Phospholipid fatty acid composition and cold tolerance in two species of barnacle, Balanus balanoides (L.) and Elminius modestus Darwin. II. Isolation and phospholipid fatty acid composition of subcellular membrane fractions

Abstract

Homogenates were prepared from whole bodies of the intertidal barnacles, Balanus balanoides (L.) and Elminius modestus Darwin in winter and summer. The homogenates were subject to fractionation using differential and sucrose density gradient centrifugation. Three principle fractions were obtained which, according to marker enzyme analysis, were enriched with respect to plasma membranes, mitochondria, and endoplasmic reticulum, respectively. Contamination of plasma membranes and mitochondria fractions by other organelles was low. This was confirmed by examination by electron microscopy of fractions prepared from E. modestus. The fatty acid compositions of total phospholipids isolated from subcellular fractions of both species were determined. In general, polyunsaturation decreased in the order mitochondria > endoplasmic reticulum > plasma membranes. In the mitochondria and endoplasmic reticulum fractions from Balanus balanoides the saturated fatty acid level increased from winter to summer while the monounsaturated fatty acids decreased. The level of polyunsaturation in mitochondria also decreased. In the plasma membrane fraction the level of saturated fatty acids increased markedly in summer while that of mono- and polyunsaturated fatty acids decreased. Changes in the total phospholipid fatty acid composition of mitochondria and endoplasmic reticulum fractions prepared from Elminius modestus were similar to those noted in Balanus balanoides except that in Elminius modestus, the level of saturated fatty acids decreased in mitochondria. In the plasma membrane fraction from E. modestus the level of polyunsaturation increased in the summer in contrast to the results obtained with Balanus balanoides. This study suggests that the differences between the two species with respect to their ability to acquire freezing tolerance may be related to seasonal changes in total phospholipid fatty acid composition of the plasma membrane.