, which until recently was thought to be extinct. If and when the last specimen dies, those fungi will disappear too.

Now, some 15 years on from what Weinbauer called his “quite speculative” paper, the evidence is mounting to suggest we are in the midst of an actual decline in the abundance and variety of microbes. “There is an emerging realisation that Earth’s microbial biodiversity is under threat,” says Averill.

Most of the evidence comes from soil fungi, many of which spend much of their life cycle as microorganisms, but also produce the bulbous fruiting bodies we know as mushrooms, toadstools, bracket fungi and the like. These are easy enough to spot, so they are often used as surrogates for the state of forest biodiversity, especially of the underground mycorrhizae – fungi that form symbiotic relationships with tree roots, taking sugars and supplying plants with water and mineral nutrients in return.

As early as the 1980s, there were signs that all was not well in the underworld. Eef Arnolds at what is now Wageningen University in the Netherlands compared historical records of the fruiting bodies of mycorrhizal fungi spotted on field trips in that country. Between 1912 and 1954, the average number of species seen per trip was 71. By the mid-1980s, that had declined to 38. Similar falls were recorded in Germany by the indefatigable mycologist Helmut Derbsch, who sampled the same piece of woodland near Saarbrücken 3500 times between 1950 and 1985. Even steeper falls were seen elsewhere.

All told, says Averill, the species-level diversity of ectomycorrhizal fungi has declined by 45 per cent across Europe over the past century.

Arnolds put the decline down to two factors: air pollution and intensive forest management, which removes fungus food – dead wood and leaf litter – and replaces native trees with non-native ones. One later study in Sweden found that the abundance of wood-decaying fungal species was negatively correlated with the intensity of forestrythere.