PhD thesis defence: Neea Hanström

Zoom-link: https://stockholmuniversity.zoom.us/ s/69876235336.

Protist parasites in marine ecosystems

Marine plankton form complex interaction networks that support the productivity and functioning of marine ecosystems. Although parasitic interactions are widespread in marine ecosystems, they have received less attention than other biotic interactions. Advances in DNA metabarcoding have revealed that parasitic protists, particularly Syndiniales, are among the most diverse and widespread components of marine microbial communities. However, their ecological roles, host associations, and environmental responses remain poorly understood. The overall aim of this thesis was to investigate the host associations, diversity, distribution, and environmental drivers of protist parasites across the Baltic Sea-Skagerrak system, using molecular methods.

The results showed that the occurrence and prevalence of host-associated Syndiniales were strongly influenced by habitat characteristics, particularly oxygen availability and salinity (I-III). Higher salinity was associated with greater Syndiniales diversity, although Syndiniales were prevalent across the brackish water gradient (II). The free-living reproductive life stages of Syndiniales, known as dinospores, also showed distinct distribution patterns, with high occurrence in environments where conditions were potentially favourable for parasite-host co-occurrence and transmission (II-III). Copepods of the genus Pseudocalanus showed associations with the Syndiniales endoparasite Hematodinium, highlighting a host association that was not previously well recognized in the Baltic Sea (I-III). Hematodinium prevalence in Pseudocalanus was associated with low oxygen concentrations and higher salinity in deep waters of the Baltic Sea. In the lower-salinity surface-dwelling copepods Acartia and Centropages, prevalence was lower, but still about half of the individuals tested positive for Hematodinium (III). Environmental Hematodinium DNA representing free-living dinospores showed higher relative read abundance in environments potentially favourable for parasite transmission (IIIII). Furthermore, fish gut dietary DNA revealed that fish can serve as natural samplers of the planktonic community and that protist DNA can be traced in consumers at higher trophic levels (IV). This shows that dietary DNA can reveal otherwise hidden associations in marine food webs and highlights the importance of protist integration into pelagic foodweb dynamics.

The findings in this thesis show that parasitic protists are an important and previously overlooked component of the Baltic Sea ecosystem, and their distributions and associations with hosts are closely linked to environmental conditions. This thesis advances our understanding of marine protist ecology by showcasing the role of environmental gradients in shaping parasite prevalence and by revealing previously hidden parasite-host and protist-consumer associations. A better understanding of these interactions is important for predicting how parasite-host dynamics and pelagic communities may respond to environmental change in marine ecosystems.

Opponent, committee and supervisors

Supervisors

• Prof. Monika Winder
• Prof. Sarahi Garcia
  

Opponent

Associate Prof. Elizabeth Harvey, University of New Hampshire, USA.

Committee

• Prof. Agneta Andersson (Umeå University, Sweden)
• Prof. Josefin Titelman (University of Oslo, Norway)
• Peter Heintzman (Stockholm University, Sweden)
• The reserve committee member will be Prof. Johan Eklöf (Stockholm University, Sweden).