Transformation and fate of microphytobenthos carbon in subtropical, intertidal sediments: potential for long-term carbon retention revealed by <sup>13</sup>C-labeling

Abstract

Abstract. Microphytobenthos (MPB) are ubiquitous in coastal sediments, but the fate of their production (carbon biomass) is poorly defined. The processing and fate of MPBderived carbon in subtropical intertidal sediments was investigated through in situ labeling with 13 C-bicarbonate. Of the added 13 C, 100 % was fixed within ∼ 4 h, suggesting that MPB productivity was limited by inorganic carbon availability. Although there was rapid transfer of 13 C to bacteria (within 12 h), a relatively small fraction of 13 C was transferred to heterotrophs (up to 12.5 % of total fixed 13 C into bacteria and 0.01 % into foraminifera). MPB was the major reservoir for 13 C throughout the study, suggesting that production of extracellular polymeric substances was limited and/or MPB recycled 13 C. This retention of 13 C was reflected in remarkably slow estimated turnover times for the MPB community (66-100 d). Over 31 d, ∼ 70 % of the 13 C was lost from sediments. This was primarily via resuspension (∼ 55 %), enhanced by elevated freshwater flow following rainfall. A further ∼ 13 % was lost via fluxes of dissolved inorganic carbon during inundation. However, 13 C losses via dissolved organic carbon fluxes from inundated sediments (0.5 %) and carbon dioxide fluxes from exposed sediments (< 0.1 %) were minimal. The retention of ∼ 30 % of the carbon fixed by MPB within one tidal exposure after >30 d, despite high resuspension, demonstrates the potentially substantial longer term retention of MPB-derived carbon in unvegetated sediments and suggests that MPB may contribute to carbon burial ("blue carbon").

The transformation and fate of sub‐Arctic microphytobenthos carbon revealed through 13 C‐labeling

“…As reported in previous studies (e.g., Middelburg et al 2000;Oakes et al 2012;Oakes and Eyre 2014), fluxes of inorganic carbon were a major pathway for loss of MPB-C. However, the portion of fixed 13 C lost as inorganic carbon was toward the lower end of the range previously reported.…”

“…Up to 75% of MPB production can be exuded as extracellular organic carbon (EOC) (Goto et al 2001) which, along with MPB, can contribute significantly to the carbon requirements of bacteria (Bellinger et al 2009;Oakes et al 2010b;Hardison et al 2011;Miyatake et al 2014) and higher heterotrophs (Middelburg et al 2000;Oakes et al 2010a;Evrard et al 2012). MPBderived carbon (MPB-C) may ultimately be removed from sediment via resuspension (de Jonge and van Beusekom 1995), grazing, or fluxes of dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) from inundated sediments (Evrard et al 2012;Oakes et al 2012;Oakes and Eyre 2014), or CO 2 from exposed sediments (Oakes and Eyre 2014), following respiration and/or bacterial and virally-mediated degradation (Glud and Middelboe 2004). Alternatively, MPB-C may be retained within the sediment record (Oakes and Eyre 2014).…”

“…MPBderived carbon (MPB-C) may ultimately be removed from sediment via resuspension (de Jonge and van Beusekom 1995), grazing, or fluxes of dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) from inundated sediments (Evrard et al 2012;Oakes et al 2012;Oakes and Eyre 2014), or CO 2 from exposed sediments (Oakes and Eyre 2014), following respiration and/or bacterial and virally-mediated degradation (Glud and Middelboe 2004). Alternatively, MPB-C may be retained within the sediment record (Oakes and Eyre 2014). The relative importance of these pathways has been assessed in very few studies, but has important implications for coastal carbon cycling.…”

“…Much of the current understanding of the processing of MPB-C derives from deliberate additions of the rare carbon stable isotope, 13 C, to trace and quantify carbon flows, but the scope of previous studies has been limited. Only two studies (Oakes et al 2012;Oakes and Eyre 2014) have investigated the relative importance of the assimilation, trophic transfer, and flux pathways for MPB-C on longer time scales. Typically the transformation and fate of MPB-C is only assessed over relatively short periods of 4-6 d (Middelburg et al 2000;Evrard et al 2008Evrard et al , 2010.…”

Fate of microphytobenthos nitrogen in subtropical subtidal sediments: A 15 N pulse-chase study

“…During biomass synthesis MPB fix carbon (C) and assimilate nitrogen (N) from both sediment pore waters and the water column e.g., (Sundb€ ack and Graneli 1988;Ferguson et al 2004;McGlathery et al 2007). A number of studies have looked at the fate of C fixed by MPB e.g., (Middelburg et al 2000;Oakes et al 2012;Oakes and Eyre 2014), but little is known about the fate of assimilated N. The most comprehensive assessment of the fate of MPBderived C in subtidal sediments showed that respiration was the major loss pathway for MPB-C (63%) (Oakes et al 2012), whereas little MPB-C was lost via DOC fluxes (3%) or resuspension (< 3%). After 33 d 31% of the MPB-C was still retained in the sediments.…”