Probing the role of parasitic fungi
Parasitic fungi are known to infect phytoplankton cells in the world’s oceans, yet their wider role and impact on global biogeochemical cycles is unclear. We spoke to Isabell Klawonn, Ph.D. about her research into the function of these fungi and how they affect interactions between phytoplankton and bacteria.
The role of marine fungi in carbon and nutrient cycling has not historically attracted much attention in research, partly due to the difficulty of identifying them under a microscope. Over the last decade or so a lot of effort has been devoted to sequencing environmental communities, indicating a widespread distribution of fungal microparasites, now Dr Isabell Klawonn and her team at the Leibnitz Institute for Baltic Sea Research (IOW) are looking to probe deeper into the impact of these fungi on global biogeochemical cycles. “On the one hand we want to look at how they drive, change or modulate interactions between phytoplankton and bacteria, while we also want to look at how they affect the carbon fixation activity of those phytoplankton, and then at the fate of the fixed carbon,” she outlines. This research is centered on a type of fungi called Chytridiomycota – or chytrids – which infect and parasitise certain kinds of plankton, with Dr Klawonn looking mostly at diatoms and cyanobacteria. “These are both types of microalgae,” she says. “Diatoms are very important when looking at the global scale, as they frequently form blooms in the global ocean. Cyanobacteria – also called blue-green algae – can be toxic and get infected by a parasite.”
This process begins when the parasite finds a suitable host and is encysted, meaning that it essentially attaches itself to the host. The fungal microparasite then penetrates the host and effectively reaches out with rhizoids, which can be thought of as part of its digestive system, and transports the nutrients from the diatom cell to its outer structure, the sporangium. “This structure grows and grows over time. Eventually it opens up and new fungal zoospores are released, and then they try to find a new host,” explains Dr Klawonn. The aim now for Dr Klawonn and her colleagues in her Emmy Noether-funded research group at the IOW is to build a fuller picture of the role of these microparasites, work in which they are using a variety of experimental approaches. “We try to go from single cells to entire populations. A population would be for example a community with only one species, and then we can go to mixed communities
which include several species,” she outlines. “We want to find out what’s going on at the single cell level, and then we are trying to extrapolate from that to a wider community.”
experiments on cultures in the lab. In these experiments, researchers compare a host culture that is infected with a fungal parasite to one that isn’t, and look at certain key
structure of the fungus, which is called the sporangium,” continues Dr Klawonn. “At the moment we are able to trace carbon shunted from phytoplankton to the sporangia, and to the zoospores that are released later on.”
A further dimension of the group’s research involves looking at the effects of fungal parasitism on the fate of sinking organic matter in the coastal ocean, in particular aggregates formed when phytoplankton cells stick together or to other material. Recent findings from lab-based research suggest that carbon fixed by the phytoplankton is remineralised faster in surface waters, rather than settling down on the seabed. “This is because it’s transferred to the fungus, then the fungus releases these zoospores, and they stay in the surface water, ” explains Dr Klawonn. We have also found that the likelihood of phytoplankton sticking to each other and forming aggregates can depend on whether they have been infected by a parasite. “These phytoplankton cells usually produce a carbon-rich mucus on their exterior, which is kind of sticky. If two cells are very adhesive they would stick together and then form these aggregates,” says Dr Klawonn.
Phytoplankton-bacteria interactions
This is a topic Dr Klawonn hopes to investigate in more detail in future, alongside continued research into the interactions between phytoplankton and bacteria and how those interactions change during chytrid epidemics.
The group’s research could also hold important implications in terms of detecting chytrids, a topic that Dr Klawonn says has been relatively neglected. “The countries around the Baltic Sea have conducted extensive research over the last few decades. It’s one of the bestmonitored areas worldwide, yet still, these chytrids haven’t attracted much attention,” she explains. While a lot of effort is devoted to monitoring and analysing the water around the Baltic Sea, it remains difficult to recognise when phytoplankton are infected with these microparasites, an issue that Dr Klawonn is working to address. “We have suggested a method of double-staining the chitin cell wall within chytrids, which we think helps people to recognise infections,” she outlines. “It’s a very simple method. We provide advice on how to best use the protocol and what to consider when a staining is positive.”
