Protection of protein from bacterial degradation by submicron particles

Abstract

This study investigated how bacteria degrade protein that is adsorbed to or trapped within submicron particles designed to model those particles produced by protists grazing on bacteria and other picoplankton. The structure for some of these particles is hypothesized to be similar to liposomes, i.e. cell-like structures with lipid bilayers surrounding an aqueous center. To examine the degradation of liposome-like submicron particles, we prepared liposomes from phospholipids, a common constituent of all cellular membranes, and examined degradation of protein freely dissolved and in various associations with liposomes. Natural bacterial assemblages responded rapidly to addition of protein and could degrade most (92%) of the truly dissolved protein (not associated with liposomes) within 72 to 90 h even though added concentrations were up to 1 order of magnitude greater than natural levels. In spite of the capacity for rapid utilization, protein degradation was substantially inhibited (3-fold) by the presence of liposomes. Protein entrapped within the liposomes was protected the most, but adsorption to the outside of liposomes also decreased the degradability of protein. Degradation of liposomes appeared to be mainly due to bacteria, not heterotrophic protists. Our data support previous studies suggesting that labile compounds may be physically protected from degradation when entrapped within submicron particles. These results help explain the presence of dissolved membrane proteins and other cell wall material in the dissolved organic carbon pool of marine ecosystems. © Inter-Research 1999.