Shell biofilm‐associated nitrous oxide production in marine molluscs: processes, precursors and relative importance

Abstract

Summary Emission of the greenhouse gas nitrous oxide (N2O) from freshwater and terrestrial invertebrates has exclusively been ascribed to N2O production by ingested denitrifying bacteria in the anoxic gut of the animals. Our study of marine molluscs now shows that also microbial biofilms on shell surfaces are important sites of N2O production. The shell biofilms of Mytilus edulis, Littorina littorea and Hinia reticulata contributed 18–94% to the total animal‐associated N2O emission. Nitrification and denitrification were equally important sources of N2O in shell biofilms as revealed by 15N‐stable isotope experiments with dissected shells. Microsensor measurements confirmed that both nitrification and denitrification can occur in shell biofilms due to a heterogeneous oxygen distribution. Accordingly, ammonium, nitrite and nitrate were important drivers of N2O production in the shell biofilm of the three mollusc species. Ammonium excretion by the animals was found to be sufficient to sustain N2O production in the shell biofilm. Apparently, the animals provide a nutrient‐enriched microenvironment that stimulates growth and N2O production of the shell biofilm. This animal‐induced stimulation was demonstrated in a long‐term microcosm experiment with the snail H. reticulata, where shell biofilms exhibited the highest N2O emission rates when the animal was still living inside the shell.

Variability in diel and seasonal in situ metabolism of the tropical gastropod Tectus niloticus

“…Variations in res piration or calcification of marine organisms have already been measured over different seasons (Lejart et al 2012), but diel variations are rarely considered since measurements tend to be taken only during the day, even for large-scale extrapolations of carbon metabolism. Furthermore, trochus shells, similar to carbonate substrates (Tribollet 2008), are colonized by epi-and endobionts such as bacterial biofilm, turf algae, encrusting calcareous algae and endo lithic flora (Heisterkamp et al 2013). Until now, the influence of these epi-and endobionts on respiration and calcification in molluscs has never been considered.…”

Fixed-Nitrogen Loss Associated with Sinking Zooplankton Carcasses in a Coastal Oxygen Minimum Zone (Golfo Dulce, Costa Rica)

“…The gastropod Cuvierina sp. reached very high abundances in GD during the time of the current study and might host microbial biofilms involved in N-cycling on their shell (Heisterkamp et al, 2013). In freshwater ecosystems, copepods and daphnids are abundant and may qualify as pelagic microbial hotspots .…”

“…The most prominent sources of N 2 O are nitrification (strictly speaking: ammonia oxidation) and denitrification. Nitrous oxide production associated with larger invertebrates has been ascribed to both nitrification and denitrification (Svenningsen et al, 2012;Heisterkamp et al, 2013). As in their benthic counterparts, the live copepods and ostracods may host denitrification activity in their anoxic gut and nitrification activity on their exoskeleton.…”

Direct Nitrous Oxide Emission from the Aquacultured Pacific White Shrimp (Litopenaeus vannamei)

“…For these animals, however, the availability of NO 3 Ϫ inside the gut and the fate of gut-produced N 2 O are currently not known. For mollusks, including Mytilus edulis, significant N 2 O production also proceeds in microbial biofilms growing on the shells of the animals (40). This phenomenon can also be expected for the richly sculptured shells of oysters, which have been recognized as keystone species for coastal nitrogen management (63), but to date has not been noted for N 2 O emission, which may be a disadvantage of nitrogen removal stimulated by benthic macrofauna (39,64).…”

“…The microoxic conditions close to the gut wall, however, may allow nitrifier denitrification, a process known to produce N 2 O in the near absence of O 2 (37). Nitrification might contribute to the total N 2 O emission by L. vannamei if oxygenated biofilms are present on the body surface of the animal, as was observed for other marine and freshwater invertebrate species (12,(38)(39)(40). For L. vannamei, however, exoskeletal N 2 O production must be very low or even absent, because the anoxic gut alone emits N 2 O at the same rate as the complete animal (Fig.…”