C25 highly branched isoprenoid alkenes in planktonic diatoms of the Pleurosigma genus

The C25 highly branched alkenes which have been widely reported in coastal and deep sea marine sediments and sedimenting particles have to date only been detected in benthic diatoms. Logically, a planktonic organism would be a much more likely source for the HBIs found in deep sea environments. Here we report the structures of C25 HBI alkenes isolated from two planktonic diatoms belonging to the Pleurosigma genus. The HBI structures have been determined using NMR and mass spectral techniques. The HBIs from Pleurosigma sp. were identical to those found in many deep sea marine samples whereas those of P. planktonicum consisted of two isomeric tetraenes whose structures are novel and which are reported here for the first time.