Transformation and fate of microphytobenthos carbon in subtropical shallow subtidal sands: A ¹³C‐labeling study

Abstract

Microphytobenthos (MPB) in photic sediments are highly productive but the fate of this production remains uncertain. Over 33 d, tracing of 13 C from added bicarbonate in subtropical shallow subtidal sand showed rapid transfer of MPB-derived carbon to deeper sediment; below 2 cm (31% within 60 h) and 5 cm (18%). Despite their high turnover (5.5 d) and only representing , 8% of sediment organic carbon, MPB represented up to 35% of the 13 C retained in sediments, demonstrating substantial carbon recycling. Carbon was rapidly transferred to heterotrophs, but their contribution to sediment 13 C was similar to their biomass contribution (, 0.1% to 3.8%), with the exception of the foraminifera Cellanthus craticulatus, which accounted for up to 33% of the 13 C within sediment. There was little loss of MPB-derived carbon via dissolved organic carbon (DOC) effluxes (3%) or resuspension (minimal). Respiration was the major loss pathway (63%), reflecting the high microbial biomass typical of lower latitudes. Given that MPB take up dissolved inorganic carbon (DIC) from overlying water, and the carbon they fix is released as DIC, MPB in subtropical sands are unlikely to substantially alter the form of carbon transported offshore (i.e., there is no conversion to DOC), but processing within the sediment may alter its d 13 C value. Given that 31% of fixed carbon remained in sediments after 33 d, subtropical sands may act as a carbon sink, thereby affecting the quantity of carbon transported offshore.

Reviews and syntheses: to the bottom of carbon processing at the seafloor

“…It is only on the timescale of ocean bottom-water renewal (100-1000 years) that nutrients regenerated by benthic organisms may impact primary producers in the sunlit upper part of the ocean (Soetaert et al, 2000). This is obviously different for sediments in the photic zone that make up about one-third of the coastal ocean (Gattuso et al, 1996) because animals can directly graze and consume the benthic primary producers at the sediment surface Evrard et al, 2010Evrard et al, , 2012Oakes et al, 2012;Fig. 5).…”

“…Various types of experimental evidence have shown that carbon flow through the living compartment is much higher than through the nonliving sediment organic matter pool. Short-term in situ experiments using 13 C-and/or 15 N-labeled organic matter (e.g., phytodetritus) revealed rapid incorporation of 13 C / 15 N in physically separated organisms (macro-and meiofauna and foraminifera) and microbes, the latter via incorporation of tracers in biomarkers specific for certain microbial groups Boschker and Middelburg, 2002;Veuger et al, 2007;Oakes et al, 2012;Woulds et al, 2007Woulds et al, , 2016 Similarly, ammonium isotope dilution studies have shown that net ammonification (ammonium release) is only a fraction of the total ammonium regeneration because a substantial part of the ammonium liberated is re-assimilated by the microbial community (Blackburn and Henriksen, 1983). Clearly the microbes and animals living in sediment assimilate carbon and synthesize new biomass (Veuger et al, 2012).…”

Stable isotopes dissect aquatic food webs from the top to the bottom

“…The in situ approach has been successfully applied to elucidate food webs in lakes (Pace et al, 2004(Pace et al, , 2007, streams (Hall and Meyer, 1998), estuaries , tidal marshes (Gribsholt et al, 2005(Gribsholt et al, , 2009), tidal flats Rossi et al, 2009), coral reefs (de Goeij et al, 2013 and deep-sea sediments (Blair et al, 1996). Heavy isotope-enriched materials have also been added in situ to benthic chambers in coastal (Oakes et al, 2010(Oakes et al, , 2012 and deep-sea sediments (Moodley et al, 2002;Witte et al, 2003) and to cores implemented at the seafloor (Nomaki et al, 2011).…”

Microphytobenthic extracellular polymeric substances (EPS) in intertidal sediments fuel both generalist and specialist EPS‐degrading bacteria

“…13 C-bicarbonate stable isotope probing (SIP) has demonstrated differential labeling of constituent sugars in diatom carbohydrate fractions from MPB (Bellinger et al 2009;Oakes et al 2010); with subsequent incorporation of 13 C-labeled carbon into the phospholipid fatty acids (PLFAs) of various bacterial groups Bellinger et al 2009;Gihring et al 2009). Though bacterial utilization may result in significant mineralization of diatom EPS to dissolved inorganic carbon (Oakes et al 2010(Oakes et al , 2012, a proportion of this carbon is incorporated into bacterial biomass, e.g., peptidoglycans, where it can contribute to the longer term retention of organic carbon within sediments (Veuger et al 2006).…”