Abstract
This study examined the ability of a Mediterranean demosponge Chondrosia reniformis to oxidize exogenous ammonium, simulating N-rich conditions that occur near finfish farms. We hypothesized that as the concentration of ammonium increases in the surrounding seawater, nitrification mediated by microbes associated with C. reniformis will lead to enhancement of ammonium uptake, nitrate excretion and oxygen consumption by the sponge holobiont. To test this hypothesis, we conducted laboratory experiments with C. reniformis explants exposed to ammonium enrichments (300–6667 nM) and to ambient seawater (45–1511 nM ammonium). We analyzed inhaled (IN) and exhaled (EX) water samples for dissolved oxygen, ammonium, nitrates and retention of picoplankton cells. We observed ammonium uptake in nearly half the cases and excretion of nitrate in most experimental outcomes. Yet, the consumption of ammonium and oxygen, as well as the excretion of nitrate by C. reniformis were not related to the concentration of inhaled ammonium, which suggests that the nitrification activity of sponge-associated microbes is not necessarily related to the concentration of ammonium in the surrounding seawater. Further research is required to reveal the sources of nitrate released from sponges and the fate of this nitrate in natural and manipulated ecosystems.
Highlights
Marine sponges feed on microorganisms and host dense and diverse microbial communities in their bodies (Taylor et al, 2007; Fan, 2012; Webster and Taylor, 2012; Pita et al, 2018)
The differences in NOx concentrations between the inhaled and the exhaled samples were negative in most cases (63 out of 68 IN-EX pairs), i.e., nitrates were excreted by the sponge (Supplementary Material, Data Sheet 1)
Our findings suggest that we cannot generalize about high microbial abundance (HMA) sponges serving as a source or a sink of ammonium
Summary
Marine sponges feed on microorganisms and host dense and diverse microbial communities in their bodies (Taylor et al, 2007; Fan, 2012; Webster and Taylor, 2012; Pita et al, 2018). Sponges produce ammonium (NH+4 ) as a metabolic waste (endogenous ammonium), which makes the sponge body an attractive niche for microorganisms in nitrogen-limited marine environments (Han et al, 2013). Nitrification is an exclusively prokaryotic process whereby ammonium is oxidized to nitrite (NO−2 ) and subsequently to nitrate (NO−3 ), and it is utilized to Effect of Ammonium on Sponge Holobiont Nitrification obtain energy by different groups of bacteria and archaea associated with sponges (Kowalchuk and Stephen, 2001; Schläppy et al, 2010; Ribes et al, 2012; Pita et al, 2018). Several sponge species were found to consume exogenous ammonium while excreting nitrate, thereby resembling microbe-mediated nitrification (Corredor et al, 1988; Diaz and Ward, 1997; Jiménez and Ribes, 2007; Schläppy et al, 2010; Ribes et al, 2012)
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