Aquatic respiration and ocean metabolism • Remember what life is all about: • Energy (ATP) • Reducing power (NADPH) • Nutrients (C, N, P, S, Fe, etc., etc.)
Photosynthetic organisms use sunlight,
H2O, and dissolved nutrients
Heterotrophic microorganisms rely mostly on dissolved nutrients-both organic and inorganic (with some energy from sunlight) • Where does primary production go? –Grazing –Bacteria – Dissolved organic matter –Export
The vast majority of marine primary production is respired during microorganism growth. Quantifying fluxes of carbon/nutrients through bacteria, requires knowledge of bacterial growth and respiration
+ 3- PP CO2, NH4 ,PO4 DOM Bacterial Biomass
Sink
BP ~10-30% PP
Higher trophic levels Link 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 CO2 and H2O.
C6H12O6 +6O2 → 6CO2 + 6H2O + heat
36 ATP produced in the complete oxidation of one glucose molecule. Most aquatic respiration is microbial
Note that 50-100% of plankton respiration in this example passes through a 10 µm filter; ~60% of the total respiration is by organisms <1 µm. Williams (1984) Energetic costs of growth
DOM Excretion Uptake
Catabolism Anabolism
b
a c ATP µ d e Product Biomass (CO2)
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. Major energy consuming processes for growth
• Solute transport
• Synthesis of macromolecules
• Maintenance
• Growth and reproduction •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 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 CO2 and O2 fluxes, biomass production, organic matter consumption. Methods of determining BGE
• Bacterial uptake and respiration of “model” DOM substrates (14C-labeled DOC).
• Estimate increases in bacterial biomass relative to utilization of ambient DOC.
• Measure bacterial production in conjunction
with respiration (changes in O2 and/or CO2 over time). Examples of bacterial growth efficiencies based on uptake of model dissolved substrates
Region Substrate BGE (%) N.E. Atlantic, Amino Acids 66-87 Mediterranean Sea Glucose 51-76 North Sea Amino acids 16-50 Glucose 20-50 Coastal California Amino acids 60-90 Link vs Sink 1. Measure growth efficiency. 2. Add 14C DOM (glucose) and follow its transfer through the food web (into larger size fractions).
In this example, 14C-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. Bacteria appeared to be a SINK
Ducklow et al. (1986) for carbon in this example. Measuring bacterial growth efficiency on naturally occurring DOC
Evaluating BGE based on changes in
CO2, 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) 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 Factors influencing BGE
• DOM composition (energetic and nutritional value)
• Temperature
• Growth rate 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) 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) BCD relative to PP (%) or DOM uptake/Primary production (%)
BGE BP/PP 0.50 0.20 0.15 0.30 60% 150% 200% 0.20 40% 100 % 133% 0.10 20% 50% 67%
Current estimates: BP/PP= 0.10 and BGE= 0.15 Motivation Bacterial growth regulates fluxes of carbon and nutrients, and dictates energy flow in marine ecosystems.
+ 3- CO2, NH4 ,PO4 DOM (C, N, P, Fe, O, S, Zn, etc., etc.) Bacterial Biomass Sink 30-50% DOM uptake to support µ BGE 10-30% of PP into BP
Higher trophic levels Link Net Community Production (NCP): Gross primary production less all autotrophic and heterotrophic losses due to respiration
(RA+H). NCP = PG –RA+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 1015 mol C yr-1) Productivity and respiration by
changes in O2
•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 = ∆O2 (light) - ∆O2(dark) 1 : 1 line • Net heterotrophy throughout the upper ocean
Duarte et al. (2001) Serret et al. (2001) The structure of the food web matters!
Classic Food web The more links- the less Inorganic efficient… Phytoplankton Nutrients
Heterotrophic bacteria
Dissolved Herbivores organic matter Protozoa
Higher trophic levels (zooplankton, fish, etc.) Mesozooplankton Microzooplankton
Mixed layer O2 is in equilibrium with the atmosphere
Rate of subsurface
O2 accumulation provides information on NCP
Riser and Johnson (2008)
But remember this study? Net autotrophic not heterotrophic…
Organic matter production
% 0 Dissolved Particulate 6 ~ Organic Organic Matter Matter
Heterotrophic Bacterial Grazing Growth ~1-10% of net Organic matter export organic from the upper ocean DOM does not matter sink, but can be production is physically exported to implies that production deep sea transported exceeds respiration. Export 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 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 Figure courtesy of Craig Carlson, UCSB 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 multi- decadal…perhaps longer…