ω20-Hydroxy and ω9,ω10-dihydroxy biomarker lipids in ferns from the Salviniaceae family

Abstract

All seven species of floating ferns from the genus Azolla (family Salviniaceae) produce a unique series of long chain mid-chain ω20-hydroxy compounds (ω20-alkanols, 1,ω20-diols, ω20-hydroxy fatty acids) and structurally related ω9,ω10-dihydroxy compounds (ω9,ω10-diols, 1,ω9,ω10-triols and ω9,ω10-dihydroxy fatty acids). These very long chain fatty acid (VLCFA) derivatives occur in the ferns’ waxes in free and esterified form. The specific distribution of these lipids differed between species belonging to each of the two sections in the Azolla genus: in species of the section Azolla and Rhizosperma, the ratio of C31 over C35 ω20-alkanols averaged 7.0 and 0.40, and the ratio of C26 over C28 ω20-hydroxy fatty acids averaged 2.7 and 1.0, respectively. Similar compounds were identified in species of another genus in the Salviniaceae family, Salvinia, suggesting that their biosynthetic pathway evolved early during Salviniaceae evolution (>89 Ma). Salvinia species contain ω20-hydroxy and ω9,ω10-dihydroxy compounds in smaller concentrations and in a much different distribution compared to Azolla; the C31 1,ω20-diol is unique to Salvinia species. Closely related fern species from the genera Marsilea, Pilularia and Regnellidium did not contain these compounds, nor did unrelated aquatic plants from the genera Lemna and Pistia. All mid-chain hydroxy compounds detected in extant Azolla have been traced previously in Arctic Eocene sediments from the so-called ‘Azolla Event’ (48.5 Ma), implying that they are well preserved in the geological record and may therefore serve as Azolla biomarkers. Our findings indicate that ω20-hydroxy and ω9,ω10-dihydroxy compounds in sediments could be used as biomarkers of the whole Salviniaceae family. Subsequently, the clear differences in compound distribution between the Azolla and Salvinia genera and the more subtle ones between the two Azolla sections, may allow assigning the compound's origin at the genus (and possibly section) level, depending on the preservation of compound classes in the sediment and the timing of the Azolla or Salvinia deposition. This is exemplified by a sediment interval of the so-called ‘Salvinia bed’ (Eemian), which contained trace amounts of the C31 1,ω20-diol, but none of the ω20-hydroxy and ω9,ω10-dihydroxy compounds common to Azolla, indicating the value of C31 1,ω20-diol as a biomarker for distinguishing Salvinia from Azolla.