C25 AND C30 HIGHLY BRANCHED ISOPRENOID ALKENES IN LABORATORY CULTURES OF 2 MARINE DIATOMS

C25 and C30 highly branched isoprenoid (HBI) alkenes have been isolated from laboratory cultures of the marine diatoms Haslea ostrearia and Rhizosolenia setigera. At least seven C25 isoprenoid alkenes, with carbon skeleton II, were found in the culture of H. ostrearia. A C25:4 HBI alkene was the major hydrocarbon present. Other hydrocarbons included a C25:3 alkene, a different C25:4 alkene, two C25:5 alkenes and two unidentified minor components that exhibited properties consistent with alkenes. Three C30:5 HBI alkenes and smaller amounts of two C30:6 alkenes, all with carbon skeleton III, were identified in R. setigera. These isoprenoid hydrocarbons were not present in thirteen other species of diatom examined. All species possessed the alkene n-heneicosa-3,6,9,12,15,18-hexaene (n-C21:6), and eight also had n-heneicosa-3,5,9,12,15-pentaene (n-C21:5). Highly branched isoprenoid alkenes are commonly reported in marine sediments and waters, but this is their first identification in marine microalgae. These data support previous suggestions that these alkenes might be derived from diatoms in many contemporary marine environments.