Context, material and methods

Seafoams, often associated with decaying marine algal blooms, are ubiquitous and increasingly acknowledged as a potential environmental issue (Lancelot 1995, Spilmont et al. 2009, Schilling & Zessner 2011, Peperzak and van Wezel 2023, Das et al. 2024, Seuront et al. 2024, Cazalis-Henry et al. 2025). The composition, structure, origin, and trophic functions of eukaryotes community trapped in seafoams are still, however, largely unexplored (Schilling & Zessner 2011, Das et al. 2024). Yet, such knowledge is essential for assessing the role of foams in the ecosystem functioning of the aquatic environment.

On the seashore of the eastern English Channel and North Sea during the spring bloom of Phaeocystis globosa (Haptophyte), a recurrent phenomenon of massive seafoam accumulation following agitated seawater and crashing waves is well documented (Lancelot 1995, Blaw et al. 2010, Peperzak and van Wezel 2023). Besides unpleasant visuals and odors (Lancelot 1995, Blaw et al. 2010), these seafoam deposits may concentrate pollutants (The Guardian 2023, Castagno 2025), various microorganisms (Schilling & Zessner 2011), and may provoke sediment anoxia (Spilmont et al. 2009). In addition, recent studies have reported that seafoams impair the behavior and metabolism of intertidal gastropods and are avoided by these organisms, which led to the identification of a new taxis, i.e. aphrotaxis, coined from the ancient Greek word aphrós, ‘seafoam’ (Seuront et al. 2024, Cazalis-Henry et al. 2025).

Figure 1

In this study, we analyzed the community composition of eukaryotic microorganisms using high throughput sequencing of 18S rRNA marker gene on seafoam deposits (N = 30) and adjacent seawater (N = 32) at the beach of Wimereux (North of France) collected over the duration of the phytoplankton spring bloom, i.e. March-May 2023 (Fig. 1). A comparative analysis was performed to investigate differences between seafoam and seawater samples in microbial diversity, community structure, potential microbes origin along a seawater-land gradient, and trophic modes (e.g. autotroph, mixotroph, phagotroph, saprotroph, parasite). The origin and trophic modes were assessed according to available scientific literatures. Samples were also examined by light microscopy (Nikon Eclipse TE2000-S) and scanning electron microscopy (LEO 438 VP).

Results and discussion

Our results notably showed that eukaryotic communities trapped in seafoams were different from communities in adjacent seawater. Indeed, only 10 % of Amplicon Sequence Variants (ASVs, the molecular equivalent of species) were common between the two habitats (Fig. 2A). The result of diversity analysis showed that seafoams accommodate a greater richness of eukaryotic ASVs compared to seawater. These results support the hypothesis that seafoam is a transition habitat and is strongly influenced by interactions with (i) seawater and wave action, (ii) sands during seafoam formation and accumulation, and (iii) the continent under the action of winds, precipitation and animals (Gobet et al. 2012, Whitman et al. 2014, Probandt et al. 2018, Okamoto et al. 2022). In addition, seafoams act as a sticky, sponge-like matrix trapping enormous organic contents.

Figure 2.

More importantly, when considering the trophic modes of microorganisms, seafoams have 7-fold more ASVs affiliated to saprobes and parasites (corresponding to 53% of all ASVs, Fig. 2B) compared to seawater. However, considering the predominant land-origin of saprobes and parasites (i.e. associated to trees, lichens, plants including vegetables, soil, human gut and skin, and fresh water; 87% of the total ASVs, Fig. 2C), their viability in the intertidal zone and seafoam remain unknown. Such unknown also apply to about 25% of auto- mixotrophs, including pollen and remains of wild and cultivated terrestrial plants. Interestingly, certain fungi, amoeba, Rhizaria, dinoflagellates, ciliates, pollen, euglenid, chlorophytes and diatoms were present only in seafoams (Fig. 3), which may indicate that seafoam acts as selective filters accumulating species and taxa otherwise present at very low concentration in the bulk phase seawater, hence below the detection limits of our analytical approach.

Figure 3

Conclusion

Eukaryotic communities in seafoams are distinct from those in adjacent seawater. Given that seafoams typically occur at the sea-land interface, the seafoam-trapped eukaryotic communities were enriched by organisms originating from land, especially saprobes and parasites. Future studies are needed to investigate their variability as well as their biological activity in a habitat different from the ones they originated from.