Penicillin

1 mL of exponentially growing culture was inoculated in F/2 medium containing final concentrations of 0.1 mg/mL Streptomycin (PanReac Applichem, A1852), 0.06 mg/mL Penicillin (PanReac Applichem, A1837), 1 mg/mL Ampicillin (PanReac Applichem, A0839), 0.1 mg/mL kanamicine (PanReac Applichem, A1493) and 0.02 mg/mL cefotaxime (Sigma-Aldrich, C7039) and allowed to grow for 5–6 days under standard growth conditions. Bacterial contamination was checked in two ways (SI1: Supplementary Fig. 6): (i) by staining DNA with DAPI and examining cultures under the microscope to check for the presence/absence of bacterial nucleoids; (ii) by performing peptone tests. For DAPI staining, 1 µL of DAPI stock solution (4′,6-diamidino-2-phenylindole, 1 mg/mL, Roche, Basel, Switzerland) was added to 1 mL of formalin preserved culture, incubated for 10 minutes and observed under the epifluorescence microscope. For peptone tests, 1 mL of diatom culture was added to a tube containing a peptone solution (1 mg/mL), incubated in the dark and checked after 2–3 days and 1–2 weeks; growth of bacteria in the tubes indicated contamination. If bacterial contamination persisted, the treatment was repeated. Large volume cultures used for DNA extraction were grown with antibiotics and the contamination tests were always performed on an aliquot of the culture.