Light extinction. Photosynthesis. Algal growth

Transcription

1 Light extinction Photosynthesis Algal growth

2 Light extinction Light is the main energy source for ecosystems and has a key role in ecological processes: photosynthesis, transpiration, evapotranspiration, Hence, the quantification of the energy reaching the Earth s surface and living organisms is important.

3 Depth (m) Physical processes light extinction Lambert-Beer s (Bouguer s) law: most famous model I: intensity or irradiance W m -2 di = - k I dx Modello: I(z) = I(0)e -kz PAR(prof) = *e -(4.50*prof) R 2 = 0.97 HP: Homogeneous medium I out (z) = I in (0) e k z works well for low concentration media. K depends on medium, direction, wavelength

4 Physical processes light extinction Lambert-Beer s law (or Bouguer s law) I out (z) = I in (0) e k z Iout / Iin : trasmittance Several applications (e.g. spectrophotometers) k*z = optical depth (a measure of the ability of the layer to block light) Hetereogeneous medium: apply relationship separately to layers using different k s

5 Light extinction in the atmosphere Not all solar radiation approaching earth s surface actually reaches the ground. Exctinction = scattering + absorption Absorption photons hit atmospheric gases (O2, O3,N2, H2O, CO2) and aerosols (natural and anthropogenic): energy transformed into heat or radiated. Scattering photons are deviated by gases / aerosol without energy loss, diffusion Rayleigh: particles with d<1/10 wavelength (N2 e O2 for visible radiation) Mie: particles with d up to 10*wavelength

6 Light extinction in the atmosphere I out = I in e k m L m = 1 / cosφ optical air mass (relative length ) to take into account the sun is not at zenith (φ=0) k = k scat gas +k scat aerosol +k abs gas +k abs aerosol In inhabited regions, usually ksuspended particles >> kgas Empirical relationships for φ >60 (refraction, non-uniform T, clouds and other substances, Earth s curvature, air density)

7 Light extinction in water Too few (or too much) light can limit primary production (phytoplankton, macroalgae, etc.) Scattering and absorption due to water, suspended and dissolved substances Also the other way: primary producers can influence extinction (phytoplankton shading and self shading). Feedback!

8 Light used in photosynthesis mainly in the visible range ( nm) - PAR measured as: W m -2 Light extinction in water PPDF (photosynthetic photon flux density, number of incident photons in the visible range per unit time per unit area, i.e. μmol m -2 s -1 ) Euphotic (photic) zone: RPP = photosynthesis (1% PAR; zone where photosynthesis takes place, etc.), from few cm to hundreds of meters

9 Light extinction in water k depends on wavelength; can be determined using photometers at several z, Iin=56% (or less, depends on wavelength) of I incident on the water surface After few meters, light becomes monochromatic (green) k can be computed in several ways k = kw & diss + kpart kpart =a*[cpart] When phyto has the biggest influence (i.e. eutrophic lakes, shading, self shading): k = b + c*[a] k = b + c*[a] + d*[a] e In oligotrophic ecosystems b (background turbidity) dominates

10 Light exctintion in water k k 0 a1a k a 1 A btss k k0 a1a a2 k b TSS b A k cost SD

11 Depth [m] Estinzione della luce in un mezzo secodo la The legge role di of Lambert-Beer K 0 Transmittance 0 0,2 0,4 0,6 0, k = 0,2 k = 0,

12 Physical processes light extinction January 2002 Worst, but predictive February 2002

13 Light extinction in terrestrial ecosystems The canopy of trees can attenuate light (i.e. tropical forests) Influence underlying vegetation / undergrowth / animals / nutrient cycling etc. Agriculture, crop spacing, water balance for irrigation

14 Light exctinction in terrestrial ecosystems Incoming light (direct or diffuse) Reflected light Through gaps Absorbed light Transmitted light Complex, multi-scale problem: few general models Modified from Barausse, A. Light extinction. Chapter to be included in S.E.Jorgensen (ed.), Encyclopedia of Ecology, Elsevier, Amsterdam. Accepted.

15 Light exctinction in terrestrial ecosystems Effects of vegetation I z (LAI) = I in e k LAI(z) Image from LAI(z) is the cumulated Leaf Area Index from the top of the canopy to depth z. The leaf area index is the ratio of the total one sided green leaf surface to the surface of the ground underneath the canopy (or the projected needle area per unit of ground surface). Iin: incoming PPDF on the top of the canopy (where z=0) k connected to the average leaf orientation, tree species, Often, hypotheses of Lambert Beer s law are not realistic (homogeneous and isotropic medium?) Other models, even complex (3D vegetation models) and field measurements (e.g. hemispherical photography)

16 Photosynthesis 6 CO2 + 6 H20 + hν C6H12O6 + 6 O2

17 Photosynthesis 6 CO2 + 6 H20 + hν C6H12O6 + 6 O2 Limiting factors: Empirical model PHOTO = k f (max demand for limiting factors)

