Analysis by gas chromatography-mass spectrometry of the fatty acid composition during temperature adaptation in Aphanizomenon flos-aquae, a diazotrophic cyanobacterium from the Baltic Sea.

Filamentous cyanobacteria such as AphunrzometiotiJl(~*~-aquae which are buoyant due the presence of gas vacuoles [ I ] float near the water surface ofthe Baltic Sea and form blooms where they experience optimum light conditions. However, as a result of turbulence in the water column, they may be mixed to greater depths and as a result experience a decrease in temperature from about 20°C at the surface to some 15°C at the thermocline between the warm upper layer and the colder lower layer It has been demonstrated in a range of cyanobacterial species that as growth temperature is lowered, the amount of more unsaturated fatty acid increases and the amount of less unsaturated fatty acid decreases [2-51, which may be an adaptive response enabling a downward shift in the phase transition temperature of the thylakoid membranes [6] However the majority of these experiments involved non-marine cyanobacteria and have examined the effect of much larger temperature differences than those that are experienced by Baltic populations as a result of mixing. As part of an investigation into the physiological adaptation mechanisms which influence the development and fate of cyanobacterial populations in the Baltic, we have determined the changes that occur in the fatty acid composition of the membrane lipids of A. flo.suquue in response to a reduction in the growth temperature of laboratory cultures similar to that which would be experienced when mixing from the surface layer to the thermocline occurs. Batch cultures (500 ml) ofA. flos-upue were grown for 10 days at 20°C in modified ASM-I medium lacking a fixed nitrogen source on a 12 h light ( 9 0 h E m-*<')/l2 h dark cycle, and the growth temperature was then reduced to 15°C (0 h in Fig. I ) . Cells from 75 ml of culture were harvested by centrifugation (10,OOO x g, 15 min) and the total lipid extracted. Fatty acid methyl esters were prepared by transesterification with MeOH. H,SO,, and purified by preparative tlc (silica gel-G, diethyl etherhexane I : 10 v:v). Fatty acid methyl esters were analysed by GC-MS using a Varian 3400 gas chromatograph equipped with a 0.25& film thickness BPX-70 column (25m x 0.22mm i.d.) and an Incos-50 mass spectrometer (70 eV electron impact ionisation). GC-MS of the fatty acid methyl esters prepared from A. flosaquae cultured at 20°C indicated that in addition to saturatedand monoenoic-C-I6 and C18 components, the fatty acid composition of A. flo.s-ayuae was characterised by the presence of 18:2 (9,12) and 18:3 (9,12,15). Both 18:l (9)and 18:l ( l l ) , a s w e l l a s 16:2 (3%) and 16:3 (8%) were present, but neither 18:3 (6,9,12) nor 18:4 were detected (Fig. 1).