Christa Schleper

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Christa Schleper

How did complex life evolve on Earth? This is still one of the greatest mysteries of biology, and Christa Schleper is one step closer to finding the answer. The microbiologist succeeded in cultivating tiny unicellular organisms, which may represent the missing link in the evolution to complex living organisms such as plants and animals. Archaea are among the first living things on Earth and offer new insights into our own evolution. At the same time, they play a crucial role in our future.

Thundering and crashing. Violent volcanic eruptions, meteorite impacts, and lightning strikes. Enormous heat and radioactive radiation drive chemical reactions: This is how scientists imagine the Earth around four billion years ago. The first chemical compounds are formed from sulfur and hydrogen, the primordial soup bubbles and boils. Bacteria and archaea emerged about 3.8 billion years ago, existing between fire and ash. During the following three billion years, the earth cools down, water vapor is produced, and oxygen is released. The landscape becomes calmer. Around 2 billion years ago, the first more complex cell structures developed from the original life forms, which later gave rise to a huge diversity of plants and animals. These tiny single-celled organisms formed the substrate for the development of all further life. How this evolutionary transition to the first more complex cells succeeded has not yet been deciphered.

A first decisive step towards solving the mystery was taken by the US microbiologist Carl Woese in the late 1970s: He discovered that not all microorganisms are bacteria, but that there is a second, independent class of microorganisms. Many of these tiny single-celled organisms live under extreme conditions: Some feel at home in boiling sulfur vapor at 100 degrees Celsius, others live in concentrated salt solutions or in a highly acidic environment. Because of their preference for archaic habitats that resemble the living conditions of primordial Earth, these life forms are called archaea. With their tiny size of 400 nanometers (0.4 thousandths of a millimeter), many of them are among the smallest living creatures on our planet.

This discovery meant that the classification of biology had to be rewritten. Today, all living things on earth are divided into three major domains: Eukaryotes, bacteria, and archaea. Eukaryotes include animals, plants, protists, and fungi. Their cells are usually larger and more complex than the cells of bacteria and archaea. They have a nucleus containing the cell’s genetic information, and a cytoskeleton, which determines both the cell shape and the transport within the cell.

In a nutshell

Since the beginning of her career, Christa Schleper has been working on Archae—these microorganisms are, together with bacteria, among the first living things on earth. Her studies on Archae have led to groundbreaking discoveries and have improved our understanding of the nitrogen cycle. Her findings are intended to help us better understand the role of microorganisms in the soil and how they can be used in future, for example, to develop more sustainable agriculture.

Christa Schleper taking a sediment sample from the Danube — They are among the first living creatures on Earth. Christa Schleper discovered archaea, previously known only from hot springs and other inhospitable places, in our everyday environment for the first time – for example here in sediments on the banks of the Danube. © Ulrich Zinell

Satellite image of the mouth of the Mississippi River showing sediments discolored by phosphorus and nitrogen — Mouth of the Mississippi River in a 1999 satellite image. Due to the massive use of artificial fertilizers in the agricultural industry, the sediments that the Mississippi River washes into the Gulf of Mexico contain enormous amounts of phosphorus and nitrogen. This allows algae to grow explosively, consuming oxygen. This creates “death zones” where higher life forms can no longer exist. © Science Photo Library picturedesk

“Without archaea, we wouldn't exist. The more we learn from them, the more we understand our own evolution.”

Based on molecular studies, especially genomics, scientists already knew that both groups, bacteria and archaea, must each have played a central role in the evolution to more complex, eukaryotic cells. Archaea and bacteria evolved separately almost four billion years ago. Today, many researchers assume that after about two billion years, two representatives of each group reunited and that a eukaryotic primordial cell formed from this symbiosis, from which plants and animals eventually evolved. The bacterial partner in this symbiosis is known (alpha proteobacteria were the precursors of today’s mitochondria in eukaryotic cells). But which archaea were involved in this important event?

A milestone in the search for this missing link was reached in 2015 by microbiologist Christa Schleper: Her lab team took samples from a depth of 3,000 meters near a hydrothermal field off the coast of Norway. These samples were found to contain a new species of archaea called Asgard archaea. Thanks to genetic markers, she and her colleagues were able to show that these Asgard archaea living today are the closest relatives of eukaryotes and share a common ancestor with them.

