In Insights we discover the story behind and beyond a recent publication in Functional Ecology. What inspired the authors to do the research, how did the project develop and what implications might their results have on the scientific community and on society?

This week, we talk to Florian Altermatt from Eawag (the Swiss Federal Institute of Aquatic Science and Technology) in Dübendorf, Switzerland, and the Department of Evolutionary Biology and Environmental Studies at the University of Zurich, Switzerland, and corresponding author on Leaf litter diversity and structure of microbial decomposer communities modulate litter decomposition in aquatic systems (plain language summary here.)

What is your paper is about and how does it links to other work from your lab?

In our lab, we are interested in how ecological dynamics are shaped by local properties of communities, such as the diversity of organisms within the community and regional properties like the flow of organisms or materials across ecosystem. A typical example of where local and spatial properties may interact is the flux of leaf litter from forests into streams, where these leaves get decomposed and spur dynamics in the aquatic communities.

In this paper, we were interested in how diversity of leaves (and thus the inflow from the terrestrial system) affects decomposition rates, and if and how the structure of the microbial community in the aquatic ecosystem plays a role. We found that different leaf types, such as beech, oak or poplar, decompose at different rates, that there is a somewhat higher decomposition rate in some mixtures of leaves, and that decomposition is faster when the whole microbial community is present compared to cases where larger microorganisms, potentially containing certain feeding types, are absent.

Much of my lab’s work is centred around microcosm experiments, where we can mimic aspects of the real world (but with a high level of control) to get a causal understanding of processes. In this study we especially highlight that there are interlinked dynamics, where both the terrestrial input as well as the aquatic component affect ecological processes.

 

The first author of the paper, Fabienne Santschi, was a master student in your lab at the time of the experiment. I think it’s great that MSc students get this opportunity; it can certainly be a good way for student to launch a scientific career and land a PhD studentship.

Of course this can be a great start into an academic career, but writing a paper is a good learning experience in general, which can also be useful for other career paths! The paper is indeed based on the MSc thesis of Fabienne. I usually give my MSc students the freedom to develop and plan their own research project, in close discussion with me and other lab members involved in the project (some of them are the co-authors). Fabienne not only conducted the experiment, but also led the analysis of the data, prepared the figures and text for her thesis. While I was then helping to bring these elements together in a manuscript, the paper is still very much based on her individual thesis parts, so it is well-deserved that she is the first author.

 

Your results show a very important impact of litter quality on decomposition rates. Most of the litter combinations had a synergistic effect on decomposition rates. Would it be to bold of me to conclude that your research indicates a priming effect?

Actually, most of the effects we observed were additive, and not synergistic. Thus, decomposition of the leaves is strongly affected by their individual quality, such as the nitrogen or phosphorus content, even in mixtures. Only in some cases we had some synergistic effects, where there could be a priming of microbial communities on one leaf type, which then could positively affect decomposition in other leaves. However, this is a speculative interpretation, as we were not looking at changes in the microbial communities on the individual leaves themselves, but rather in the overall microbial community present in the water around the leaves.

 

What I think is especially great from your paper is the finding about the role of the structure of the microbial community. Your research stresses that the functional completeness of the microbial community is important for efficient litter decomposition. Under natural circumstances, how would we end up with a non-complete microbial community, and if lost, can we restore them?

We were especially interested if the presence (or lack) of certain functional groups or size classes of microbes affects decomposition rates. We therefore sampled water and the respective natural microbial community in a relatively pristine, forested stream in Switzerland. For half of the leaf treatments, we used this microbial community, while in the other half we removed the larger microbes by sieving. The removal of these larger size classes resulted in a small, but significant reduction in decomposition rates. While we obviously can’t generalize to all microbial communities, our work highlights that changes in the microbial community structure affect essential ecosystem processes.

In natural ecosystems we can imagine both natural but also anthropogenic causes of how the structure of a microbial community is affected. For example, the presence of certain filter feeders may disproportionately remove certain size classes. Alternatively, chemical pollutions may differentially affect microbial communities, such that bacteria and small protozoa are differently affected than generally larger metazoans, such as rotifers.

 

Reading your paper I had to immediately think of a recently published paper by Elly Morriën and her colleagues in Nature Communications, where she show that the structure of the soil food web becomes increasingly interlinked with ecological restoration, subsequently improving ecosystem functions. Can we safely assume that the structure of microbial communities is important for many more ecosystem processes across a range of ecosystems? Do you have indications from your own lab that shows that?

I think that the role of microbial communities and microbial species on many ecosystem properties can be hardly overestimated. We may very much have ecosystems without large organisms, but I can’t imagine an ecosystem without an important role of microbial communities. It is also more and more recognized that microbial communities are not only important in aquatic ecosystems, but also in terrestrial ecosystems, for example affecting plant growth, or – within the microbiome ­– for individual organisms. While I would not be surprised if the structure of the microbial communities is thereby playing an important role, it would be not possible to directly extrapolate from our work to these other systems, as the microbial communities can be radically different. Also, in our study we did not look at how microbial communities change over time, for example in ecosystems at different states of a succession or restoration.

 

And one last question: when you’re not in the lab, where can we find you?

Honestly, I am not so often in the lab myself these days. While I try to be involved in the data collection of all projects, the heavy lifting of the lab work is often in the hand of my students, postdocs and lab-technicians. So you would probably find me in my office, at least during working hours, or in some meetings. As the field site in this specific case is not far from where I’m living, you might also find me there when I go for a walk with my family on the weekend.