Temperature rise globally are likely to increase the rate of organic matter decomposition resulting in substantial CO2 release. The marine carbon cycle in Arctic regions currently occupies a prominent role in the study of climate warming effects as there is evidence from observational and modelling studies for strong seasonal temperature increases in northern high-latitude environments (Serreze et al. 2000). Further, biogeochemical processes react very sensitively to temperature changes (Denman et al. 2007; Anisimov et al. 2007). Since more than 90 per cent of the global sea floor is cold at 4°C (Levitus and Boyer 1994), sulfate reducers must be able to metabolise and grow at such low ambient temperatures.
Dissimilatory sulfate reduction is the most important bacterial process in anoxic marine sediments, accounting for up to half of the total organic carbon (TOC) remineralised (Canfield et al. 1993; Jorgensen 1982; Nedwell et al. 1993). Sulfate reduction rates (SRR) in polar sediments may be similar to those of temperate environments (Jorgensen et al. 1990; Nedwel et al. 1993; Sagemann et al. 1998; Thamdrup et al. 1994). Numerous factors have been shown to influence rates of bacterial sulfate reduction which include temperature, salinity, concentration of dissolved sulfate and quality and quantity of organic matter available for decomposition (Goldhaber and Kaplan 1975; Jorgensen 1983).
Kongsfjorden is a unique marine coastal system in the northwest coast of Svalbard, with a length of about 26 km surrounded by mountains and glaciers. Kongsfjorden itself consists of several zones from calving glacier fronts to rocky shores and soft bottom, caused by strong deposition of sediments from glaciers. Rivers and glaciers discharge high amounts of sediments and fresh water loaded with fine sediments resulting in high water turbidity and salinity stratification of the water body during the summer months (Hanelt et al. 2004). The inner fjord is generally free of ice cover during summer due to the mild climate influenced by the west Spitsbergen current. Thin pack ice is shifted out of the fjord by wind and the fjord surface maintains open water characteristics throughout the winter. The annual mean water temperature is generally slightly above 0°C (Ito and Kudoh 1997), which supports the abundance of psychrophilic bacteria. Among the most important is the capacity of these cold-adapted bacteria to catalyse biogeochemical processes at low temperatures (Feller and Gerday, 2003; Hoyoux et al. 2004).
In the present study, we therefore address the question whether an increase in temperature by 4oC has an impact on sulfate reduction (a dominant process in marine sediments) in cold sediments of Kongsfjord in Svalbard. We have also evaluated the substrate utilisation and the importance of salinity variations on the SRR with temperature increase and in turn the CM.
Materials and Methods
Kongsfjorden is a glacial fjord in the Svalbard, Arctic, about 40 km long and 5 to 10 km wide in the northwest coast of Spitsbergen. The main island of Svalbard, it is a site where the warmer waters of the Atlantic meet the colder waters of the Arctic. The melt water during summer not only stratifies the upper water column but significantly alters the turbidity. The locations of the 4 sampling sites are depicted in Figure 1.
Sediment samples were collected on the 4 July 2009 from Kongsfjorden, Svalbard, Arctic Ocean (Station 1; 79°02.126’ N, 11°17.014’ E), (Station 2; 79°0.594’ N, 11°25.471’ E), (Station 3; 78°57.216’ N, 12°10.527’ E) and (Station 4; 78°58.530’ N, 11°41.487’ E). Station 2 and 3 lie in the vicinity of glacier water inflow, while at Station 1 mixing of warm Atlantic water takes place with Arctic cold waters. The sediment temperature was on an average 0°C, while the surface water temperature was ca. 5°C. Sediment samples, collected with a grab from various depths (Table 1) were sub sampled and kept at in-situ temperature during transport. The pH was measured immediately, (Thermo Orion model 420A, USA) as per the standard procedure. TOC was estimated by titrimetry with a precision of 0.01 per cent (Allen et al., 1976) that involves complete wet oxidation of organic matter in the sample with chromic acid. The TOC was expressed as a percentage. Glucose was used as the standard.
Pore water analysis
Sediment was centrifuged at 5000 rpm for 10 minutes. The supernatant- pore water was collected with a 5 ml syringe and salinity was measured using a hand refractrometer (S/MillE, ATAGO, Co. Ltd, Japan).
Sulfate reduction rates
SRR was measured using King’s assay (King GM, 2001). Sodium sulfate 10 µl (74KBq) from Sigma Aldrich (USA) was injected into the section to distribute the label evenly, incubated at 5oC and the activity was arrested at the end of 10 hours by adding 5 ml (5 per cent wt/vol) zinc acetate and frozen at -20oC till the analysis. Radiotracer assay of the sediment was carried out using single step chromium reduction assay (King GM, 2001) using Packard 1600 CA Tri Carb (USA) liquid scintillation counter.
SRR were calculated using the equation:
SRR = (H235S/35SO4-2) ×32SO4-2 × IDF/T
H235S = radioactivity of reduced sulfur (DPM),
35SO42- = radioactivity of sulfate at the beginning of incubation,
32SO4-2 = Pore water sulfate concentration (µMSO4-2cm-3),
Where IDF (isotopic discrimination factor) = 1.06,
T = time of incubation in hours.
SRR was expressed as nM.g-1.d-1.