The researchers at the University of Vienna have now been able to find similar archaea in many places, even in sediment samples taken from the banks of the Danube river in Vienna. However, in order to study Asgard archaea in more detail, it was necessary to cultivate them in greater enrichment in the laboratory. Cultivating these extremely sensitive organisms turned out to be extremely difficult and tedious work requiring great precision.

Short bio

Christa Schleper is Professor at the Department of Functional and Evolutionary Ecology at the University of Vienna. She studied biology in Aachen and Constance, received her PhD in Munich, and has researched and taught in the USA (Caltech, UC Santa Barbara), Norway (Bergen), and Germany (Darmstadt). She has been studying archaea since the beginning of her career and was one of the first to use metagenomic methods in her field of research. She is one of the most cited researchers in the world. Among numerous other awards, she has received the ERC Advanced Grant from the European Research Council in 2016 and the FWF Wittgenstein Award in 2022.

Microbiologist Christa Schleper has dedicated her research career to archaea – the tiny unicellular organisms that counted among the first life forms on Earth. Not only has the 2022 Wittgenstein Award winner discovered numerous of these microorganisms, but she also pioneered in cultivating them in the laboratory.

Then, after six years of “hard and frustrating” efforts, finally success: For the first time worldwide, Schleper’s group managed to produce a highly enriched, stable culture of Asgard archaea derived from marine sediments taken from the coast of Piran, Slovenia. This allows the cells to be examined in detail with a modern cryo-electron microscope. “These archaea take entirely new cell forms. They are complex and interwoven, and some have very long, tentacle-like appendages,” says the researcher from the University of Vienna, describing the bizarre structures. But above all: Cells have a cytoskeleton, the structure that gives the eukaryotic cell its external shape and is responsible for movement and transport. Until now, it was assumed that only higher organisms developed cytoskeletons. “There are still some exciting discoveries to be made in this field,” says the professor from the Department of Functional and Evolutionary Ecology. When the German-born scientist began using metagenomics in the 1990s to search our everyday environment for archaea, which until then had only been known from hot springs and other inhospitable places, she was a pioneer.

As early as 2005, she made an important discovery with the help of metagenomics: Archaea were found in very large numbers in soil samples, which play an important role in the nitrogen cycle. At its former location in Vienna-Alsergrund, her research group again succeeded in cultivating and studying a representative of these archaea in the laboratory. “Nitrososphaera viennensis,” named after the place where it was found, is an ammonia-oxidizing microorganism that forms nitrite and thus plays an important role in the nitrogen cycle. “In natural, pristine habitats, this nitrogen cycle is a healthy and important process. It’s agriculture that causes problems,” explains the microbiologist.

On average, only 30% of the nitrogen used in agriculture is absorbed by plants; the rest is flushed out and ends up in our groundwater, rivers, lakes, and oceans. With fatal ecological consequences: The high nitrogen load leads to explosive growth of algae, which consume oxygen in the process. “Death zones” emerge, where no higher life forms can exist. A recently published study by Chinese scientists led by Lei Liu shows how dramatic this development is: Annual inputs of ammonia to the oceans in 2018 were about 89% higher than in 1970.

Field research on volcanoes and hot springs

Agriculture is responsible for two-thirds of the global load of reactive nitrogen. While global food production has doubled over the past four decades, the use of artificial nitrogen fertilizer has tripled. “If we can better understand archaea and regulate their activity, hopefully we will need less fertilizer in the future and can also reduce greenhouse gas emissions in arable soils,” says Schleper.

Helping the environment with her work is is one of the researcher’s key priorities. For example, she holds a lecture series in cooperation with the “Fridays for Future” movement, allowing her to reach many young people. “All researchers – but especially those of us who work on environmentally relevant topics – have a responsibility,” Christa Schleper says, citing her motivation. “We need to speak up even louder and dare to communicate beyond the boundaries of our disciplines to draw attention to the climate crisis.”

She was never a “common garden-variety biologist who always collected bugs,” laughs the 60-year-old. She came to biology by chance through a fellow student. But then it took ahold of her, and the fascination is still with her today. She also has coincidence to thank for her choice of research fields. Through a temporary job at the Max Planck Institute of Biochemistry in Martinsried near Munich, the biology student joined the group of a pioneer in archaea research – an almost esoteric field at the time, as Schleper recalls, and not the booming field of study it is today. She was fascinated by the open, diverse working atmosphere, and ended up sticking with it. She describes this decision as a key moment: the realization that she had found her place.