Sleeping witnesses: time travel through sediment egg bank

By bringing waterfleas from decades ago back to life, we could track adaptive processes and uncover reversed evolution in response to man-made environmental changes.

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As a child I had a prehistoric Triops kit. For everyone who did not: You get a small aquarium and some sand with Triops eggs inside. Add water and after a couple of days those prehistoric animals come to life. Amazing! I guess I am still doing the same, at least in some way our study started very similar.

Shortly after I kicked off my PhD research at beautiful Lake Constance, Germany, my supervisor and me went to get sediment cores to resurrect Daphnia, or waterfleas. Waterfleas have so many interesting features that this would be worth an article by itself. One that made this study possible is their amazing survival strategy. In nature, when conditions in the lake become unfavourable, waterfleas produce resting eggs. Typically, this is the case in autumn when food starts becoming scarce, the days get shorter and the temperatures drop. Resting eggs are enclosed in protective shells, which we call ephippia. They look a little bit like ravioli filled with two tiny eggs.

on the left: Can you find the Ephippia? Look for the little Ravioli stuffed with two eggs. This is how a washed sediment sample looks like; on the right: This is one of our waterfleas (Daphnia galeata).

Once the waterfleas shed the ephippia off, they slowly sink down to the bottom of the lake, where they lay dormant until conditions become favourable again. When temperatures rise and the days get longer, tiny waterfleas hatch from the resting eggs to repopulate the lake. But what happens when the eggs sink down to the bottom of a deep lake, where it is always cold and the sun never reaches the bottom? Then the eggs are unlikely to hatch and become archived in the sediments.

This allows scientists like us to go out there, take sediment cores and collect decade-old resting eggs. By exposing them to cozy lab conditions we can bring them back to life: Isolate them from the sediment, place them in the warmth and add some water. Just what I did as a child.

on the left: the sediment corer is about to be deployed; on the right: Me and my PhD supervisor and senior-author on this paper Dominik are processing sediment cores that contain our waterfleas we are about to bring back to life.

Why be so excited about old eggs of tiny zooplankton? The sediment basically is a time capsule and the longer your sediment core, the further you can travel back in time. I will never forget how awestruck I was when we opened the first sediment core and I could see all those pretty laminated layers. Not only looking at the layer that was deposited in the year that I was born but also layers from World War II and I all the way back to layers that were deposited during the French revolution – that would be 1799.

We do not know yet what determines if you can bring resting eggs back to life, but it seems to depend on the lake. For Lake Constance this evidently means, no waterfleas older than 40 years can be resurrected. Still, pretty amazing, no? It would be challenging to maintain Daphnia cultures in the lab for so long and, most likely, they would have adapted to the lab conditions. But our newly hatched individuals, whose mothers lived decades ago, are adapted to different environmental conditions they experienced in the lake at that particular time.

 In our case, the different conditions were man-made. Many freshwater bodies in the 1970s and 1980s were polluted by untreated waste-water, run-off from agriculture and industry. The resulting increase in nutrients, especially phosphorus and nitrogen, had serious consequences for the lake. A particularly undesirable one was the mass development of cyanobacteria. Cyanobacteria are also called blue-green algae. You know a lake that is closed for bathing in summer? Most likely, you can blame cyanobacteria for that. They can produce toxins that can be harmful not only to us when we bath in the lakes but also to all organisms living in the lake.

Beautiful Lake Constance with a view on the German, Austrian and Swiss Alps. We are excited to find out what lessons we will continue to learn from that lake.

From studies by our co-author Nelson we already know that the Daphnia galeata population was able to evolve resistance to cyanobacteria when Lake Constance was polluted and cyanobacteria biomass increased in the 1970s and 1980s. Luckily, humanity is trying to reverse some impacts it had on this planet and the various ecosystems. For nutrient-polluted lakes this means installing or improving wastewater treatment plants and regulating the anthropogenic nutrient input. In Lake Constance, those efforts were successful; the nutrient load was reduced and consequently the cyanobacteria biomass decreased. 

Development of phosphorus concentration and cyanobacteria biomass over the past decades in Lake Constance.

When we are reversing our impacts on ecosystems, we automatically assume that it will reach the natural state again. However, we often do not know how organisms react to reversed anthropogenic changes, because these changes impose new selection pressures on the inhabitants of the respective ecosystems.

 If you want to find out how the waterflea population in Lake Constance reacted once the nutrient load was reduced and the cyanobacteria biomass decreased, check out our paper:

Reversed evolution of grazer resistance to cyanobacteria

Or continue reading, attention spoiler alert:

 The resistance to cyanobacteria that waterfleas evolved when the lake was polluted and cyanobacteria increased in biomass was lost again. Individuals whose mothers lived at the beginning of the 21st century can’t handle cyanobacteria and are, for example, unable to reproduce when they are exposed to cyanobacteria in their food. Just like the evolution of the resistance, the loss of resistance happened very fast at high evolutionary rates within only few Daphnia generations. This story, the evolution of grazer resistance, is an example for how fast evolution can occur. Within one human-life span our impacts on ecosystems can lead to evolving a new trait- here the resistance to cyanobacteria- and also to the loss of it. This should make all of us, once more aware of the extend our impacts have on our surrounding ecosystems and the organisms that live in it.

 

Jana Isanta-Navarro

Postdoc, Flathead Lake Biological Station, University of Montana