Role of Gut Flora in the Transfer of Amino Acids Through a Marine Food Chain

Abstract

The gut of the sea urchin Strongylocentrotus droebachiensis has between 2 × 108 and 6 × 109 bacteria/mL of gut contents. When these bacteria are present, radioactive carbon fed to the urchins as either glucose or cellulose appears in all of the protein amino acids of the gonads. However, when the gut flora is suppressed by a mixture of antibiotics, injection of the sea urchins with radioactive glucose results in the labeling of some of the amino acids (nonessential amino acids) but not others (essential amino acids). These results imply that: (a) sea urchins have the ability to digest cellulose, probably with the aid of the gut flora; (b) intestinal microorganisms can synthesize essential amino acids; and (c) amino acids of microbial origin are available to the sea urchins. The amino acid compositions of (i) kelp (Laminaria longicruris), (ii) sea urchin intestinal microflora, and (iii) sea urchins, were determined over a yearly cycle. The amino acid composition of kelp and microbes were compared as a source of nutrient for the sea urchins, using as indications: overall similarity in compositions, essential amino acid index, and percentage of total amino acid that is essential. All three comparative methods show that the microbial protein is of higher nutritive value than the kelp protein.Key words: amino acids, nitrogen, bacteria, kelp, Laminaria longicruris, sea urchin, Strongylocentrotus droebachiensis