Effect of carbon source and temperature on SRR: slurry experiment
Sediment samples for the preparation of slurries were collected. Aliquots were prepared by taking approximately 2 g of sediment slurry into 5 mL vials. These samples (in triplicates) were then amended separately with lactate (5mM), acetate (5mM) and a combination of lactate and acetate to a final concentration of 5mM. Incubation was carried out in two separate sets at 0oC and 4oC, respectively. All the vials were flushed with oxygen free nitrogen gas. The resulting slurry samples were acclimatised for 10 minutes and supplemented with 10 µL sodium sulfate (74KBq) and incubated at room temperature for 10 hours. Samples were fixed and analysed by the radiotracer using single step chromium reduction King’s assay (2001) as described previously.
The TOC content in the sediments showed an increasing trend from the inner zone (0.21 per cent) to the outer zone (1.28 per cent) at a depth of 75 m and 340 m, respectively—the highest amount of organic carbon being in the outer zone (Table 1). SRR generally increased with an increase in the TOC at S1, S3 and S4.
Similarly the SRR increased by 45 per cent with a 4°C rise in temperature. The percentage increase in the CM was calculated and found to increase with a 4oC rise in temperature. However, there was a substantial decrease in the CM in the middle zone S2 of the Kongsfjorden. Addition of substrates to the slurries resulted in a marked increase in the SRR at 4oC than at 0oC. Lactate when supplemented as a sole carbon source was more preferred as compared to acetate at station S1, S2 and S3. Acetate was the most preferred at station S4. However when both substrates were supplemented together, the SRR was lower than the control at 0oC (Figure 2). At 4oC, the SRR were almost two times higher than the values at 0oC when supplemented with carbon substrates. Lactate was preferred to acetate at 4oC. A combination of lactate and acetate amended, elicited lower SRR in most of the stations.
Figure 3 shows the effect of salinity on the SRR and in turn on the CM rates. CM was calculated as increased rates subsequent to a 4oC rise in temperature which is expressed as per cent increase of the value at 0oC. The increase in CM rates through SRR at 30 psu was 53 per cent at station S4 . It was observed that with a decrease in salinity, the rate of CM decreased and was negative at station S2 which coincided with the minimum salinity recorded.
Coastal and estuarine sediments are important sites for the mineralisation of biomass produced in the photic zone (Jorgensen, 1983). Anaerobic sulfate reduction is an important process in the degradation of organic matter in these sediments. SRA in marine ecosystems, especially the coastal regions contributes as much as 50 per cent organic carbon turnover (Jorgensen, 1982). Organic carbon levels also influence SRR which is an important reaction for the production of reactive byproducts and the regeneration of nutrients. Naturally occurring organic polymers are stated to have vastly different susceptibilities to bacterial attack. In the present study we have used two volatile fatty acids viz. acetate and lactate which are the normally preferred substrates by sulphate reducing bacteria (SRB) in marine sediments. It is widely accepted that acetate is one of the important substrates for sulfate reduction in Smeerenburgfjorden, Svalbard sediments (Finke et al. 2007) and intertidal sediments (Taylor and Parks, 1985) accounting for 70 per cent of the total sulfate reduced in marine sediments and lactate is a widely utilised substrate by most SRB (Kerkar and Bharathi, 2007), hence chosen for the present study. We have observed that although a range of different substrates are used by SRB, the individual substrates utilised in different environments and SRB populations vary as seen in the above four stations which are located in Kongsfjorden within an approximately 26 km length. Kongsfjorden is a fjord with several zones from calving glaciers to rocky shores and soft bottom habitats. We suggest that the susceptibility of organic substrates to metabolic attack exerts an important control on the temperature dependence of sulfate reduction. Thus seasonal variations need to be considered in quantifying annual rates of sulfate reduction in these polar regions. From our observations, we infer that deeper sediments have smaller thermal fluctuations than the surface sediments. However depth may also be an important factor influencing SRR. The SRR in the Konsfjorden at 0oC were in the range of 1.73 to 6.12 nM.g-1.d-1 which are much lower than the rates observed in the Smeerenburgfjorden in Svalbard which varied from 48 to 70 nM.cm-3.d-1 where incubation temperatures have not been mentioned. Interestingly SRR in the Konsfjorden increased by 45 per cent with a 4°C rise in temperature. Relatively higher activities in SRR were observed with higher TOC and increasing salinities. Jorgensen (1977) has stated that on an average 2 moles of organic carbon (C) are oxidised for every mole of sulphate reduced. Redfield’s stoichiometry for organic matter oxidation by sulphate reduction Corg: SO4-2 = 2.1 (Volkov et al. 1998). Thus SRB contribute to the CO2 flux. Based on this relationship, the annual CM rate estimated by SRR in sediments ranged from 3.46 to 12.2 g. C d-1 which amounts to a 20.3 to 53.19 per cent increase in the CM .When the salinity decreases to 9 psu, due to the melting glaciers at S2 , the per cent CM was below the value at 0 degrees by 31 per cent. This change could be attributed to the decrease in salinity having a negative impact on the SRR and thus the CM. Consequently, a temperature rise would mean that the SRB would contribute to an increase in CO2 flux and thus contribute to global warming. However, due to mixing of sea water with fresh water by ice and glaciers melting, the SRR decreases neutralising the above effect.
The present work suggests that the temperature dependence of sulfate reduction may have predictable substrate dependence. Salinity variations also play an important role in the SRR and in turn the CM. However there is a need to determine the seasonally averaged rates of sulfate reduction in these polar marine environments with reference to the variations in temperature, salinity and organic matter.
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