Methodology:

Microalgae cultures:

In 2022, we conducted FA analysis on 32 cold-water strains that were isolated in the Arctic (25 strains), Southern Ocean (2 strains) or North Atlantic (5 strains), and included diatoms, chlorophytes, haptophytes, cryptophytes, chrysophytes, dinoflagellates and cyanobacteria. These strains were either obtained from commercial culture collections (CCAP or RCC) or provided by the AWI. The CCAP cultures were inoculated in 50 ml Erlenmeyer conical flasks with 25 ml of F/2 medium and added 60 micromol L-1 silicate for diatoms (Guillard and Ryther 1962). The algal strains were grown for four weeks at either low temperature - low light intensity (3-4 degrees Celsius, 10 micromol m-2 s-1) or higher temperature - higher light intensity (8 degrees Celsius, 20 micromol m-2 s-1) and a light:dark cycle of 12:12 hrs.

The culturing conditions at RCC resembled those at CCAP, but only for the low temperature - low light intensity (3-4d degrees Celsius, 10 micromol m-2 s-1). The algae growth rates were not monitored, but with these culture conditions and time span, the strains usually reach late exponential-early stationary growth (C. Rad-Menendez, I. Probert, pers. comm.). From each strain, two 5 ml technical replicates were filtered via a vacuum pump (-20 kPA) onto pre-combusted (12 h, 450 degrees Celsius) 25 mm Whatman GF/F filters, freeze dried and stored in aluminium foil at -20 degrees Celsius until FA analysis.

Note: Three of the strains from CCAP (CCAP-1023/3, CCAP-1029/29, CCAP-1029/30) were obtained and analysed as 'Fragilariopsis sp.' but subsequently identified as 'Grammonema sp.'.

Experiments with Melosira arctica:

The experiments with Melosira arctica were carried out at the AWI in 2021 and 2022, using a strain that was isolated in the Central Arctic Ocean in 2015 (79.56 degrees N, 4.84 degrees W; strain PS93.1_030). The strain was grown as semi-continuous batch cultures under 6 different environmental conditions including the species' natural range of temperature, light and nutrient availability (Spilling and Markager 2008, Fernandez-Mendez et al. 2014). Therefore, low temperatures (0-1 dgrees Celsius) were combined with high and low light intensity (10 and 100 micromol m-2 s-2), each with high and low nutrient supply (details below), and higher temperatures (3 and 6 degrees Celsius) with high light and nutrient availability. Experiments were performed with 4 biological replicates in sterile 1-L Schott bottles in temperature-controlled rooms, with bottles at 3 and 6 degrees Celsius being immersed in water-filled aquaria for additional temperature stability. Day light lamps (Biolux T8, 6500K, Osram) provided continuous light and irradiance levels were adjusted with a black mesh fabric and measured using a 4 pi spherical sensor (Li-Cor) and data logger (ULM-500, Walz). Cells were cultivated in 0.2 micrometerm sterile-filtered Arctic seawater (salinity 33.5), with or without added macronutrients, vitamins and trace metals according to F/2 or F/20 media (Guillard and Ryther, 1962). Initial nutrient concentrations in set-ups with F/20 media were 10.4 +/-0.24 micromol L-1 nitrate-and-nitrite, 18.1 +/-1.36 micromol L-1 silicate and 1.2 +/- 0.19 micromol L-1 phosphate. In set-ups without added medium, initial nutrient concentrations were 1.8+/-0.07 micromol L-1 nitrate-and-nitrite, 14.4 +/-2.3 micromol L-1 silicate and 0.4 +/-0.01 micromol L-1 phosphate. To minimize changes in carbonate chemistry and to remain close to natural population densities, cultures were diluted every 1-2 weeks with Arctic seawater, with or without medium. Sufficient algal biomass for subsequent lipid analysis was grown after 4 weeks for cultures that received media, and after 7 weeks for those without media. At the end of the experiment, 2 technical replicates were taken from each biological replicate, filtered onto pre-combusted (12 h, 450 degre...(24)

Data collection:

Fatty acid analysis: FAME were quantified using an Agilent 6890N gas chromatograph (Agilent Technologies, USA) with a DB-FFAP capillary column (60 m, 0.25 mm I.D., 0.25 micrometer film thickness, Agilent Technologies, USA) supplied with a splitless injector and a flame ionization detector using temperature programming.

Chromatogram data evaluation: Clarity chromatography software system (version 8.8.0, DataApex, Czech Republic)

Data quality:

The fatty acid profiles were compared to several commercial- and self-produced standards (e.g. Arctic algae standard, Bacteria standard, Calanus spp. standard), and fatty acid peaks were identified accordingly. In a few cases, samples were also analysed with the mass spectrometer and peaks were identified via (1) the mass of the compound, (2) the retention time of the compound and (3) the equivalent chain length method. Unusual peaks were also compared with published FA profiles from related species (e.g. Leblond et al. 2006 for cold-adapted dinoflagellates).