Primary producers. Primary consumers (herbivores) Secondary consumers (carnivores) Tertiary consumers

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1 Primary producers Primary consumers (herbivores) Secondary consumers (carnivores) Tertiary consumers The way biomass is distributed among trophic levels in the food web provides clues to the efficiency of energy transfer through the ecosystem. Note: this is a static depiction-it does not provide information on how fast biomass turns over within each trophic level.

2 Commonly used determinations of bacterial biomass (mg C L -1 ) Cell abundance and cell volume (by microscopy and more recently by flow cytometry)

3 Typical bacterial cell densities in aquatic ecosystems Habitat Cell density (cells ml -1 ) Estuaries >5 x 10 6 Coastal (near shore) 1-5 x 10 6 Open Ocean x 10 6 Deep Sea <0.01 x 10 6

4 Remember that cell abundance carbon biomass Size matters Fagerbakke et al. (1996)

5 Determinations of cell volume Need to measure cells per sample for statistical precision. Assumes spherical cells No standards; calibrated with microspheres that likely have different fluorescence and shape characteristics than bacterial cells Image analyses is highly dependent on edge determination

6 Depth (m) Bacterial cell volumes by depth in several ocean ecosystems Polar Front Ross Sea EQ-PAC (March) EQ-PAC (September) Cell Volume (µm 3 cell -1 ) 0.08 Location Escherichia coli Vibrio natriegens Sargasso Sea Cell Volume (µm 3 ) Study Watson et al. (1977) Fagerbakke et al. (1996) Carlson et al. (1996) North Sea Fagerbakke et al. (1996) Ross Sea, Antarctica Sargasso Sea Ducklow et al. (2001) Gundersen et al. (2002) Cell abundance (cells ml -1 ) x Cell volume (µm 3 cell -1 ) = Biovolume (µm 3 ml -1 )

7 Why are oceanic bacteria so small? Inactive or dormant low growth rates? High S/V advantageous for growth in limiting nutrient media? Predation defense?

8 Consider a spherical microbe: SA= 4πr 2 V= 4/3 πr 3 r = 0.50 µm SA = 3.1 µm 2 V = 0.52 µm 3 SA : V = 15.7 r = 1.0 SA = 12.6 µm 2 V = 4.2 µm 3 SA : V = 3.0 The smaller the cell, the larger the SA : V Greater SA : V increases number of transport sites (per unit biomass), and may allow smaller cells to out compete larger cells under limiting nutrient conditions.

9 To get from abundance and volume to biomass requires conversion factors Location Density C content Method Study (fg C µm -3 ) (fg C cell -1 ) E. coli CHN analyses Watson et al. (1977) E. coli, P. putida, B. subtilius CHN analyses Bratbak et al. (1985) Natural plankton Station ALOHA Southern Ocean CHN Lee and Fuhrman (1987) Biomass constraint 12 Direct Measure by TOC Christian and Karl (1994) Fukuda et al. (1998) Most studies converge ~10 fg C cell -1 Ross Sea, Antarctica Sargasso Sea C mass balance X-Ray micro. Carlson et al. (1999) Gunderson et al. (2002)

10 45 o N 25 o N 5 o S Buck et al. (1996)

11 Generalized relationships between bacterial population size and chlorophyll Slope = Slope = 0.52 Bird and Kalff (1984) Cole et al. (1988) Relationships between bacterial abundance and Chl a across diverse aquatic ecosystems. 1) At low concentrations of Chl a, bacterial abundance becomes increasingly important. 2) Along gradients in productivity, increases in Chl are proportionality greater than bacterial abundance.

12 Bacterial and phytoplankton biomass in the upper ocean in various ecosystems Location Bacterial Biomass (mg C m -2 ) Phytoplankton Biomass (mg C m -2 ) Bact:Phyto Sargasso Sea North Atlantic Subarctic North Pacific Station ALOHA Arabian Sea Average Stand dev. CV (%) % % % Bacterial biomass constitutes a large pool of living carbon in marine ecosystems. Note greater variation between ecosystems in phytoplankton biomass relative to bacterial biomass. Bacterial biomass calculated assuming 10 fg C cell -1 ; phytoplankton biomass calculated assuming C:Chl of 50:1.

13 Plankton biomass relationships Over wide range of aquatic ecosystems, bacterial biomass appears correlated with phytoplankton biomass. Bacterial biomass is generally less variable than phytoplankton biomass across large gradients in productivity. With increasing oligotrophy, bacterial biomass becomes a proportionality larger fraction of total plankton biomass.

14 An inverted food web in low productivity ecosystems? Primary producers Primary consumers (Bacteria) Secondary consumers (microzooplankton) Tertiary consumers (mesozooplankton) Total carbon consumed by bacteria = (biomass x turnover rate) growth efficiency We need information on turnover and efficiency

15 To determine the importance of microbes to ocean food webs we need to: 1. Quantify population size and mass Biomass, abundance, cell sizes 2. Quantify growth rates and production Biomass production, respiration, cell division, and turnover 3. Understand factors limiting microbial growth Metabolic flexibility, physiology

16 What is it we want to know? Biomass (biogenic carbon, trophic linkages) Carbon fluxes (production, respiration, DOC utilization rates) Rates of growth (cell physiology, nutritional status)

17 The Microbial Loop : A central theme in marine microplankton ecology Classic Food web Phytoplankton Inorganic Nutrients A simplified depiction of the microbial loop Heterotrophic bacteria Herbivores Dissolved organic matter Higher trophic levels (zooplankton, fish, etc.) Protozoa

18 Growth in a closed system Closed system; variable growth rate cells are inoculated into media and grow until resources are depleted (logistic growth model).

