Although particulate organic carbon (POC) in sediments is derived from diverse sources, characteristics of bulk POC are frequently used as indicators of the ‘quality’ of organic matter potentially available to sedimentary microbial communities. In order to investigate the extent to which characteristics of POC relate to sedimentary metabolism, the rates of the initial and terminal steps of organic carbon remineralization (extracellular enzymatic hydrolysis, and sulfate, iron, and manganese reduction, respectively) were compared at three sites in Skagerrak dominated by different terminal remineralization processes. In parallel, dissolved organic carbon (DOC) inventories and bulk POC characteristics were assessed. At all three sites, bulk characteristics of POC were similar, with C/N close to 12, low sedimentary amino acid content, and moderate concentrations of total hydrolyzable carbohydrates. On average, just 12% of POC was characterizable as carbohydrates or amino acids. These characteristics are frequently considered typical of unreactive or ‘low quality’ organic matter. At all three sites, however, organic carbon remineralization (measured as CO 2 production and sediment O 2 uptake) was quite high relative to other locations with similar bulk characteristics. A comparison of DOC inventories with rates of terminal remineralization demonstrated that at the three sites, on average 27, 8, and 31% of the sedimentary DOC pool must be turned over on a daily basis in order to support terminal respiration. Extracellular enzymatic activity, calculated as potential carbon turnover, was sufficient to support these rates. At these sites, standard chemical characterization of bulk POC does not reflect the reactivity and availability of substrates to the sedimentary microbial community. Carbon remineralization is likely fueled by a small fraction of POC, not distinguishable by measurement of bulk parameters, which is rapidly cycled through the DOC pool.

Carbon cycling in a continental margin sediment: contrasts between organic matter characteristics and remineralization rates and pathways

Measurements of sediment oxygen consumption were made during spring and summer in the North Water, the polynya that forms between Greenland and Ellesmere Island, and used in conjunction with sediment trap data to assess benthic–pelagic coupling in this system. Bottom sediments ranged from cobble in the north to soft muds in the southern part of the sampling grid. Muddy sediments were often pelletized as shown by disaggregation. Sediment photopigments were generally lower in coarse sediment stations to the north than in finer sediments stations to the south. Shipboard incubation of intact cores provided rates of 0.07–0.17 mmol O 2  m −2  h −1 , with significantly greater oxygen consumption in summer than in spring. Additional incubation of macrofauna-free sediment aliquots in vials demonstrated significantly lower oxygen consumption in summer than in spring. Partitioning of benthic metabolism via these selective exclusion experiments showed a seasonal change in the response of the benthos to pelagic input, with meio-microbenthos dominating oxygen consumption in spring and macrofauna dominating in summer. Increased oxygen demand in the western polynya is suggested to coincide with the highest rates of carbon input measured by sediment traps and highest levels of sediment pigments. This region is an advective sink for particles produced in the east and subsequently transported by net polynya circulation. Although the benthos of the North Water does not display enhanced rates of carbon processing compared to other Arctic sediments, including other polynyas, the protracted production season of North Water provides a longer period over which the benthos can receive and mineralize organic carbon.

Sediment properties and benthic-pelagic coupling in the North Water

Organic matter ‘quality’ has long been recognized to affect rates of microbial remineralization of organic matter (e. g. WESTRICH & BERNER, 1984), but factors determining ‘quality’ remain elusive. WESTRICH & BERNER (1984) defined organic matter components of their multi-G model empirically. ‘Quality’ has also been associated with specific characteristics of sedimentary POC (particulate organic carbon) such as C/N ratio, amino acid composition or contribution to sedimentary N, or carbohydrate and amino acid contribution to total organic carbon (e. g. COWIE & HEDGES, 1994; DAUWE et al., 1999). The extent to which bulk composition of sedimentary POC reflects the substrates actually used by sedimentary microbial communities is unclear, however, since a small, rapidly cycled fraction of POC – whose identity is obscured by a background of bulk POC – may fuel microbial metabolism. Furthermore, the methods typically used (e. g. COWIE & HEDGES, 1994; WAKEHAM et al., 1997; DAUWE et al., 1999) to measure POC composition provide no information on the manner in which these components are linked in macromolecular structures. Macromolecular structure is an important consideration since heterotrophic microbes must use extracellular enzymes to hydrolyze substrates to sizes sufficiently small (600 daltons; WEISS et al., 1991) for transport across the membrane. The types of substrates accessible to a sedimentary microbial community therefore depend also on the activities and substrate specificities of the extracellular enzymes of the organisms. We sought to measure carbon flow between the initial (enzymatic hydrolysis) and terminal steps in the carbon remineralization pathway in sediments dominated by different terminal remineralization processes: sulfate, iron, and manganese reduction. We also compared specific components of POC and DOC inventories with rates of remineralization processes. Do sediments dominated by different pathways differ from one another in bulk POC characteristics?

