Sensing Danger: Inducible defenses and chemical signaling in marine phytoplankton

Abstract

Copepods play a central role in marine food webs, acting as an important link between primary producers and higher trophic levels. Predator-prey interactions between copepods and their prey drive the evolution of defense traits in phytoplankton, including chemically mediated inducible defenses. While our understanding of how predator alarm cues structure plankton communities is developing rapidly, the molecular mechanisms underlying predator recognition remain hidden beneath the ocean surface. In this study, I target the first part of the signal transduction pathway: the receptor proteins to which grazer cues bind. I hypothesize that dinoflagellates and diatoms possess different types of G-protein coupled receptor (GPCR) proteins to detect copepodamides. Using a combination of dose-response experiments and in silico binding assays between the copepodamide ligand and GPCR proteins identified by bioinformatics from the transcriptomes of the diatoms Skeletonema marinoi and Skeletonema dohrnii, I have identified copepodamide candidate receptors. Finally, the expression of two top receptor candidates in response to ligand exposure were confirmed by dPCR analysis. The results represent a strong lead towards the identification of the first receptor-ligand pair in phytoplankton. Furthermore, this could provide new insights into the evolution and molecular mechanism of grazer-induced defense traits in plankton.

Popular Abstract

How Tiny Ocean Plants Smell and Survive Danger

The ocean is full of microscopic life, and one key group is phytoplankton—tiny, plant-like organisms. But even they face danger: they are a favorite food for copepods, small crustaceans that life in the ocean too. Surprisingly, phytoplankton can defend themselves. Some can smell copepods based on the chemicals they release, called copepodamides. Phytoplankton reacts by changing shape into smaller chains of cells, becoming too small to catch, or producing toxins that make them unappetizing. Yet much remains unknown. Do different phytoplankton respond differently to these signals? Does the type of chemical matter? And how do they detect these cues—what receptor do they use?

To test this, I exposed different species of phytoplankton to different mixtures and concentrations of copepod signals, observing their reactions. I also used computer-based tools to explore how potential receptor proteins might interact with these signals. Finally, I checked whether these receptors appeared more or less often in phytoplankton that had been exposed to the signals compared to those that hadn’t.

I found out that different phytoplankton species react differently to copepod signals. Some respond quickly and strongly to all mixtures, while others react more slowly and show more distinct responses depending on the type of signal. This suggests that different species may have different “noses”—that is, different smell receptors that detect these copepodamides in unique ways. By testing how well potential receptors could interact with the chemical signals, I was able to rank them from most to least likely to be involved. When I looked at how many copies of these receptor genes were present in exposed versus unexposed phytoplankton, two stood out—as they were expressed in all samples, making them promising candidates for further study (Fig. 1).

Previous studies suggest that diatoms, my species of phytoplankton of interest, likely have class C G-protein-coupled receptors (GPCRs). These receptors have a large part outside of the cell that interacts with the signal and seven domains that are lying within the cell membrane. Inside the cell, a shorter part interacts with G-proteins, which help to pass the signal along (Fig. 2). By analyzing the structure of my receptors, I was able to confirm these typical features. I found that the two GPCR candidates (532 and 638) had the correct shape, bound well to a copepod signal molecule and may become more active in the presence of the smell. This suggests that these proteins could help diatoms to smell nearby copepods.

Why does it matter?
Phytoplankton take up carbon dioxide and produce the oxygen, helping to slow down climate change. When they die or are eaten by organisms such as copepods, some of the carbon they have absorbed sinks to the deep ocean where it can stay trapped for a long time. This process helps to balance the Earth's carbon levels. Understanding how phytoplankton detect and respond to copepods at a molecular level could help us to predict changes in phytoplankton behavior, bloom formation and carbon cycling. Knowing which receptors diatoms use to smell chemical signals also provides insight into how cells evolved to sense their environment, revealing how ancient organisms adapted to life in the ocean. This could help us to better understand how modern marine life evolved the ability to smell and survive danger and respond to their surroundings.

Master’s Degree Project in Biology, 60 credits
Department of Biology, Lund University

Advisor: Erik Selander
Advisors Unit/Department: Lund University, Department of Biology, Aquatic Ecology

• author: Knappe, Hannah C.
• supervisor: Erik Selander; Dan-Dan Zhang
• organization: Degree Projects in Biology
• course: BION02 20242
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 9212849
• date added to LUP: 2025-09-19 11:13:24
• date last changed: 2025-09-19 11:13:24