Can the protein/carbohydrate (P/C) ratio of exopolymeric substances (EPS) be used as a proxy for its ‘stickiness’ and other biophysical properties?

Microbially secreted exopolymeric substances (EPS), rich in polysaccharides, make up an important part of natural organic matter, especially marine snow, in the ocean. While the attention in the oceanographic literature is mainly directed towards the role that polysaccharides play in EPS functions, less is known about the role of proteins, especially the protein/carbohydrate (P/C) ratio on particles, in marine systems. EPS associated with particles forms a biofilm, whereby proteins are not only involved in cell surface attachment, but also in the stabilization of the biofilm matrix, and the development of a three-dimensional biofilm architecture. Here, we provide a perspective based on the recent literature on the relationship between the P/C ratio and a number of biophysical properties related to biopolymer aggregation, (e.g., their relative hydrophobicity, surface activity and surface tension, attachment efficiency, light-induced chemical crosslinking) and sedimentation efficiency of marine snow in marine environments. The accumulated evidence discussed in this review points to the fact that the P/C ratio is predictive for the relative hydrophobicity and ‘stickiness’ of EPS as well as coagulation efficiency of suspended particles. If such particles have sufficient specific weight, this aggregation process can then lead to sedimentation. These extensive observations should allow us to use the P/C ratio in the future as a surrogate for aggregation propensity, sedimentation efficiency or a chemical basis for the “fudge or stickiness factor” in scavenging and particle cycling models.