Sponge species from New Zealand may transform and degrade dissolved organic matter

Sponges are an important component of shallow- and deep-water ecosystems enhancing eukaryotic biodiversity via diverse endo- and epibiota and by providing three dimensional habitats for benthic invertebrates and fishes. Sponge biodiversity is particularly high in the waters around New Zealand (Southwest Pacific), where we collected two shallow- and two deep-water sponge species (Tedania sp., Suberea meandrina, Farrea raoulensis, Artemisina sp.) for ex-situ incubation experiments to measure processing of dissolved organic matter (DOM). Several sponge species take up DOM and make it available to other fauna as detritus or as sponge biomass, a process known as sponge loop. However, it is unknown whether the selected sponge species are able to consume dissolved organic carbon (DOC) and/or total dissolved nitrogen (TDN).

We measured DOC and TDN fluxes and linked it to the bacterial communities of the sponge holobiont to address research hypothesis 1. It stated that high-microbial abundance (HMA) sponges consume more DOM than low-microbial abundance (LMA) sponges. Changes in fluorescent dissolved organic matter (FDOM) over time were investigated to address research hypothesis 2. It proposed that the fluorescence intensity F_max of fluorophores decreased in incubations that showed a significant loss in DOM. We assessed the biochemical and phospholipid-derived fatty acids (PLFAs) composition of sponge tissue to address hypothesis 3. It suggested that the PLFLA composition of sponges differs between sponge classes. Finally, we tried to better understand the role of these sponges in nutrient cycling around New Zealand by combining data from all analyses.

Based on the community composition of the sponge-associated bacteria, we classified Tedania sp., S. meandrina, and Artemisina sp. as HMA sponges and F. raoulensis as LMA sponge. We did not measure a significant DOC flux and only the release of TDN by Tedania sp. was significantly different from 0 μmol TDN g org. C^-1 d^-1. The presence of four fluorophores were detected in the FDOM pool: 2 tryptophan- and protein-like fluorophores (C1, C2), 1 humic-like fluorophore (C3), and 1 tyrosine-like fluorophore (C4). However, we could not validate hypothesis 2, because F_max of C1 decreased significantly in S. meandrina incubations, whereas F_max of C2 grew in the same incubations. F_max of C3 increased in Tedania sp. incubations, in which F_max of C4 decreased. In comparison, F_max of C4 in S. meandrina rose. The PLFA composition of sponge tissue was dominated by long-chain fatty acids, saturated fatty acids, and monosaturated fatty acids, and most PLFAs were sponge- and bacteria-specific. We could not confirmed hypothesis 3, either, because the PLFA composition of the hexactinellid sponge included seven identified PLFAs, whereas the PLFA composition of the demosponges ranged from three to 29 identified PLFAs.