Disputation i ekologi: Thomas Mollica

För att erhålla en inbjudan till den digitala disputationen vänligen kontakta fakultetshandläggare Mirjam Lingkrans: mirjam.lingkrans@lnu.se

Abstract

The Baltic Sea is a unique aquatic system with a North-South gradient in temperature, salinity and nutrient limitations. The northern regions, like the Bothnian Bay and Bothnian Sea, have lower salinity and temperatures, with phosphorus (P) as limiting nutrient. In the central Baltic Proper, the environment is characterized by a higher temperature and salinity and nitrogen (N) is the limiting factor. Throughout the year, different species of phytoplankton – microalgae – can dominate the ecosystem: diatoms and dinoflagellates in spring, cyanobacteria in summer and small flagellates in autumn. Both N and P are essential for these communities, but research has historically prioritized N. However, with the growing concerns of climate change and eutrophication - the excessive input of N and P from human activities - P is increasingly becoming a focal point of study.

This thesis explores how phytoplankton and bacteria acquire P in different regions of the Baltic Sea, which exhibits a unique north-south gradient that may lead to variations in P acquisition across basins. To investigate this, I conducted a year-long sampling of water at the Linnaeus Microbial Observatory (LMO) in the Baltic Proper and participated in three research cruises in the Bothnian Bay and Bothnian Sea, covering spring and summer periods.

The study reveals spatial and temporal differences in community composition across the basins. Notably, diatoms dominate the community during spring, i while dinoflagellates prevail in the Baltic Proper, and cyanobacteria thrive in the summer, except in the Bothnian Bay. Despite these changes, small plankton (< 20 µm) consistently contribute to biomass and nutrient uptake across the basins. Gene expression analyses indicate that phytoplankton and bacteria employ different P acquisition strategies, with phytoplankton primarily remodelling their membranes to cope with P fluctuations, whereas bacteria express both P transporters and genes involved in membrane remodelling. The range of strategies employed to acquire P was largely consistent across different basins. These community-wide trends are accompanied by variations within bacteria and phytoplankton, highlighting their adaptability and resilience. Ultimately, the complex interplay between environmental parameters and biological adaptability deepens our understanding of ecological dynamics in the Baltic Sea and marine ecosystems.