Fatty acids as biological markers for bacterial symbionts in sponges from the Great Barrier Reef

Samples were taken from three phototrophic, cyanobacteria containing sponge species (Dysidea herbacea, Phyllospongia papyracea and Pseudaxinyssa sp) and two nonphototrophic sponge species (Phakellia aruensis and Rhopaloeides odorabile) at Davies Reef.Lipids were extracted and a subsample saponified. The resultant fatty acids were converted to methyl esters, which were analysed on both apolar and polar capillary columns. Structures were confirmed by chromatographic comparisons with authentic standards, by comparison of equivalent chain lengths for the methyl esters with those of secondary standards and by capillary gas chromatography-mass spectometry (GC-MS).Pseudaxinyssa sp. was collected from Rib Reef and the sponge was separated into surface tissue, to a depth of 0.5 mm (including exopinacoderm and a distinct reddish-brown band of cyanobacteria) and interior tissues. Each portion was lyophilized prior to extraction and the isolated phospholipids were converted to methyl esters, which were analysed using gas chromatography. Electron microscopy was used to confirm the large volume of cyanobacteria within the surface tissue fraction and the almost complete absence of cyanobacteria within the interior tissue fraction.The contributions of symbiotic bacteria were determined on the basis that fatty acids extracted from sponge tissue originated in either the cell membranes of the sponge, or nonphototrophic bacteria or cyanobacteria. The minimum abundance of any specific acid in a suite of sponge samples was used as an estimate of the sponge contribution of that acid in other sponges.
This study was initiated to estimate the cyanobacterial and nonphototrophic bacterial abundance in sponges using a bacterial biological marker technique based on fatty acid content.