Dissolved organic matter fluxes and fatty acid composition from shallow-water and deep-water demo- and hexactinellid sponges from New Zealand

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 detect whether these sponges may process dissolved organic matter.

We assessed the biochemical and phospholipid-derived fatty acids (PLFAs) and measured dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) fluxes. Changes in fluorescent dissolved organic matter (FDOM) over time were analyzed and the results were linked to the bacterial communities of the sponge holobiont.

Dried sponge tissue consisted of 17.5 ± 3.75% organic (org.) C and of 4.34 ± 1.02% total nitrogen (TN) with a natural stable isotope composition of -19.0 ±0.25‰ for δ ¹³ org. C and of 10.2 ±2.43‰ for δ ¹⁵ TN. None of the DOC fluxes was significant and only the release of TDN by Tedania sp. was significantly different from 0 μmol TDN g org. C ^-1 d ^-1 . We detected the presence of four fluorophores in the FDOM pool: 2 tryptophan- and protein-like fluorophores (C1, C2), 1 humic-like fluorophore (C3), and 1 tyrosine-like fluorophore (C4). The maximum fluorescence intensity 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. The bacterial community of the demosponges Artemisina sp., S. meandrina and Tedania sp. consisted mostly of Proteobacteria and Chloroflexota, whereas the dominating bacteria phylum of the hexactinellid F. raoulensis was Proteobacteria.

We proposed that the holobionts of S. meandrina and Tedania sp. contain bacteria that are involved in the transformation and degradation of DOM. In S. meandrina , Chloroflexota and Poribacteria may degrade tryptophan-like fluorophores to a chemically modified tryptophan-like and protein-like fluorophore, while producing a tyrosine-like fluorophore. In Tedania sp., Chloroflexota may contribute to the release of significant amounts of TDN by producing humic-like fluorophores, while degrading tyrosine-like fluorophores. Farrea raoulensis may not take up DOM due to a lack of Poribacteria and Chloroflexota or may use colloidal instead of truly dissolved DOC.