Fig 1.
Anatomical locations of the organs sampled from Antarctic krill, Euphausia superba.
Modified after The Curious Life of Krill by Stephen Nicol (Drawing by Marcia Rackstraw). Copyright © 2018 Stephen Nicol. Reproduced by permission of Island Press, Washington, DC [32].
Table 1.
Characteristic fragment ion and neutral loss masses indicative for all phospholipid classes analysed in this study for both positive and negative electrospray ionization.
Fig 2.
High-resolution mass spectrometry-based structural identification of intact phospholipids.
a. Measured standard solution of an IPL (PC(38:6)) with 16:0 and 22:6 as the fatty acids in sn-1 and sn-2 position. b. Unknown IPL detected in samples identified as PC(58:14). Characteristic traits measured in ESI+ are coloured orange, whereas they are coloured blue for ESI-. In positive mode, the head group of the PC can be detected as ion. The exact mass of the [M+H]+ ion is used to denote the overall composition of the IPL. For PCs, an [M+HCOO]- ion in ESI- can be expected to occur at the same retention time as the [M+H]+ ion in ESI+. However, for PCs, the most abundant ion in MS2 will be the one formed due to the neutral loss of methyl formate. Additionally, in ESI-, fatty acids can be assigned by finding their ion signals in the lower mass range. In the case of the unknown IPL depicted in b, 22:6 and 36:8 fatty acid signals were identified. The combination of these two fatty acids has to match the overall composition determined in ESI+.
Fig 3.
Principal component analysis of the percentage distribution of the IPLs.
IPLs were grouped according to their characteristics as summarized in S3–S5 Tables of the Supporting Information. Colours indicate the sampling months while shade and symbol represent the organ sampled from krill. LMW, low molecular weight; MMW, medium molecular weight; HMW, high molecular weight; LDU, low degree of unsaturation; MDU, medium degree of unsaturation; HDU, high degree of unsaturation.
Fig 4.
Comparison of the PC composition between Antarctic summer and autumn.
Values given reflect the ratio of the percentage distribution of PCs. Lipids represented by bars pointing upwards are more abundant during Antarctic autumn, whereas those pointing downwards are more abundant during Antarctic summer. Colour intensity reflects the percentage of the PCs in summer samples on a logarithmic scale.
Fig 5.
Compositional comparison of the 15 PCs with the highest molecular weight between January and March.
Values given reflect the ratio of the percentage distribution of PCs between months. Lipids represented by bars pointing upwards are more abundant in January, whereas those pointing downwards are more abundant in March. Colour intensity reflects the percentage of the PCs in summer samples on a logarithmic scale in accordance with Fig 4. Labels on bars depict the fatty acid composition of the PCs, with the fatty acid of higher intensity in ESI- experiments provided first. For PC(46:9) and PC(48:12) two different combinations were possible, with the more abundant pairing listed first.
Fig 6.
Ternary diagrams of fatty acid compositions.
Saturated (SFA), monounsaturated (MUFA) and polyunsaturated (PUFA) fatty acid compositions in the total lipid extracts (TLE) were measured with GC-MS and in phosphatidylcholines (PCs) and phosphatidylethanolamines (PEs) measured with LC-MS. In cases were multiple fatty acid combinations for a given intact phospholipid were observed, the ratio was estimated based on the intensity distribution of the more abundant fatty acid signal in ESI- MS2 experiments. All fatty acids listed in S2–S4 Tables were taken into account for the corresponding ternary diagrams.
Table 2.
Comparison of PC and PE diversity and abundance between this study and data provided in Holm et al. [57].
Fig 7.
Degree of unsaturation and abundance of EPA and DHA in PCs and PEs.
Comparison of the weighted mean of unsaturation per fatty acid (a and b) as well as the percentage of EPA- (c and d) and DHA-containing IPL species (e and f) for PCs (a, c and e) and PEs (b, d and f) between the planktonic lipidome at the Antarctic Peninsula derived from Holm et al. [57] (orange lines, upper and lower limit; dashed line, mean value) and krill data presented in this study (green dots; mean values displayed as diamonds).