Culture conditions used for growth of phytoplankton, and bacteria, and details of terrestrial material analysed.

Phytoplankton and bacteria strains used for this study were obtained from different culture collections and universities. They were cultured using optimal media for each strain. Phytoplankton cultures were maintained under either a 14:10 or 16:8 h light:dark cycle. Temperature of all phytoplankton cultures was 18−20°C (average±SD) and bacteria 23−30°C (average±SD). Terrestrial matter includes finely ground particles of leaves from one grass (Phragmites) and two tree (Betula and Alnus) taxa. Due to the high number of strains we were not able to perform all analysis from all cultured strains. UWCC: Algal and Fungal Culture Collection of University of Washington, Seattle, Washington, USA; Peltomaa: Lammi Biological Station, University of Helsinki, Finland; CCAP: Culture Collection of Algae and Protozoa, Ambleside, Cumbria, UK; Gilbert: Dartmouth College, New Hampshire, USA; NIVA: Norwegian Institute for Water Research, Oslo, Norway; UTEX: University of Texas Culture Collection, University of Texas at Austin, Texas, USA; CPCC: Canadian Phycological Culture Centre, University of Waterloo, Ontario, Canada; CCMP: National Center for Marine Algae and Microbiota, Bigelow Laboratory for Ocean Sciences, Maine, USA; Carolina: Carolina Biological Supply Company, Burlington, North Carolina, USA. CFA = compound-specific SIA, TLS = lipid stable isotope, PLS = phospholipid stable isotope, C% = carbon content (%), TL% = lipid content (%) and PL% = phospholipid content (%).