University:
Widener University
Course:
CHEM-255 | Organic Chemistry I
Academic year:

2023
Views:

434

Pages:

33
Author:

hprox12345ddg

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? Dysaerobic and anoxic environments are devoid of higher heterotrophic organisms. Biogeochemical cycling, Including photosynthesis and C respiration are carried out by bacteria 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.). These communities are widespread in the Great Sippewissett Marsh in West Falmouth. Oxidation of organic, rnatt:er in marive sediments Reaction Capacity (mrnolesJL sed) 0-85 Sulfate reduction CH3C06H -t 5042' +2 COr -I-2 H z 0 4 COz + 4 CH3COOH +52- - oxic degradatian nitrate redwct~on saluble c w r ~ s t ~ t u e n t s 0 3 2 .N O j - , M P 0 3 - - pmteins [strrrrXural], peptides; pmlysacchadde5 Cstructurat), oligosa~cfiar-id-~ sugars, nucldc add&lipids qxtracellular ,.,,-hydrolysis ; ferrnemfa%&gm ' J= I- ~larP;iIe fatty acids sulfate redueion rrwmmp?mesks Acetate = CH3COOH Volatile Fatty Acid (VFA) = CH3RCOOH Double reciprocal plot of rate vs Acetate concentration C6mpetltion far acetate between 5ewdHirbocterparagorsiandMerhararrinobBkeii competition for hydrogen between sulfate reducing and methanogenic bacteria - M. ~rblriphilus 1 % ' it& 80 30 20 10 % H2in gas stream 0 Changes in pore water concentrations during a 114 day incubation Anaerobic degradation of pectin 9 0 15 20 Time (Days) Degradation of pectin in Knaack Lake sediments Sulfate Present Sulfate Absent complex organic matter R 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? Biogeochemical cycling in Cape Lookout Bight Can we achieve chemical balance if the system is at steady state? CO2 Nutrients (N/P/S) methane Organic matter sulfate (permanently anoxic below a few mm) Sulfate and Methane in CLB sediments (August) 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 How can we calculate C remineralization from sulfate reduction? Flux = k (δC/δz) Measurement of sulfate Oxidation using 34S Methane fluxes from CLB sediments - O N D J F M A M J 1 A S O N O J F M A M J J A S O N D J F MA 1977 0800 'F 978 0900 1000 Methane concentrations build up to saturation values and form bubbles that remove CO2 and methane from sediments ZC02 fluxes prn rn-2 hr-1 Month I Jan Feb March April May lu(nc July AQCJ Qpt ~ Qct Now n~r: Average ibubbl~ Total diff 1900. 0 1000. Q 1000 0 3 800~ 8 ~ 5 0 : .6 SWQ 5&i ~ a ~ j a 2a 16300, 4WO' 3.7QtJ 22ag ; 1win' 3460 1900 1 m lww V m 3 . ~ 6 ssa mza 6326 18 6,s. aa. 0 6.5 493.8~ 2707 2201 190U ~. trgg7 mean monthty Rux [male rnm2 yfl] 29.8 0.057 29*9 diff $uhbl.e Total I Total C remineraliztion in CLB sediments out of top out of bottom Total C remineralization remineralization = C flux 41.4 molesm-2yr1 lrotal Carbon flux = 41.4 Methane flux = 5.8 total C rerniaemlirtSandue to methawgenesis: C reminerealization due to sulfate reduction is: Sulfate reduction measured or calculated frum tuk/tracer incubations and sulfate gradient is 30-36 moles yr-1 Production of methane from acetate and CO2 in CLB sediments. 14C tracer studies. Rates of C remineralization in CLB sediments Integrated (0-35 cm)rates of methane production and sulfate reduction in CLB sediments Summary: Terminal electron acceptors are used in the order of free E yields (O2, Fe/Mn, SO4, CO2) Anoxic sediments are biogeochemically zoned according to e- acceptors Organic matter is oxidized by microbial consortia - no single organism degrades complex organic matter to CO2. Instead, fermentation produces VFAs which are Used by acetophiles to yield CO2 and methane. Methane is produced by two reactions CO2 redn with Hydrogen, and disproportionation of acetic acid.

Biogeochemical Cycling in Anoxic Marine Sediments I

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