[permaculture] How Lichens Explain (And Re-explain) the World - The Atlantic
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Tue Feb 19 03:48:40 EST 2019
The Overlooked Organisms That Keep Challenging Our Assumptions About Life
Gorgeous and weird, lichens have pushed the boundaries of our understanding
of nature—and our way of studying it.
Science is sometimes caricatured as a wholly objective pursuit that allows
us to understand the world through the lens of neutral empiricism. But the
conclusions that scientists draw from their data, and the very questions
they choose to ask, depend on their assumptions about the world, the
culture in which they work, and the vocabulary they use. The scientist Toby
Spribille once said to me, “We can only ask questions that we have
imagination for.” And he should know, because no group of organisms better
exemplifies this principle than the one Spribille is obsessed with: lichens.
Lichens can be found growing on bark, rocks, or walls; in woodlands,
deserts, or tundra; as coralline branches, tiny cups, or leaflike fronds.
They look like plants or fungi, and for the longest time, biologists
thought that they were. But 150 years ago, a Swiss botanist named Simon
Schwendener suggested the radical hypothesis that lichens are composite
organisms—fungi, living together with microscopic algae.
It was the right hypothesis at the wrong time. The very notion of different
organisms living so closely with—or within—each other was unheard of. That
they should coexist to their mutual benefit was more ludicrous still. This
was a mere decade after Charles Darwin had published his masterpiece, *On
the Origin of Species*, and many biologists were gripped by the idea of
nature as a gladiatorial arena, shaped by conflict. Against this zeitgeist,
the concept of cohabiting, cooperative organisms found little purchase.
Lichenologists spent decades rejecting and ridiculing Schwendener’s “dual
hypothesis.” And he himself wrongly argued that the fungus enslaved or
imprisoned the alga, robbing it of nutrients. As others later showed,
that’s not the case: Both partners provide nutrients to each other.
Today, such a relationship is called a “symbiosis,” and it’s considered the
norm rather than the exception. Corals rely on the beneficial algae in
their tissues. Humans are influenced by the trillions of microbes in our
guts. Plants grow thanks to the fungi on their roots. We all live in
symbiosis, but few organisms do so to the same extreme degree as lichens.
If humans were to spend their lives in the total absence of microbes,
they’d have many health problems but would unquestionably still be people.
But without its alga, a lichen-forming fungus bears no likeness to a
lichen. It’s an entirely different entity. The lichen is an organism *created
by symbiosis*. It forms only when its two partners meet.
Or does it?
Lichen-forming fungi mostly belong to a group called the ascomycetes. But
in 2016, Spribille and his colleague Veera Tuovinen, of Uppsala University,
that the largest and most species-rich group of lichens harbored a second
fungus, from a very different group called *Cyphobasidium*. (For
simplicity, I’ll call the two fungi ascos and cyphos). The whole organism
resembles a burrito, with asco fillings wrapped by a shell that’s rich in
algae and cyphos.
For many, it was a game-changing discovery. “The findings overthrow the
two-organism paradigm,” Sarah Watkinson
<https://www.plants.ox.ac.uk/people/dr-sarah-watkinson> of the University
of Oxford told me at the time
“Textbook definitions of lichens may have to be revised.” But some
lichenologists objected to that framing, arguing that they’d known since
the late 1800s that other fungi were present within lichens. That’s true,
Spribille countered, but those fungi had been described in terms that
portrayed them as secondary to the main asco-alga symbiosis. To him, it
seemed more that the lichens he studied have three core partners.
But that might not be the whole story, either.
Look on the bark of conifers in the Pacific Northwest, and you will quickly
spot wolf lichens—tennis-ball green and highly branched, like some
discarded alien nervous system. When Tuovinen looked at these under a
microscope, she found a group of fungal cells
that were neither ascos nor cyphos. The lichens’ DNA told a similar story:
There were fungal genes that didn’t belong to either of the two expected
groups. Wolf lichens, it turns out, contain yet *another *fungus, known as
Read: Is this fungus using a virus to control an animal’s mind?