An effective long-term chytrid monitoring regime would require high-throughput methodologies, yet the focus for Dr Klawonn is more on in-depth research into chytrids and their function. This research is very much ongoing, with Dr Klawonn planning to explore different avenues of investigation to build a deeper picture of fungal parasitism and its wider importance. “For example, in future we will probe deeper into the molecular biology of these fungal microparasites,” she says.
FunPhy
Fungal infections on phytoplankton — cryptic perturbation of phytoplankton growth, recycling and sedimentation
Project Objectives
The Emmy Noether Junior Research Group will elucidate the functional and quantitative role of parasitic fungi on phytoplankton blooms and element cycling in brackish and marine waters. Our comprehensive approach will include experimental work on phytoplankton–fungi co-cultures as well as field-sampled plankton communities, spanning from single-cell up to mesoscale-scale flux measurements across the water column.
Project Funding
This project is funded by the DFG - Deutsche Forschungsgemeinschaft.
Project Partners
• Silke Van den Wyngaert, University of Turku, Finland
• Luca Zoccarato, University of Natural Resources and Life Sciences, Vienna, Austria
• Maliheh Mehrshad, Swedish University of Agricultural Sciences, Uppsala, Sweden
• Martin Whitehouse, Swedish Museum of Natural History, Stockholm, Sweden
Contact Details
Project Coordinator, Isabell Klawonn, Ph.D Junior Research Group Leader, Emmy Noether Program (German Research Foundation) Microbial Plankton and Biogeochemistry Leibniz Institute for Baltic Sea Research, Warnemuende (IOW), Office 404
T: +49 381 5197-0
E: isabell.klawonn@io-warnemuende.de W: https://www.io-warnemuende.de/ isabell-klawonn-en.html
Klawonn, I., S. Van den Wyngaert, A. E. Parada, N. ArandiaGorostidi, M. J. Whitehouse, H.-P. Grossart and A. E. Dekas (2021). Characterizing the “fungal shunt”: Parasitic fungi on diatoms affect carbon flow and bacterial communities in aquatic microbial food webs. Proc. Nat. Acad. Sci. U.S.A. 118: e2102225118, doi: 10.1073/pnas.2102225118 Klawonn, I, Van den Wyngaert, S, Iversen, M H, Walles, T J W, Flintrop, C M, Cisternas-Novoa, C, Nejstgaard, C, Kagami, M, Grossart, HP (2023) Fungal parasitism on diatoms alters formation and bio–physical properties of sinking aggregates. Commun. Biol. 6, 206, doi.org/10.1038/s42003-023-04453-6
Isabell Klawonn, Ph.D
Plankton communities
Researchers in Dr Klawonn’s group are both working with natural plankton communities and also isolating cells and conducting
parameters. “One crucial consideration is how different populations develop in terms of cell abundance, in terms of how many cells get infected. We also look at chlorophyll content, which is usually a sign of photosynthetic activity, then we can use more sophisticated methods to probe deeper into other aspects,” says Dr Klawonn. One major aspect of interest is the so-called fungal shunt, the process by which carbon is transferred from a host cell to the fungal parasitic cell. “In one publication we quantified how much carbon is transferred from one phytoplankton cell –the diatom Asterionella formosa – to the outer
Isabell Klawonn is a Junior Research Group Leader at the Leibniz Institute for Baltic Sea Research. She gained her PhD from Stockholm University and was a postdoctoral scholar at Stanford University in the US before taking up her current role.
We want to look at how fungal microparasites drive, change or modulate interactions between phytoplankton and bacteria, while we also want to look at how they affect the carbon fixation activity of those phytoplankton.Illustrated based on SEM images taken by K. Schulz (Medical Biology and Electron Microscopy Centre, Medical Faculty, University of Rostock) and I. Klawonn. Sampling of plankton communities on-board the research vessel R/V Sonne. Credit: N. Choisnard. Isolates of model pathosystems growing under controlled laboratory conditions.