Factors influencing the distributions of polyunsaturated terpenoids in thediatom, Rhizosolenia setigera

Polyunsaturated highly branched isoprenoid (HBI) hydrocarbon distributions of laboratory cultures of five strains of the planktonic diatom Rhizosoleni a setigera (Brightwell) are shown herein to be highly variable. Some strain s produced both haslenes with from three to five double bonds and rhizenes. The haslenes comprised not only Delta5 alkenes but also those with C7(20) unsaturation, including hasla-7(20),9E,Z, 23-trienes and hasla-7(20),9E,Z-1 3, 23-tetraenes. The rhizenes contained C7(25) unsaturation and the vinyl m oiety common to all algal haslenes so far characterised. The effects of tem perature and salinity on HBI composition, along with isotopic content, were determined in strain CS 389/A. Increase in growth temperature from 18 to 2 5 degreesC increased the degree of unsaturation in the haslenes and E to Z isomerisation in the triene. There was also an increase in unsaturation in the rhizenes at the highest growth temperature, with hexaenes dominant over the pentaenes but in the rhizenes, Z to E isomerisation increased. Increas ed salinity from 15 to 35 psu increased cell growth and rhizene production but decreased haslene production. Unsaturation in haslenes was not changed by increased salinity but unsaturation in the rhizenes decreased. These may reflect growth rate differences. The carbon isotopic compositions of the h aslenes and rhizenes were similar to that of the major sterol at 18 degrees C, but the major HBI isomers were 3-4 per mil depleted relative to phytol r eleased by saponification from chlorophyll a. This suggests biosynthesis of HBIs from a different isotopic pool of isopentenyl biphosphate to that fro m which phytol is biosynthesised. At 25 degreesC, further isotopic differen ces were observed. The variables controlling HBI distributions in R. setige ra are still not fully understood and rationalisation of the environmental controls on the sedimentary distributions of the HBIs from R. setigera may only be possible once such factors are established. (C) 2001 Elsevier Scien ce Ltd. All rights reserved.