Increasing P limitation and viral infection impact lipid remodeling of the picophytoplankter Micromonas pusilla

Abstract. The intact polar lipid (IPL) composition of phytoplankton is plastic and dependent on environmental factors. Previous studies have shown that phytoplankton under low phosphorus (P) availability substitutes phosphatidylglycerols (PGs) with sulfoquinovosyldiacylglycerols (SQDGs) and digalactosyldiacylglycerols (DGDGs). However, these studies focused merely on P depletion, while phytoplankton in the natural environment often experience P limitation whereby the strength depends on the supply rate of the limiting nutrient. Here we report on the IPL composition of axenic cultures of the picophotoeukaryote Micromonas pusilla under different degrees of P limitation, i.e., P-controlled chemostats at 97 and 32 % of the maximum growth rate, and P starvation (obtained by stopping P supply to these chemostats). P-controlled cultures were also grown at elevated partial carbon dioxide pressure (pCO₂) to mimic a future scenario of strengthened vertical stratification in combination with ocean acidification. Additionally, we tested the influence of viral infection for this readily infected phytoplankton host species. Results show that both SQDG : PG and DGDG : PG ratios increased with enhanced P limitation. Lipid composition was, however, not affected by enhanced (750 vs. 370 µatm) pCO₂. In the P-starved virally infected cells the increase in SQDG : PG and DGDG : PG ratios was lower, whereby the extent depended on the growth rate of the host cultures before infection. The lipid membrane of the virus MpV-08T itself lacked some IPLs (e.g., monogalactosyldiacylglycerols; MGDGs) in comparison with its host. This study demonstrates that, besides P concentration, also the P supply rate, viral infection and even the history of the P supply rate can affect phytoplankton lipid composition (i.e., the non-phospholipid : phospholipid ratio), with possible consequences for the nutritional quality of phytoplankton.