Microbial iron reduction and greenhouse gas production in response to organic matter amendment and temperature increase of periglacial sediments, Bolterdalen, Svalbard

Figures & data

Figure 1. Adventdalen and the sampling site in Bolterdalen (www.toposvalbard.npolar.no).

Figure 2. Detail of the section sampled for the study, showing three discrete sedimentary units.

Table 1. Main sedimentological characteristics of the three sedimentological units.

	Unit 1 (diamicton)	Unit 2 (beach)	Unit 3 (delta)
H2O content (%)	13 ±0.02	19 ±0. 01	8.4 ±0.01
pH (actual)	6.6 ±0.02	7.7 ±0.08	7.5 ±0.01
pH (potential)	5.2 ±0.1	6.8 ±0.1	6.6 ±0.01
SO4^2− (mM)	5.3	45.9	32.7
NO3^− (mM)	18.2	4.6	0.2
Total Fe (µmol g^−1 dry weight)	178	233	239
Fe(II) (% of total Fe)	27.8	58.4	42.4
Fe(III) (% of total Fe)	72.2	41.6	57.6
Particle size distribution (%)
Clay	10.9	10.1	16.3
Silt	48.6	47.8	51.4
Sand	40.5	42.1	33.2
Organic C (mg kg^−1)	27,316	6,304	12,110
Organic N (mg kg^−1)	1,491	164	656
Inorganic C (mg kg^−1)	26,377	13,077	16,805
Inorganic N (mg kg^−1)	1562	618	676
Organic C/N	21	45	22
Inorganic C/N	20	25	29

Figure 3. Fe(II) concentration data and linear models for iron reduction rates for incubations from three geological units with three treatments; 21°C (red circles), 4°C (blue circles), and 4°C with OM amendment (magenta circles). Points show mean values (n = 3 for each point), with error bars showing a single standard deviation. The rates (slopes) were significantly different for Unit 1, where the 4°C treatment resulted in a significantly slower rate than the other two treatments. No other rates were significantly different from one another for a given unit. Note that the linear model is based on the entire data set, not just the mean values displayed here.

Table 2. Production rates (µmol g⁻¹ dw d⁻¹) of CO₂ from incubation experiment 2.

Treatment	Unit 1 (diamicton)	Unit 2 (beach)	Unit 3 (delta)
4°C	0.11 (±0.01)	0.13 (±0.02)	0.05 (±0.01)
21°C	0.34 (±0.04)	0.63 (±0.01)	0.15 (±0.01)
4°C + OM	0.46 (±0.02)	2.40 (±0.06)	0.75 (±0.02)

Notes. Values shown are means of triplicates ±SD. CO₂ was only measured at the end of the incubation period after thirty days. The rate determined in in the 4°C incubation of Unit 3 was significantly different from rates in Units 1 and 2, which were not different from one another. Rates obtained at 21°C and after OM amendment were significantly different from each other and from the unamended 4°C incubation (p < .05). OM amendment also resulted in the highest rates.

Table 3. 16S rDNA gene copy number of Bacteria and Archaea per gram of sediment dry weight quantified after thirty days of incubation under three conditions, 4°C, 4°C with addition of organic matter, and 21°C, in the three units: Unit 1 (diamicton), Unit 2 (beach), Unit 3 (delta).

	Unit 1 (diamicton)		Unit 2 (beach)		Unit 3 (delta)
	Bacteria	Archaea	Bacteria	Archaea	Bacteria	Archaea
Treatment
4°C	8.9 * 10^5 ± 1.9 * 10^5	3.0 * 10^4 ± 2.6 * 10^3	4.8 * 10^4 ± 2.0 * 10^2	7.7 * 10^2 ± 48	5.2 * 10^3 ± 5 * 10^3	1.6 * 10^2 ± 1.5 * 10^2
4°C+ OM	1.1 * 10^6 ± 3.3 * 10^5	2.8 * 10^4 ± 8.4 * 10^3	5.4 * 10^5 ± 3.5 * 10^5	1.9 * 10^3 ± 1.0 * 10^3	2.4 * 10^5 ± 8.1 * 10^4	9.8 * 10^2 ± 210
21°C	1.4 * 10^6 ± 5.9 * 10^5	4.8 * 10^4 ± 1.9 * 10^3	4.5 * 10^4	6.1 * 10^2	2.0 * 10^4	6.1 * 10^2

Note. Each data point is a mean, calculated from triplicates (except for 21°C treatment of Unit 2 and Unit 3, where the value is based on a single measurement due to failed DNA extraction).

Table 4. The ratio between amount of CO₂ determined from Fe(II) production rates assuming that Fe(III) reduction was exclusively heterotrophic and CO₂ production determined from CO₂ measurements in the headspace of the slurries vessel.

Treatment	Unit 1 (diamicton)	Unit 2 (beach)	Unit 3 (delta)
4°C	2.00	1.56	5.33
21°C	1.54	0.37	4.28
4°C + OM	0.73	0.16	0.88

Note. For the conversion of Fe(II) into CO₂ equivalents it was assumed that the average oxidation state of organic carbon was 0.

Figure 4. CH₄ production under three conditions (magenta circle: 4°C with addition of organic matter, blue circles: 4°C, red circles: 21°C) in the three units: (a) Unit 1 (diamicton), (b) Unit 2 (beach), (c) Unit 3 (delta). Each data point is a mean, calculated from triplicates. Concentration is shown in nanomoles per gram dry weight.

Figure 5. Taxonomic assignment of relative abundance of bacterial 16S rRNA gene sequences at the phylum level of the three units (1 = diamicton, 2 = beach, 3 = delta). “Others” combines minor phyla never exceeding 5 percent of any sample.

Figure 6. Taxonomic assignment of relative abundance of bacterial 16S rRNA gene sequences at the class level of the three units (1 = diamicton, 2 = beach, or 3 = delta). “Others” combines minor classes never exceeding 5 percent of any sample.

Figure 7. Fractional abundance of potential Fe(III)-reducing bacterial genera identified using 16S rRNA gene sequencing.

Figure 8. Fractional abundance of potential Fe(II)-oxidizing bacterial genera identified using 16S rRNA gene sequencing.