Thraustochytrid fungoid protists in faecal pellets of the tunicate Pegea confoederata, their tolerance to deep-sea conditions and implication in degradation processes

Abstract

During a swarm of the tunicate Pegea confoederata (salp) in the northern Arabian Sea, we examined their faecal pellets for thraustochytrid protists and bacteria to understand the role of the former in decomposition processes in the sea. Fresh faecal pellets from surface waters contained on average 6.58 × 10 6 thraustochytrids g -1 dry wt, while bacterial numbers were about 3 orders of magnitude higher. Highest numbers of thraustochytrids were observed when the faecal pellets were incubated at 25°C for 6 d using unsterilized surface sea water, as compared to 10°C using sterilized or unsterilized water collected from 100 m depth. Our results indicate that thraustochytrids in the water column may further colonize faecal pellets. A thraustochytrid isolate cultured from such faecal pellets grew on pine pollen, Artemia larvae and nutrient broth, when subjected to 10°C and a pressure of 10 MPa, corresponding to 1000 m depth. It also produced proteases when subjected to combinations of 2 pressures, 0.1 (1 atm) and 10 MPa (100 atm), and 2 temperature conditions, 30 and 10°C. The results suggest that thraustochytrids found in such particulate organic matter may actively contribute to decomposition processes not only in the surface waters, but also under deep-sea conditions.