Relations entre les compositions en acides amines des particules en voie de sedimentation, du contenu intestinal des holothuries abyssales et du sediment environnant

Abstract

The amino acid composition of the gut content of two species of deposit-feeding holothurians ( Deima validum validum and Pseudostichopus villosus) is compared with that of the sediment trap particles and the surrounding sediment collected from the Demerara abyssal plain in the equatorial Atlantic Ocean at a depth of 4800 m. Amino acid concentrations are ∼ 10, ∼ 50 and ∼ 100 μmol g −1 in superficial sediment, sediment trap particles and ingested sediments, respectively. Compositions of the latter two are nearly identical and differ slightly from that of the former. No selective absorption of amino acids is noticed through the digestive tract. The acid-soluble (cold 2 N HCl) hydrolysed category, nearly absent in sediment trap particles from that depth as well as in superficial sediment, increases in gut contents for both species as a result of digestive system action on particulate matter. Glutamic acid which is largely predominant in this fraction may be partly secreted by the organisms or may result from bacterial activity. Similarly diaminopimelic acid (DAP) is important in gut contents only, but entirely in the free form. The bacterial cell numbers calculated from DAP concentrations are 100-fold that of direct epifluorescent microscopy counts but cannot be considered as the numbers of visible cells because the cell-wall mureid complex containing DAP is normally insoluble in water. Nevertheless, DAP concentrations lead pointedly to the fact that remains of larger bacterial populations or more intense bacterial activity exist in gut content compared to that of sinking particles and superficial sediment. In the superficial sediment, β-alanine and γ-aminobutyric acid account for about 8 mol%, almost entirely in the residual hydrolysed category. In sinking particles and gut contents they account for 1.5 mol% only, but their total amounts are similar to that of the superficial sediment. In gut contents about 40% of β-alanine and γ-aminobutyric acid are found in acid- and alkali-soluble hydrolysed categories, maybe as a result of the general decomposition of ingested organic matter. Absolute concentrations do not appear to change through the digestive tract. The inefficiency of organism assimilation systems and the intimate contact between mineral particles and digested organic compounds which occur in the digestive tract of deposit-feeders may be partly responsible for their increased relative abundance in pelagic sediments.