Potential digestive function of bacteria in krill Euphausia superba stomach

Abstract

Evidence is presented that bacteria in the stomach of live Antarctic krill Euphausia superba participate in the digestion of host dietary components. Total (AODC: Acridine Orange Direct Count) and culturable (CFU: Colony Forming Units) bacteria in this organ in fresh adult krill were compared in numerical and physiological terms with those in contemporaneously collected seawater samples in Admiralty Bay, King George Island, South Shetland Islands. On a per unit volume basis, AODC and CFU numbers in the stomach were greater than in the seawater, and higher than could be accounted for through selective prey filtration alone. CFU in the stomach and seawater averaged ∼4% and <0.01% of the respective AODC, and in the stomach ∼6% of AODC were dividing, compared to less than 1% in seawater, observations suggesting that bacterial growth occurs in the stomach. Bio-volume of cells in the stomach exceeded that of cells in seawater (t2854 = -6.262, p < 0.001), but the former were too small to have been selectively filtered. Maltose and gluconate assimilation, and lipase (C14), 'trypsin'-like, and acid phosphatase activities were significantly more prominent in culturable bacteria from the stomach than seawater, further indicating selection. Conversely, α-fucosidase production appears host derived. Yeasts isolated from the stomach were identified as Leucosporidium antarcticum and Metschnikowia australis.