Algal and bacterial strains and cultivation

Lobomonas rostrata (SAG 45‐1) was obtained from the Experimental Phycology and Culture Collection of Algae at the University of Goettingen (EPSAG), Germany. It was grown autotrophically on TP⁺ medium (Kazamia et al., 2012). Vitamin B₁₂ was provided as cyanocobalamin (Sigma‐Aldrich) at 100 ng l⁻¹, as this supports the maximum carrying capacity of L. rostrata (Kazamia et al., 2012). The L. rostrata–M. loti coculture was an established coculture that had been maintained over many generations without a source of organic carbon or vitamin B₁₂. Cultures were maintained on a 16 : 8 h, light : dark cycle, with shaking (140 rpm) at 25°C. M. loti (MAFF 303099) was a gift from Prof. Allan Downie at the John Innes Centre, Norwich, UK. It was maintained axenically in TP+ with 0.1% v/v glycerol at 28°C. Cells were harvested by centrifugation, and, if not analysed immediately, cell pellets were frozen in liquid N₂ and stored at −80°C. Algal cell counts were determined using a Dual Threshold Beckman Coulter (Z2) Particle Counter and Size Analyser (Indianapolis, IN, USA). Bacterial cell numbers were determined by plating on solid media.