Metabolic plasticity of HMA sponges allows them to completely oxidize all reduzed sources in the water: an energetic view

Abstract

The contribution of DOM to sponge feeding is now well documented1, 2 and uptake of labile DOC occurs in both high (HMA) and low (LMA) microbial abundance sponge3. In contrast, considerable DON uptake and nitrate excretion are observed only in HMA species. We used in situ and laboratory techniques to measure the dissolved and particulate nitrogen fluxes mediated by HMA and LMA sponges and constructed a partial ni- trogen budget for 14 sponges species from four different oceanic areas: North-East Pacific, North Sea, North- West Mediterranean Sea, and the Red-Sea. Regardless of the oceanographic settings, HMA sponges exhibited high plasticity in their ability to use the available dissolved nitrogen compounds. Under normal oxygenated conditions, NH4+ levels are low and most of nitrate excreted by HMA sponges is derived from full oxidation of DON; when NH4+ concentrations are high, its uptake represent an addition to the nitrification rates. The ubiquity of nitrification among the different geographical zones and the high rates of nitrate production as- sociated with HMA sponges suggest that it represents a relevant process for energy income and potentially microbial production within these sponges. We discuss the energetic consequences of these results considering the contribution of full oxidation of the organic matter for the sponge holobionts and the consequences for nutrient cycling. Our data suggest that the HMA sponges are unique among the metazoans in their nutritional plasticity and their ability to take advantage of the entire range of reduced compounds available in the water, provide them with a relevant additional energy source