*4.1. Algal Culture and 13C Labelling*

This study was conducted following the experimental design described by Remize et al. (2020) [34]. The marine prymnesiophyte *Tisochrysis lutea* (T-iso, CCAP 927/14) was cultured in 2 L batch condition in balloons under continuous light (24 h light cycle, 100 μmoles photons m−2·s<sup>−</sup>1) at 20 ◦C and with pH regulation at 7.50 ± 0.05 by CO2 injection. Filtered seawater was previously enriched with Conway medium [64] and inoculated with *T. lutea* preculture in a growing stage (exponential phase, four days old). The experimental setup was composed of two cultures (Tl1 and Tl2) receiving the labelling 13C-CO2 gas (Sigma-Aldrich, <3%atom 18O, 99.0%atom 13C) and one culture (TlT) receiving petrochemical CO2 gas. 13C-incorporation in Tl1 and Tl2 began after inoculation (t0) and was maintained for 24 h (t24).

During the first hours of the experiment, the 13CO2 injection tube of balloon TI1 had been temporarily disconnected from the system. Consequently, balloon Tl2 received earlier the 13CO2 and thus started to incorporate 13C before balloon Tl1.

## *4.2. Samples Collection*

Sampling was performed as described in Remize et al. (2020) [34], i.e., at 30 min, 1 h, 2 h, 3 h, 4 h, and then every 2 h. A total of 16 samples was collected during the 24 h monitoring. At each sampling time, a total volume of 30 to 70 mL was collected for (i) flow cytometry analysis of cellular parameters, (ii) bulk isotopic analysis of particulate organic carbon (13C-POC) and dissolved inorganic carbon (13C-DIC) by EA-IRMS, (iii) fatty acid (FA) analysis in neutral lipids (NL) and polar lipids (PL) by GC-FID, and (iv) compound specific isotope analysis (CSIA) of FA (13C-FA) by GC-IRMS, as described in the following paragraphs.
