Aquatic respiration and ocean metabolism

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Nathaniel Neal
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1 Aquatic respiration and ocean metabolism

2 Remember what life is all about: Energy (ATP) Reducing power (NADPH) Nutrients (C, N, P, S, Fe, etc., etc.) Photosynthetic organisms use sunlight, H 2 O, and dissolved nutrients Heterotrophic microorganisms rely mostly on dissolved nutrients-both organic and inorganic (with some energy from sunlight)

3 Where does primary production go? Grazing Bacteria Dissolved organic matter Export The vast majority of marine primary production is respired during microorganism growth.

4 Quantifying fluxes of carbon/nutrients through bacteria, requires knowledge of bacterial growth and respiration PP CO 2, NH 4+,PO 4 3- DOM Bacterial Biomass Sink BP ~10-30% PP Higher trophic levels Link

5 Energy via oxidation of organic matter Glycolysis, the citric acid cycle (TCA cycle), and electron transport together generate ATP and NADPH via the oxidation of reduced carbon. End products are CO 2 and H 2 O. C 6 H 12 O 6 +6O 2 6CO 2 + 6H 2 O + heat 36 ATP produced in the complete oxidation of one glucose molecule.

6 Most aquatic respiration is microbial Williams (1984) Note that % of plankton respiration in this example passes through a 10 µm filter; ~60% of the total respiration is by organisms <1 µm.

7 Energetic costs of growth DOM Uptake Excretion Catabolism Anabolism b a ATP c µ e d Product (CO 2 ) Biomass del Giorgio and Cole (2001) a. Oxidation of organic matter to form ATP, b. energy expense of active transport, c. anabolic reactions utilize energy, d. maintenance energy expenditures, e. degradation of biomass via endogenous metabolism.

8 Major energy consuming processes for growth Solute transport Synthesis of macromolecules Maintenance Growth and reproduction

9 The total amount of carbon required for growth includes that used for creating new biomass (production) and carbon that gets metabolized for energy (respiration). Total flux of carbon supporting growth: Carbon demand = Production + Respiration

10 Bacterial growth efficiency (BGE) is the term used to describe the amount of biomass produced relative to total carbon required for growth. = BGE = BP / (BP + Respiration) Usually reported as % The efficiency of cell growth determines the flux of carbon through bacteria in aquatic food webs. Variability in growth efficiencies (in space or time) could have large influences on CO 2 and O 2 fluxes, biomass production, organic matter consumption.

11 Methods of determining BGE Bacterial uptake and respiration of model DOM substrates ( 14 C-labeled DOC). Estimate increases in bacterial biomass relative to utilization of ambient DOC. Measure bacterial production in conjunction with respiration (changes in O 2 and/or CO 2 over time).

12 Examples of bacterial growth efficiencies based on uptake of model dissolved substrates Region Substrate BGE (%) N.E. Atlantic, Mediterranean Sea North Sea Amino Acids Glucose Amino acids Glucose Coastal California Amino acids 60-90

13 Link vs Sink 1. Measure growth efficiency. 2. Add 14 C DOM (glucose) and follow its transfer through the food web (into larger size fractions). In this example, 14 C-glucose was added to seawater and passage of this DOC through the food web was monitored over time. Conclusion: very little of the DOC channeled into biomass passes to higher trophic levels the vast majority is respired. Ducklow et al. (1986) Bacteria appeared to be a SINK for carbon in this example.

14 Measuring bacterial growth efficiency on naturally occurring DOC Evaluating BGE based on changes in CO 2, DOC, and cell biomass. This approach requires eliminating the sources of DOM production in order to determine the net change over time. BGE = BB / BB + BR or BGE = BB / DOC Carlson et al. (1999)

15 Estimates of BGE in various ocean ecosystems Ecosystem BGE (%) Sargasso Sea 4-9 Coastal /Shelf waters 8-69 Central North Pacific 1-33 North Atlantic 4-6

16 Factors influencing BGE DOM composition (energetic and nutritional value) Temperature Growth rate

17 The source of the DOM influences BGE Growth efficiencies vary widely depending on the DOC source. DOC exudates from actively growing cells appears to support higher growth yields (40-70%) than growth on cellular constituents. The source of the DOC partly determines its suitability as a growth substrate. del Giorgio and Cole (1999)

18 Bacterial growth efficiencies across aquatic ecosystems BGE in the open ocean ~10-30%. Growth efficiency tends to increase along a productivity gradient from oligotrophic to eutrophic environments. del Giorgio and Cole (1999)

19 BCD relative to PP (%) or DOM uptake/primary production (%) BGE BP/PP % 150% 200% % 100 % 133% % 50% 67% Current estimates: BP/PP= 0.10 and BGE= 0.15

20 Motivation Bacterial growth regulates fluxes of carbon and nutrients, and dictates energy flow in marine ecosystems. DOM (C, N, P, Fe, O, S, Zn, etc., etc.) DOM uptake to support µ Bacterial Biomass 10-30% of PP into BP BGE Higher trophic levels CO 2, NH 4+,PO 4 3- Link Sink 30-50%

21 Net Community Production (NCP): Gross primary production less all autotrophic and heterotrophic losses due to respiration (R A+H ). NCP = P G R A+H The ocean appears to be net heterotrophic, i.e. NCP is negative The ocean should soon have no free oxygen Massive carbon shortfall (upwards of 1.3 x mol C yr -1 )

community production (photosynthesis and community respiration).

22 Productivity and respiration by changes in O 2 Measures changes in oxygen concentrations in light and dark bottles following incubation Light bottle = net community production (photosynthesis and community respiration). Dark bottle: community respiration. Light + Dark = Gross primary production GPP = O 2 (light) - O 2 (dark)

23 1 : 1 line

24 Duarte et al. (2001) Net heterotrophy throughout the upper ocean

25 Serret et al. (2001)

26 The structure of the food web matters! Classic Food web Phytoplankton Inorganic Nutrients The more linksthe less efficient Heterotrophic bacteria Herbivores Dissolved organic matter Protozoa Higher trophic levels (zooplankton, fish, etc.) Mesozooplankton Microzooplankton

30 Mixed layer O 2 is in equilibrium with the atmosphere Rate of subsurface O 2 accumulation provides information on NCP Riser and Johnson (2008) But remember this study? Net autotrophic not heterotrophic

32 Organic matter production ~60% Dissolved Organic Matter Particulate Organic Matter Organic matter export from the upper ocean implies that production exceeds respiration. Heterotrophic Bacterial Growth DOM does not sink, but can be physically transported Grazing ~1-10% of net organic matter production is exported to deep sea Export

33 What processes control DIC gradients in the sea? Abiotic and biotic processes control carbon distributions in the sea. Need to understand controls on the variability of these processes

34 Total organic carbon at BATS Remember that DOC = ~98% of the TOC. Note the build up in DOC through the spring and summer, with subsequent export the following winter. Figure courtesy of Craig Carlson, UCSB

35 Figure courtesy of Craig Carlson, UCSB

36 DOC has been characterized based on the turnover times of the various constituent pools. Labile pools cycle over time scales of hours to days. Semi-labile pools persist for weeks to months. Refractory material cycles over on time scales ranging from decadal to multidecadal perhaps longer

Examine annual or seasonal scale changes in

Primary production approach 5: Estimate Net community production based on in situ variations in oxygen, nutrients, carbon, or biomass (often chlorophyll) Examine annual or seasonal scale changes in O 2,