Unravelling the versatile feeding and metabolic strategies of the cold-water ecosystem engineer Spongosorites coralliophaga (Stephens, 1915)

Abstract
Sponges are often major players in the functioning of shallow-water ecosystems through their high
biomass and high capacity in filter feeding. In comparison, little is known about the feeding and
metabolic strategies of deep-sea sponges, although they can also form dense aggregations with high
biomass. This situation hinders our understanding about how some sponge species thrive under the
often food-limited conditions of the deep sea. In the present study we examined the feeding and
metabolic strategies of 1) the massive demosponge Spongosorites coralliophaga, which was recently
described as an important ecosystem engineer in cold-water coral reefs (CWCRs) and 2) the anthozoan
Parazoanthus anguicomus and the ophiuroid Ophiura ophiura, i.e. two dominant epibionts on S.
coralliophaga. All three benthic species have high density at CWCRs of the North-East Atlantic and
knowing their feeding strategies facilitates future studies on carbon (C) and nitrogen (N) cycling at
CWCRs. The on-board feeding experiments examined the processing of four isotopically-labelled food
sources, namely 15N-ammonium chloride, 13C-glucose, 13C/15N-labelled microalgae, 13C/15N-labelled
bacteria by S. coralliophaga and its symbiotic bacteria and the epibionts P. anguicomus and O. ophiura
from the Mingulay reef complex and the Logachev mound (North-East Atlantic). There were no
significant differences among the three species in terms of biomass-specific C and N assimilation rates;
however, there were differences among S. coralliophaga, P. anguicomus and O. ophiura in how they
processed the food sources and this is may be linked to interspecific variability in metabolic needs. S.
coralliophaga preferentially assimilated particulate organic N (PON) over particulate organic C (POC)
while this was not the case for P. anguicomus and O. ophiura. We did not detect the 15N tracer in the
bacterial biomarker D-Alanine suggesting that the preferential assimilation of N over C in S.
coralliophaga was mediated by sponge cells instead of the bacterial symbionts. S. coralliophaga
assimilated C and N from all four food sources and this versatile feeding strategy was accompanied by
an ability for de novo synthesis of essential and non-essential hydrolysable amino acids (HAAs). We
suggest that the recorded feeding and metabolic flexibility of S. coralliophaga plays an important role in
the survival of this massive sponge under food-limited conditions in the deep sea.