Distribution of labile organic matter in the Eastern Brazilian Margin sediments

Resumo

Here we address the distribution of labile organic matter in the sediments of Eastern Brazilian Margin. High-quality organic matter accumulated in two sediment cores hypothesized to a local downslope transport of the sand sediments, restraining heterotrophic action. Protein and lipid concentrations showed an attenuation profile through the column water, exhibiting a significant relationship to depth, which is thought to be the main responsible factor for the quantity and quality of the labile organic matter in the sediments. Water column depth and sedimentary grain size seem to play a key role in determining the amount and quality of carbon sequestered in the Eastern Brazilian margin sediments.

Palavras chaves

Biopolymeric carbon; Organic matter source; Degradability

Introdução

Continental margins play remarkable roles in global biogeochemical cycles and form sites of significant biomass and biodiversity production (LIU et al, 2010). Autochthonous and allochthonous organic carbon from shallow waters is partially stored in sediments, turning these areas responsible for considerable global carbon reservoirs (HENRICHS, 1992; HEDGES AND KEIL, 1995; DELL'ANNO et al, 2013). Degradation of organic carbon in marine sediments influences a wide range of phenomena, including the magnitude of carbon sequestration over recent and geological timescales, recycling of inorganic carbon and nutrients, dissolution and precipitation of carbonates, production of methane and the nature of the seafloor biosphere. Although much has been learned about the composition and mechanisms of organic matter degradation stored in natural systems, controls over the distribution of organic carbon in modern and ancient sediments are still not fully understood (WATANABE AND KUWAE, 2015; LA ROWE et al, 2020). Recent observations and model projections suggest that the impact of climate change on marine particulate organic carbon is likely to be regionally heterogeneous, particularly from terrestrial sources (PASSOW AND CARLSON, 2012; REGNIER et al, 2013). Each of the well-documented factors triggered by climate change - namely: warming waters, disappearing sea ice, increasing DIC content, reduced pH, and changing organic carbon fluxes into and through the water column (LEVIN AND LE BRIS, 2015; SWEETMAN et al, 2017) - have potential implications for how organic carbon is delivered and processed in sediments. As the dynamics of benthic carbon is primarily controlled by the quantity and quality of organic carbon that is deposited on the sediment, the assessment of the biochemical composition of sedimentary organic matter inserts as an interesting analysis to understand the degradation response, as well as the reactivity of benthic carbon, in the deep ocean face to projected environmental changes. Regarding the Brazilian continental margin, the knowledge of biogeochemical processes, as well as the main factors that act in the degradation of organic matter in sediments, have increased in the last decade, but concentrated in southeastern latitudes (YOSHINAGA et al, 2008; OLIVEIRA et al, 2013; CARREIRA et al, 2010; 2015; CORDEIRO et al, 2018). Considering the distribution analyzes of lipid biomarkers, autochthonous material was considered as the main organic component in the sediments, including some amount of terrestrial organic matter in the shallower sediments of the inner shelf (CARREIRA et al, 2010, 2015; CORDEIRO et al, 2018) and partially degraded organic matter transported from the platform to the upper slope (CORDEIRO et al, 2018). Despite the wide application of lipid biomarkers, the quantification of major compounds classes of sedimentary organic matter – such as proteins, carbohydrates, and lipids – has not yet been addressed in Brazilian marginal sediments. Furthermore, the effects of physical environmental conditions that can alter the bioavailability and biodegradation of these biopolymers are unknown. Here we address the distribution of biopolymers in the sediments of the Eastern Brazilian and discuss about physical and environment processes that determine the quantity and quality of labile organic matter in the sediments. The results obtained raise the importance of considering not only chemical and biological processes, but also physical oceanographic and sedimentological processes in the bioavailability and biodegradation of organic matter in Brazilian Margin sediments.

Material e métodos

-Study area The study area comprises the latitudes between 10º to 22ºS in the Eastern Brazilian Margin (table 1). The Brazil Current, which is a weak western boundary current, is the main conduit of upper ocean waters in the region (GONI et al, 2011). -Bulk geochemistry and sediment grain size The contents of total organic carbon, total nitrogen, and carbon (δ13C) and nitrogen (δ15N) stable isotopes organic matter were measured in decarbonated sediment samples macerated in agate grail and analyzed in the PDZ Europa ANCA- GSL elementary analyzer coupled to a 20-20 PDZ Europe isotope ratio mass spectrometer. Isotopic ratios were reported relative to the international standards of Vienna Pee Dee belemnite for carbon and atmospheric N2 for nitrogen. For grain size analysis, decarbonated sediment samples were treated with 30% hydrogen peroxide to remove the organic contents. Analyses were performed in a laser diffraction particle size analyzer CILAS 1064 model. - -Biopolymeric composition of sedimentary organic matter CHO contents were measured spectrophotometrically using the phenol sulfuric acid assay for sediment samples (DUBOIS et al, 1956; GERCHACOV AND HATCHER, 1972). PRT concentrations were determined according to Hartree (1972) and Rice (1982). LIP contents were extracted by chloroform and methanol following the procedure of BLIGH AND DYER (1959) and MARSH AND WEINSTEIN (1966). The sum of the carbon equivalents of CHO, PRT, and LIP was referred as biopolymeric carbon (BPC) considering the conversion factors of 0.40, 0.49, and 0.75, respectively (FABIANO et al, 1995). -Data treatment and statistical analysis To test the relationships between the various parameters, a Spearman-rank correlation analysis was performed. The Kruskal-Wallis test was performed to evaluate significant differences in the distributions of values among the stations.

