L isoleucine standards

From the freeze-dried biomass samples, total C and N levels (termed Particulate Organic Carbon, POC and Particulate Organic Nitrogen, PON) were measured using an ANCA-GSL 20-20 stable isotope analyzer (PDZ Europa, Sercon Ltd., Crewe, UK). Freeze-dried samples at 1–2 mg micro-algal DW biomass were combusted in pre-weighed aluminum capsules with helium as a carrier gas. The instrument was calibrated using l-isoleucine standards (Sigma) over the range 5–200 μg N and 33–1320 μg C, delta calibrated N-7.89, C-12.18 (Slocombe et al., 2013a (link), 2013b (link)).

4-HI and l-isoleucine standards were procured from Sigma Aldrich (St. Louis, MO, USA). The membrane filter (0.22 μm) was purchased from Merck Limited (Mumbai, India). LC-MS grade solvents like methanol, acetonitrile, and formic acid were obtained from Sigma Aldrich (St. Louis, MO, USA). Syringe filters (13 mm × 0.22 μm) were procured from Chromatopak Pvt. Ltd. (Mumbai, India). Deionized ultra-pure water was obtained from a Millipore Milli-Q Plus system (Millipore Bedford Corp., Bedford, MA, USA). Commercial kits for the determination of TG and TC were purchased from Coral Clinical Systems.