18 Phosynthesis 6 CO2 + 6 H20 + hν C6H12O6 + 6 O2 Limiting factors: Empirical model PHOTO = k f (max demand for limiting factors) 1. Light (optimum irradiance: energy is needed but not too much; Chlorophyll absorbance spectrum nm and depending on species, bacteria bacteriochlorophyll also longer wavelength)

19 Photosynthesis 6 CO2 + 6 H20 + hν C6H12O6 + 6 O2 Limiting factors: Empirical model PHOTO = k f (max demand for limiting factors) 1. Light 2. Inorganic carbon

20 Photosynthesis 6 CO2 + 6 H20 + hν C6H12O6 + 6 O2 Limiting factors: Empirical model PHOTO = k f (max demand for limiting factors) 1. Light 2. Inorganic carbon 3. Water

21 Photosynthesis 6 CO2 + 6 H20 + hν C6H12O6 + 6 O2 Limiting factors: Empirical model PHOTO = k f (max demand for limiting factors) 1. Light 2. Inorganic carbon 3. Water 4. Temperature (optimum)

22 Photosynthesis 6 CO2 + 6 H20 + hν C6H12O6 + 6 O2 Limiting factors: Empirical model PHOTO = k f (max demand for limiting factors) 1. Light 2. Inorganic carbon 3. Water 4. Temperature (optimum) 5. Other: plant characteristics and state (e.g. LAI, reproductive state), nutrients (N in chlorophyll)

23 Net photosynthesis Photosynthesis 6 CO2 + 6 H20 + hν C6H12O6 + 6 O2 Acclimation: environmental fluctuations (light, T, humidity, nutrients (P, N, Si) ) light acclimated 0 shade acclimated irradiance Photosynthesis at saturation level depends on species, T, ph, etc.

24 Photosynthesis Not only chlorophyll photosynthesis: some autotroph bacteria with bacteriochlorophyll pigments 6 CO H2S C6H12O S + 6 H2O Electron donor: hydrogen sulfide instead of water. Reaction takes place only without oxygen (toxic), no oxygen production Also some cyanobacteria (chlorophyll) can perform this reaction

25 Chlorophyll photosynthesis - cyanobacteria (blue-green algae, Cyanophyta, etc) Unicellular prokaryote, can form colonies. They can: reduce N (fix N2 into NH3, often symbionts) reduce S (only some) reduce C (chlorophyll photosynthesis) Atmospheric O2 and cyanobacteria Photosynthesis in cytoplasm, not in specialized organs (chloroplasts, probably derived from cyanobacteria embedded as endosymbionts)

26 Photosynthesis 6 CO2 + 6 H20 + hν C6H12O6 + 6 O2 Photosynthesis (respiration) vs net photosynthesis In chlorophyll photosynthesis there are two separate reactions Light dependent: light energy to create high energy molecules, O2 released Light independent: CO2 transformed into organic compounds using such molecule energy

27 Biological processes photosynthesis PAR is about 56% of total incident radiation. In water, Lambert Beer s law: I h I e 0 dove I 0 : light intensity on surface [W/m 2 ] =0,56 : light exctinction coefficient [1/m] h: depth [m]

28 Biological processes photosynthesis P P MAX I I k I 1 I k 2 where P: photosynthesis rate [mgo 2 /g/h] I k : light adaptation parameter Saturation P also depends on T (enzyme kinetics) and on ph (carbonate equilibrium) P 20 P T P MAX ph P 6.5 MAX 1 1e e ( T 20) ( ph 6.5) 2

29 Biological processes algal growth

30 Plankton Image source:

31 Image source:

32 Biological processes algal growth da dt r es m sa G where A: weight concentration of algae [mgx/l] : growth rate [1/d] r: respiration rate [1/d] es: exudation rate [1/d] m: non-predatory mortality rate [1/d] s: sedimentation rate [1/d] G: grazing [mgx/l/d] da dt m sa G where A: abundance (i.e. number) concentration [cell/l]

33 Biological processes algal growth Respiration and exudation: r r T ref f T can also depend on the physiological state of the cell: r T r T k T f L, N, P, C, Si,... ref min ref r ref

34 Biological processes algal growth Non-predatory mortality: m mt ref f T Some authors point out to a carrying capacity-like effect: m T k T A ref m ref Settling: s vs h

35 Biological processes algal growth MAX f T f Lf N, P, C, Si,... T ref f f N, P, C, Si,... f N f P f C f Si... N, P, C, Si,... min f N, f P, f C, f Si f N, P, C, Si,...,... f 1 1 n 1 N f P f C f Si 1... f N, P, C, Si,... f N f P f C f Si n...

36 Biological processes algal growth Temperature limitation Linear: f T T T T T ref min min Exponential: f T T T ref Optimum: f T e T T 2.3 Tx T opt opt 2

37 Biological processes algal growth max (1/day) Temperature C

38 Biological processes algal growth