The trouble with lichen

Walking through the bush, one cannot help noticing varieties of lichen on rocks, trees and other places where it could achieve a foothold. Reading the lichens informs us about soil chemistry, rain patterns and plant nutrients. Recently a species was noticed growing on a hemlock, unusual because conifer bark is usually too acidic to support these kinds of lichens. So why are they there?

In a 2000 paper biologists found that the answer lies in a mature trembling aspen nearby. Water dripping from its branches becomes a leachate, which, when it falls onto the conifer’s bark, lessens the acidity, allowing the lichen to thrive. They dubbed this interaction the drip-zone effect

The discovery called into question the very nature of the lichen symbiosis, shedding fresh light on how symbioses across biology work, how natural selection proceeds and even how to define life-forms. Lichens are both ubiquitous and fascinating. Perhaps more than 500 million years old, they occur on every continent and can thrive in some of the most inhospitable places on earth. They even survived for a year and a half in space, fully exposed to cosmic radiation, ultraviolet irradiation and vacuum conditions.

The approximately 14,000 species of lichen come in a variety of forms: flat rounds on stones, scalloped leaves nestled among mosses, crusts clinging to tree bark, flowing strands hanging from branches, tiny trumpets tipped in red. For centuries people thought they were plants (and then fungi).

Then, in the 1860s, Swiss botanist Simon Schwendener discovered that they were a partnership between a fungus (an organism classified in its own kingdom because, unlike plants, it cannot make its own food) and an alga, an organism that feeds itself with photosynthesis but lacks the roots and stems of plants. The fungus apparently provided the structure of the lichen, and the alga provided food for the fungus via photosynthesis.

Later it was discovered that in some lichens, a cyanobacterium provided the food—and a handful of species contained both an alga and a cyanobacterium, along with the fungus. Schwendener’s discovery, at first resisted by the scientific community, ultimately made lichens the poster children for symbiosis, a mutually beneficial interaction among organisms.

Since then, science has found symbioses across nature, including among the trillions of nonhuman microbes that cling to the scaffold of our bodies. Science over the past two centuries has largely viewed molecules, cells and species as individuals. Symbiosis challenges that notion. “Within a lichen,” Spribille says, “algal cells and fungal cells may experience each other as individuals, but together they form a lichen that the feeding caribou sees as an individual: tasty.”

Natural selection happens on both scales simultaneously. Just as light is both a wave and a particle, the fungus and alga are both individuals and parts of a whole. Science’s reductionist focus has made it nearly impossible to fully understand symbiosis, Spribille says. “Ecology was supposed to be the science of natural process and synthesis, but its backbone is severely strained under the mathematics of individuality.”

Three’s company

In July 2016 Spribille and his co-authors took a major step forward in that understanding. Their big reveal in Science: many lichens have a second fungus within them.

At the heart of the study is a pair of lichens which deserve attention: Bryoria fremontii, which is hairlike and often brown and eaten by northwestern indigenous peoples, and a similar lichen, Bryoria tortuosa, which is often a yellowish green and is toxic, with high levels of vulpinic acid. The two posed a fascinating conundrum.

Despite their differences, a genetic analysis published in 2009 by Saara Velmala of the University of Helsinki and her colleagues, on which Goward was a coauthor, showed that the two species consisted of the same fungus and same alga. Spribille recalled how this perplexing finding infected them both. “[Goward] took the question of how could these two different lichens— one of which is toxic, for God’s sake—be identical.” The question would not let go of Goward. And when  he wrote about it, “by extension, it wouldn’t let go of me.”

Aside from their different appearances and levels of vulpinic acid, Goward observed that the two lichens also had slightly different ecologies. Although they grew in some of the same places, B. tortuosa was found only on the summer-dry fringes of B. fremontii’s larger territory. In 2009 he proposed that lichens are formed not by the shape of their fungal partner but by a series of decisions made during the developmental dance between fungus and alga. One lichen can look different from another that is composed of the same partners because it took different turns during development.

Goward suggested that the difference between the two species of Bryoria might stem from each of them having a different relationship with a third life-form, a bacterium. After five years of work in the lab, Spribille and his colleagues discovered that both Bryoria species did include a third partner. But it was not a bacterium; it was another fungus, known as a basidiomycete yeast. The toxic Bryoria contained a lot more of the yeast than the edible one.

The team also demonstrated that the yeast was not a contaminant but had evolved with the other partners for more than 200 million years. Expanding their search to lichens across the globe, they found the yeast in 52 other sets (genera) of lichen. The finding dramatically expanded the world’s understanding of lichens, opening the door to other insights.

“Only now are we beginning to see that lichens really have pulled off a rare feat in evolution: a large multicellular organism but built entirely of microbes—and here’s the amazing thing—without a scaffold,” Spribille says. “Self-assembling, self-replicating, generation after symbiotic generation.”

Microscopic plankton hunters

Most of us are aware of plants which obtain their nutrients from animals or insects (Venus flytraps). In other words, they are not dependent on photosynthesis for nutrition.

In the sea, plankton do photosynthesise theirs from the sun. However, they are hunted by Mesodinium which are seven times their size at 22 microns. These creatures consume their pink prey and digest most of it but not the organelles responsible for photosynthesis. They are not able to take in and use the carbon dioxide so rely on their victims to supply the necessary chloroplasts. This strategy is called mixotrophy. There are countless mixotrophs in the oceans, neither plant-like nor animal-like.

According to a recent National Institutes of Health (NIH) estimate, 90% of cells in the human body are bacterial, fungal, or otherwise non-human. Although many have concluded that bacteria surely enjoy a commensal relationship with their human hosts, only a fraction of the human microbiota has been characterized, much less identified.

The sheer number of non-human genes represented by the human microbiota – there are millions in our “extended genome” compared to the nearly 23,000 in the human genome – implies we have just begun to fathom the full extent to which bacteria work to facilitate their own survival. We are therefore symbiotic ourselves. Biologists are coming to the conclusion that all life on Earth is interconnected and we are loosing species at an alarming rate.

We, the self styled lords of creation, can no longer just refer to things as  being animal, vegetable or mineral; some are animal/vegetable; some vegetable/fungal; and surely some are yet to be discovered. It is surely about time to take better care of the environment we still know so little about.