A classification scheme for marine organic colloids in the Adriatic Sea: colloid speciation by transmission electron microscopy

Organic colloids from the Adriatic Sea, covering the full colloid size range, were characterized on a per-colloid basis by transmission electron microscopy applied to ultrathin sections of embedded colloids. To detect, assess, and minimize artifacts, a multimethod correlative approach to analysis was used. A colloid classification scheme was developed from the characterizations, with individual colloid species defined in the broad sense established by environmental analytical aquatic chemists. The basic scheme is related to derivative classification schemes in which abundant colloid species are listed according to activities known from the literature, with a focus on activities related to carbon transformations and transport. Morphological criteria show that abundant colloid species occupy broad size ranges, such that colloid fractionation protocols based on sizing alone are likely to be of questionable scientific utility. Image analysis revealed that small colloids, designated as humic substances, represented 91% of the total organic colloid burden of the bulk water during a summer period of high biological productivity. At this time, fibrils were rare in the bulk water while representing the most abundant colloid in marine snow.