/pdf/assessing-quality-from-a-microbial-perspective-contrasts-14jxv594uw.pdf

Assessing 'quality' from a microbial perspective: contrasts between organic matter characteristics and remineralization rates and pathways

Benthic oxygen fluxes and sediment properties on the northeastern New Zealand continental shelf

Importance of microbial iron reduction in deep sediments of river-dominated continental-margins

Des recherches sur la consommation d'oxygene des sediments, les productions de dioxyde de carbone et la reduction des sulfates ont ete menees dans les sediments vaseux de la marge continentale de Nouvelle Ecosse, afin d'examiner les variations saisonnieres et temporelles de la mineralisation du carbone. Les differences dans la texture des sediments sont causees principalement par les variations de taille et d'abondance des pelotes fecales, et les contenus en carbone et en azote sont plus bas au niveau du talus que sur le plateau continental. Les flux mesures sur un navire a partir d'incubations de sous-echantillons provenant d'un carottier-boite sont compares aux valeurs deja publiees obtenues au cours des memes campagnes mais avec des microelectrodes, par analyses de profiles verticaux realises sur l'eau interstitielle et par incubations d'echantillons provenant d'un carottier multitubes. La mineralisation du carbone mesuree soit par la consommation d'oxygene soit par la production de CO 2 n'est generalement pas significativement differente pour une meme station. La consommation d'oxygene est plus elevee a la station la moins profonde (Emerald Basin, 230 m) et moindre sur le talus (800 m), avec des valeurs intermediaires dans la Laurentia Trough (500 m), mais les differences temporelles dans les flux ne sont pas les memes a toutes les stations. Les taux de consommation d'oxygene mesures a bord a partir d'incubations de sous-echantillons d'un carottier-boite sont generalement identiques a ceux obtenus par incubations d'echantillons provenant d'un carottier multitubes. Les taux de consommation d'oxygene determines par microelectrodes et les flux de CO 2 obtenus pour l'eau interstitielle sont plus bas que ceux issus d'incubations, probablement en raison de la non prise en compte de la bioturbation dans les calculs realises sur les incubations. Le metabolisme benthique total est domine par la reduction des sulfates a toutes les stations jusqu'a au moins 35 cm au-dessous de la surface du sediment, avec a nouveau des taux plus importants sur le plateau que sur le talus. Malgre une plus grande profondeur au niveau du talus, comparee a Emerald Basin, la dimution de la mineralisation du carbone est faible, ce que suggere un apport lateral significatif au talus en provenance du plateau de Nouvelle Ecosse. En comparaison avec les sables qui recouvrent la plus trande partie du plateau, l'importance relative des sediments fins de celui-ci est accentuee car les processus de mineralisation dans ces substrats (particulierement la reduction des sulfates) impliquent une plus grande hauteur de la colonne de sediment que dans les sables, pauvres en matiere organique. En outre, la mineralisation relativement elevee sur le talus doit etre prise en compte dans les budgets de carbone de la marge continentale, en particulier dans le cas d'un transfer significatif en dehors du plateau continental.

Sulfate reduction and total benthic metabolism in shelf and slope sediments off Nova Scotia

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Sulfate reduction and total benthic metabolism in shelf and slope sediments off Nova Scotia