19 Cell abundance (cells ml -1 ) Calculating growth rates from a population dividing exponentially Exponential growth phase Time (hours or days) ln cell abundance µ= d -1 d = 1 day Time (hours or days) T (hours or days) N t (cells or biomass) From an exponentially growing population the specific growth rate (µ) can be derived from: dn/dt = µn N t = N o e µt or alternatively: µ =(ln N t -ln N o ) / t µ has units of time -1 Doubling time (d) is the time required for the population to increase by 100%; it is related to µ by: N t = N o e µd N t /N o = e µd = 2 d = ln 2/µ d has units of days or hours.

20 Growth in a chemostat Seawater Media Air Inlet Sampling port Open system: constant supply of limiting nutrients; growth rate is determined by the rate that a limiting nutrient is added or removed from the system. Typically use an exponential model to model growth dynamics. Overflow Growth chamber Is the ocean more like a batch culture or a chemostat?

21 Measuring the rate of growth by natural assemblages of plankton is complicated Most direct method would be to measure changes in biomass over time. Why is that difficult for naturally occuring plankton?

22 In natural seawater sample. Mixed assemblage of microbes with variable growth rates. Growth is often balanced by loss (predation or disease), this no net change in biomass with time.

23 Biomass Production Production is defined as the rate that new biomass is synthesized. Production is mathematically related to biomass and growth as: P = µb µ = specific growth rate (time -1 ) B = biomass (mg C L -1 ) **Note that µ = P/B Thus, P has units of mg C L -1 d -1

24 Two main forms of biomass production: Primary production: is the rate of biomass synthesis via reduction of CO 2 ; in the ocean mostly controlled by the growth and biomass of photosynthetic organisms. Secondary production: formation of biomass via assimilation of organic matter; controlled by growth and biomass of chemoheterotrophs (heterotrophic bacteria, zooplankton, etc.)

25 Photosynthesis Absorption of light energy by pigments or photoproteins (light antenna). Energy excites e- in the antenna and this energy is passed to the photosynthetic reaction centers via the flow of electrons. This process creates reducing power (NADPH) and chemical energy (ATP). Energy and reducing power gained from light harvesting are used to reduce CO 2 to organic matter (dark reactions).

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27 Photosynthesis CO 2 ATP, NADPH H 2 O O 2 Sunlight CO 2 + 2H 2 O CH 2 O + O 2 + H 2 O + heat 3 mol ATP and 2 mol NADPH are consumed for every 1 mol CO 2 fixed.

28 Inorganic carbon pools in the sea: CO 2 : carbon dioxide H 2 CO 3 : carbonic acid HCO 3- : bicarbonate CO 3 2- : carbonate CO 2 H 2 CO 3 HCO 3- CO % 89% 10.5% Proportion of carbon species in seawater

29 2 Key enzymes for phytoplankton photosynthesis RUBISCO (1,5-bisphosphate carboxylase/oxygenase)-key enzyme in the Calvin-Benson cycle, incorporates CO 2 into 3- phosphoglycerate. Most abundant protein on Earth. Carbonic anhydrase: converts bicarbonate to CO 2, and vice versa. Most marine phytoplankton transport bicarbonate and carbonic anhydrase dehydrates to CO 2 intracellularly near RUBISCO.

30 Net and Gross Photosynthesis Sunlight CO 2 + 2H 2 O CH 2 O + O 2 + H 2 O + heat Net photosynthesis (P N ): net organic carbon production in the light. Rate of photosynthesis excluding material lost to respiration in the light (R L ). P N = P G -R L Gross photosynthesis (P G ): The total amount of light energy converted into biochemical energy. P G = P N + R L Relatively straightforward to measure net or gross photosynthesis in pure cultures, but the oceans contain many organisms that contribute to respiration (e.g., bacteria, zooplankton).

31 Primary Production Gross Primary Production: The rate of organic carbon production via the reduction of CO 2 ; in the ocean predominately due to photosynthesis. Net Primary Production: Gross primary production, less photoautotrophic respiration (R A ), integrated over time and depth. NPP = P N R A Net Community Production: Gross primary production, less all autotrophic and heterotrophic losses due to respiration (R A+H ). NCP = P G R A+H Units for all measures of production: mg C m -2 d -1

32 Trapezoidal integration 0 50 Depth (m) Production (µg C L -1 d -1 ) Depth (m) m Int. 363 mg C m -2 d C-assimialtion rate (µg C L -1 d -1 ) Area of trapezoid = Height * avg. base [(25 m -5 m) * (6.5 mg C m -3 d mg C m -3 d -1 )/2] = 129 mg C m -2 d -1 [(45 m -25 m) * (6.4 mg C m -3 d mg C m -3 d -1 )/2] = 114 mg C m -2 d -1 [(75 m -45 m) * (5.0 mg C m -3 d mg C m -3 d -1 )/2] = 120 mg C m -2 d -1 Sum 5-75 m = 363 mg C m -2 d -1

33 Primary productivity in the sea 50 Net Photosynthesis Depth (m) Compensation depth (P cell = R cell ) Respiration Z 0 1/p dp/dt >0 Critical depth (P water = R water ) Z 0 1/p dp/dt <0

34 The Spring Bloom Sverdrup (1953) Critical depth Mixed layer Winter mixing introduces nutrients to the upper ocean; seasonal increases in irradiance results in deepening of the critical depth and shoaling of the mixed layer. The result: net accumulation of biomass.

35 Phytoplankton biomass can only accumulate (production exceeds respiration) when the depth of the mixed layer shoals above the critical depth.