Resultado e discussão

Organic matter from both terrigenous and marine sources may contain compounds that are intrinsically labile, or refractory purely based on their molecular structures (e.g., elemental composition, presence of functional groups) and the physical form in which they exist in nature (SOLLINS et al, 1996; HEDGES AND OADES, 1997). According to the ranges of δ13C (from -22 to -20.5‰), δ15N (from 5 to 7.5‰) values obtained in this work (Table 1), the bulk geochemistry indicated an accumulation of organic matter from marine algae and marine particulate organic carbon (MEYERS, 1997; RAMASWAMY et al, 2008). The vertical distribution of BPC, protein, and lipid concentration (Figure 2) seems to follow the shape of the depth-diminishing particulate organic carbon (POC) flux profile in the open ocean that is traditionally characterized by empirical fits to data obtained from particle intercepting sediment traps (SUESS, 1980; MARSAY et al, 2015) or radionuclide disequilibria (BUESSELER et al, 2006; TOMALLA et al, 2006). The attenuation of biopolymer concentrations through the water column allows us to infer the action of selective degradation, where most of the fresh organic matter is recycled in the water column shallower than approximately 750 m. The distribution of PRT/CHO ratio according to the water column depth reports sediment cores generically characterized under the unit (<1.0). The increased contribution of less reactive organic matter in deeper depths (i.e., lower PRT/CHO ratio in depths greater than ~ 750 m) is also consistent with the increased organic matter 14C-age in deep continental margins (AUSÍN et al, 2021), because of recycling during sedimentation along the water column, and hence, of a lower vertical flux of the labile organic matter mostly recycled in the column water. The offset of these two cores from the biopolymer pattern in the Brazilian margin sediment cores can be explained by regional sedimentation (Figure 2). Upper-slope areas from latitudes around 19–23oS are strongly influenced by the active deposition of sediments from the outer shelf due to the strong bottom currents (VIANA et al, 1998; CADDAH et al, 1998). The downslope transport of sand to the depths around 300 – 450 m was reported for the region by the action of BC and SACW flows reworking the shelf-derived sand due to the ocean- atmosphere changes and to the local physiography (VIANA, 2002). Although BC- driven upwelling and meanders found in the area could increment the export of labile organic carbon (SILVEIRA et al, 2008; CALADO et al, 2010; LESSA et al, 2016), we cannot confirm the improvement of biopolymers in other cores from the same latitudes (i.e. M125-50, and M125-55). Then, we hypothesize the downslope transport of sand slightly improving the preservation of labile organic matter due to the increased sedimentation and less heterotrophic consumption. The presence of labile organic matter in the middle slope in similar latitude (around 21–22ºS) was also found by CORDEIRO et al, (2018), which exhibited the presence of partially altered (sterols) and reworked organic matter (branched fatty acids) in the surface sediments. These results evidence the influence of additional sedimentation patterns in the reactivity of the sedimentary organic matter in the depths, such those found in M125-35, and M125-49, namely 400-500m. Compared to other major compound classes, such as proteins, lipid, and nucleic acids, little is known about CHO biogeochemical processing due the possibility of polysaccharides be connected in multiple ways, creating enormous molecular diversity (ARNOSTI et al, 2021). The persistence of organic carbon in oxic oligotrophic sediment is generally attributed to a combination of protective processes that involve adsorption to mineral surfaces and physical inaccessibility to the heterotrophic community (ESTES et al, 2019). Then, we believe that the non-dependence of carbohydrate content with the water column depth (Figure 2) may rely on the presence of complex polysaccharides that might be bioavailable only in the presence of organisms with specific enzymatic systems capable to hydrolyze them. Such possibility has ecological implications since that substrate availability might provide a series of niches in which specialized populations could bloom, as reported by TEELING et al (2012). Facing such a perspective, we suggest that complex structure of polysaccharides analyses, including detrital components, and separation - as well as analytical techniques that do not destroy structural information - must be carried on for further investigations concerning the polysaccharide turnover in the ocean to accurately quantify rates of organic carbon cycling.

Table 1

Location and depth of each sediment core, including data of bulk geochemistry and sediment grain size.

Figure 2

Depth profiles of organic compounds in the sediments of investigated areas. Spearman correlation coefficients (Rs) are exposed (* = p ≤ 0.05).

Conclusões

Here we evaluated the biopolymers distribution in the sediments from the Eastern Brazilian margin. The quality and quantity of the organic matter accumulated in the sediments were mainly related to the water column depth and regional sedimentation trigged by local oceanographic dynamic, in special the protein and lipids concentrations which match the conceptual model of depth-diminishing particulate organic carbon and organic matter aging in the continental margins. The carbohydrate content behavior is distinct from the other biopolymeric compounds, showing no dependence to the depth but exposing evident drop in deepest sediment cores profiles. The achieved results provide more comprehensive insights about the mainly forcing that modulate processes involved in the labile organic carbon degradation and carbon sequestration in marine sediments at the Eastern Brazilian margin. Further investigations concerning the factors or variables controlling the extracellular enzymes in the water column and sediments would vastly improve our understanding of constraints on carbon cycling at the eastern Brazilian margin.

Agradecimentos

Carolina Santos R. de A. da Silva acknowledges the scholarship from Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq).

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