Organic matter cycling in anoxic (no oxygen) Marine sediments

Transcription

1 Organic matter cycling in anoxic (no oxygen) Marine sediments There are many examples where oxygen becomes exhausted in sediments How is organic matter oxidized without oxygen? What is the origin of petroleum source rocks? What biological communities can exist in anoxic sediments and how do they function? How does biogeochemical cycling in anoxic marine sediments affect global C:N:S cycles?

2 Dysaerobic and anoxic environments are devoid of higher heterotrophic organisms. Biogeochemical cycling, Including photosynthesis and C respiration are carried out by bacteria Image courtesy of T.D. Brock

3 Bacterial communities are much more metabolically specialized than higher heterotrophs. The communities are strictly segregated according to the environmental conditions. This leads to biogeochemical zoning of microbes and chemical species (nutrients, carbon dioxide, methane, etc.). Courtesy of T.D. Brock. Used with permission.

4 These communities are widespread in the Great Sippewissett Marsh in West Falmouth.

5 Oxidation of organic, rnatt:er in marive sediments Reaction Capacity (mrnolesjl sed) 0-85

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7 Sulfate reduction CH3C06H -t 5042' + 2 COr -I- 2 Hz0 4 COz + 4 CH3COOH +52-

8 - oxic degradatian nitrate redwct~on saluble cwr~st~tuents 032. NOj-, MP03- - pmteins [strrrrxural], peptides; pmlysacchadde5 Cstructurat), oligosa~cfiar-id-~ sugars, nucldc add& lipids J= ~larp;iie I- qxtracellular,.,,-- hydrolysis ; ferrnemfa%&gm ' sulfate redueion rrwmmp?mesks fatty acids

9 Acetate = CH 3 COOH Volatile Fatty Acid (VFA) = CH 3 RCOOH

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11 C6mpetltion far acetate between 5ewdHirbocterparagorsiand MerhararrinobBkeii Double reciprocal plot of rate vs Acetate concentration

12 competition for hydrogen between sulfate reducing and methanogenic bacteria - M. ~rblriphilus 1%' it& % H2 in gas stream

13 Changes in pore water concentrations during a 114 day incubation

14 Anaerobic degradation of pectin Time (Days)

15 Degradation of pectin in Knaack Lake sediments

16 Sulfate Present Sulfate Absent complex organic matter R

17 Chemical thermodynamics and pure culture studies of anaerobic micro-organisms suggest that the oxidizing capacity of sediments is >>> than [O2]. We predict that sulfate and ultimately carbon dioxide will be used as electron acceptors during organic matter oxidation. We also predict that sulfate reduction will occur before methanogensis. Can we observe the consequences of these factors And model their impact on organic matter oxidation?

18 Biogeochemical cycling in Cape Lookout Bight Sediments Map of Cape Lookout Bight. Map removed due to copyright restrictions.

19 Biogeochemical cycling in Cape Lookout Bight Can we achieve chemical balance if the system is at steady state? CO 2 Nutrients (N/P/S) methane Organic matter sulfate (permanently anoxic below a few mm)

20 Sulfate and Methane in CLB sediments (August)

21 Seasonal distribution of sulfate and acetate In Cape Lookout Bight sediments Note that sulfate is exhausted closer to the sediment/water interface in summer due to enhanced rate of sulfate reduction. Acetate accumulates only when sulfate reaches low concentrations

22 Flux = k (δc/δz) How can we calculate C remineralization from sulfate reduction?

23 Measurement of sulfate Oxidation using 34 S

24 Methane fluxes from CLB sediments

25 - ONDJFMAMJ1ASONO JFMAMJ JASOND JF MA 1977 'F

26 Methane concentrations build up to saturation values and form bubbles that remove CO2 and methane from sediments

27 ZC02 fluxes prn rn-2 hr-1 I Month diff ibubbl~ Total diff $uhbl.e Total Jan Feb March April May lu(nc July AQCJ Qpt ~ Qct Now n~r: Q 1m lww 3800~ 8 Vm ~ : 3.~6 SWQ 5 ssa &i ~ a ~ j a 2a mza 16300, WO' ~ ~. 3.7QtJ 6,s ag ; aa win' 0 190U I Average trgg7 mean monthty Rux [male rnm2 yfl] *9

28 Total C remineraliztion in CLB sediments out of top out of bottom remineralization = C flux Total C remineralization 41.4 molesm-2yr1

29 lrotal Carbon flux = 41.4 Methane flux = 5.8 total C rerniaemlirtsan due to methawgenesis: C reminerealization due to sulfate reduction is: Sulfate reduction measured or calculated frum tuk/tracer incubations and sulfate gradient is moles yr-1

30 Production of methane from acetate and CO 2 in CLB sediments. 14 C tracer studies.

31 Rates of C remineralization in CLB sediments

32 Integrated (0-35 cm)rates of methane production and sulfate reduction in CLB sediments

33 Summary: Terminal electron acceptors are used in the order of free E yields (O 2, Fe/Mn, SO 4, CO 2 ) Anoxic sediments are biogeochemically zoned according to e- acceptors Organic matter is oxidized by microbial consortia - no single organism degrades complex organic matter to CO 2. Instead, fermentation produces VFAs which are Used by acetophiles to yield CO 2 and methane. Methane is produced by two reactions CO 2 redn with Hydrogen, and disproportionation of acetic acid.