Use of 14C-protein-labelled bacteria for primary production

Abstract

Three cultured species and mixed seawater communities of heterotrophc or mixotrophic flagellates were fed with non-cultivated aquatic bacteria labelled with I4C-protein hydrolysate, i.e. the method of radioactive labelled bacteria (RLB). Seawater bacteria incorporated protein-hydrolysate more rapidly and far more efficiently than 14~-glucose. Loss of label was less in the protein-labelled bacteria compared with those labelled with glucose. All tested flagellate species were found to ingest bacteria; individual clearance rates ranged from 0.014 $ ind.-' h-' (36 bacteria ind.-' h-'; Ochromonas minima) to 0.156 $ ind.-' h-' (156 bacteria ind.-' h-'; Paraphysomonas sp.). Uptake of bacteria by 0. minima and Bodo sp. was linear for ca 10 min, and levelled off between 10 to 40 min. Paraphysomonas sp. had a linear uptake for 20 min. Grazing rates obtained with 0. minima fed labelled bacteria and grazing rates calculated from chemostat culture were not significantly different. RLB proved rapid and efficient for estimating clearance by small heterotrophs and mixotrophs. Grazing rates in Bodo sp, and Paraphy- somonas sp, from a 'direct-count' method using DTAF-stained bacteria were 10 % of those obtained with RLR. RLB is shown to work well even with small size differences between predator and prey. This is obtained by use of pre-starved bacteria, which are reduced to a size where they may be separated from predators using a 1.0 pm filter. Field experiments with mixed seawater communities gave 10 to 30 times higher grazing rates using RLB compared to stained bacteria. The method accounts for food vacuoles containing bacteria, egested upon preservation of the bacterivory flagellates.