Lipidomics of Thalassiosira pseudonana under Phosphorus Stress Reveal Underlying Phospholipid Substitution Dynamics and Novel Diglycosylceramide Substitutes

Phytoplankton substitute phosphorus (P) containing lipids with non-P analogues boosting growth in P-limited oceans. In the model diatom Thalassiosira pseudonana, lipid headgroup substitution dynamics are well described, but those of the individual lipids, varying in fatty acids, are unknown. Moreover, the behaviour of lipids outside of the common headgroup classes, and the relationship between lipid substitution and cellular particulate organic P (POP) are yet to be reported. We investigated these through the mass spectrometric lipidomics of P replete (P+) and depleted (P-) T. pseudonana cultures. Non-lipidic POP was depleted rapidly by the initiation of P stress, followed thereafter by cessation of P-lipid biosynthesis and per-cell reduction in P-lipid of successive generations. Minor P-lipid degradative breakdown was observed, releasing P for other processes, but most remained intact. This may confer an advantage to efficient heterotrophic lipid consumers in P limited oceans. Glycerophosphatidylcholine (PC), the predominant P-lipid, was similar in composition to its betaine substitute lipid. During substitution, PC per cell was less abundant and more highly unsaturated in composition. This may reflect underlying biosynthetic processes or the regulation of membrane biophysical properties subject to lipid substitution. Finally, several diglycosylceramide lipids increased up to ten-fold under P stress. These represent novel substitute lipids and potential biomarkers for the study of P limitation in situ, contributing to growing evidence highlighting the importance of sphingolipids in phycology. These findings contribute deeply to our understanding of P-lipid substitution, a powerful and widespread adaptation to P limitation in the oligotrophic ocean.ImportanceUnicellular organisms substitute phosphorus (P) containing membrane lipids with non-P substitutes when P is scarce, allowing greater growth of populations. Previous research with the model diatom species Thalassiosira pseudonana grouped lipids by polar headgroups in their chemical structures. The significance of the research herein is in the description of the individual lipids within the headgroups during P-lipid substitution. This revealed the relationship between lipid headgroups and hints at the underlying biochemical processes. Secondly, P-lipid substitution was contextualised by measurements of total cellular P. This depicted the place of P-lipid substitution in relation to the broader response to P-stress and yields insight into the implications of substitution in the marine environment. Finally, lipids previously unknown in this system were identified. This revealed a new type of non-P substitute lipid, potentially useful as a biomarker to investigate P-limitation in the ocean.