This isn’t entirely new. Over the years, other lichenologists have
*in wolf lichens, but only ever in three specimens, and only in the context
of abnormal swollen structures called galls
<https://en.wikipedia.org/wiki/Gall>. “It was thought to be a parasite,”
Tuovinen says. “But we found it in completely normal wolf lichens that
don’t have any kinds of bumps.” *Tremella *is right there in the shell of
the lichen burrito, next to the cyphos. It seems to make extremely close
contact with the algae, hinting at some kind of intimate relationship. And
it’s *everywhere*. Tuovinen analyzed more than 300 specimens of wolf
lichens from the U.S. and Europe, and found *Tremella *in almost all of
Wolf lichens are among the most intensively studied of all lichens, so how
could such a ubiquitous component have been largely missed? The problem,
Tuovinen says, is that under a normal microscope, “the fungal cells all
look the same.” She saw it only when she tagged the lichens with glowing
probes that were designed to recognize *Tremella* genes. And she knew to do
that only after finding those genes amid wolf lichen DNA. Earlier genetic
studies, she says, might have missed them because they had specifically
focused on the genes of the ascos. “There hadn’t been a reason to expect
anything else based on the knowledge at the moment,” she says.
It’s an exciting discovery, says Erin Tripp
<https://www.colorado.edu/ebio/erin-tripp>, a lichenologist from the
University of Colorado Boulder, but it’s still unclear what *Tremella *is
actually doing. Most likely, she argues, it’s an infection, albeit a very
widespread one. The alternative is that *Tremella* is a core part of the
lichen. “This would, of course, be *very* exciting,” Tripp says, but to
demonstrate that, the team would need to try to reconstitute wolf lichens
with or without *Tremella* or, alternatively, use gene-editing techniques
to disable the fungus and check how the lichens respond. “Without this sort
of experimental approach, it seems premature to suggest that *Tremella*
represents a third, fourth, or whatever-th symbiont.”
Tuovinen agrees that one shouldn’t overplay *Tremella*’s role. But she
argues that lichenologists have too readily downplayed such organisms. More
than 1,800 species of non-asco fungi have been described within lichens,
and they’ve been labeled with terms that imply some kind of externality:
*commensalistic*. *Parasymbiotic*. *Endolichenic*. *Lichenicolous*. If
they’re not ascos, “we somehow just decided, without testing, that they’re
parts of a lichen that can be excluded,” Tuovinen says. “We really don’t
“Language matters a lot when dealing with these organisms,” Spribille, now
at the University of Alberta, adds. “If we set up our language so that our
definition of a lichen is fixed, and these other elements are extrinsic,
we’re setting ourselves up to find that they’re extrinsic.” He thinks that
researchers should move away from “the imperative of classification” and
the compulsion to shoehorn organisms into fixed buckets. He suspects that
the relationships between all the components of a lichen are probably
highly contextual—beneficial in some settings, neutral or harmful in
That’s a lesson other scholars of symbiosis should also heed. There’s a
to categorize the bacteria within an animal’s microbiome as good or bad, as
beneficial mutualists or harmful pathogens. But such labels imply an
inherent nature that likely doesn’t exist. The same microbes can be benign
or malign in different contexts, or perhaps even at the same time. Biology
is messy—as are lichens.
Tripp agrees that “we, as a community of lichen biologists, need to revisit
the role of all symbionts in the lichen microcosm.” No matter how one
describes *Tremella *and other lichen-associated fungi, it’s clear that
they do affect the form and function of the lichen as a whole. *How *they
do so is “the great unsolved problem” of lichenology, says Anne Pringle
<https://pringlelab.botany.wisc.edu/> of the University of Wisconsin at
Madison. “Are the multiple species of fungi interacting mutualistically?
With each other? With the algae? Are some parasites? Probably the answer to
all questions is yes. Regardless, the data support an emerging consensus:
Lichens are ecosystems as well as organisms.”
How many partners are there in a lichen? “I don’t know, but I think it
depends on the lichen,” Spribille says. “I don’t expect there to be any one
configuration that makes a lichen, a lichen.” That’s especially likely
because lichens have evolved many times over, from different lineages of
ascos that independently formed partnerships with different algae, over
hundreds of millions of years. To expect them all to share the same basic
plan is like expecting birds to be the same as fish.
They’re especially hard for us to understand because they’re so different
from the organisms we’re familiar with. Unlike animals and plants, lichens
don’t really have tissues. They don’t grow from embryos, and instead form
through fusion. Different combinations create different forms—brittle or
flexible, flat or round—and these traits are likely just as important to
them as wings or legs or eyes are to animals. “We don’t understand their
needs,” Spribille says. “In the absence of that, it’s difficult to say what
kinds of configurations are within the realm of the possible.” And we can
only ask questions that we have imagination for.
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