Light in the Ocean

1. Light in the Ocean .. and its influence on photochemistry

2. Light travels faster than sound. This is why some people appear bright - until you hear them speak. Disclaimer – quote probably not attributed to Einstein

3. Light is a form of electromagnetic radiation – with wave-like properties wavelength (λ) = C/ ν where λ is the wavelength in meters, C is the speed of light in a vacuum (3 x 108 m s-1) and ν is the wave frequency (# of wave crests (cycles) per second) Electromagnetic energy travels in distinct packets called photons. The energy in each photon is given by: E = h where h is Planck’s constant (6.63 x 10-34 J s) and ν is the frequency (s-1). Since ν = C/ λ we can relate energy to wavelength: E = hC/ λ Thus, the energy in a photon is inversely proportional to the wavelength (longer λ = less energy; shorter λ = more energy)

4. Units used during measurements of light Einstein = 6.02 x 1023 photons i.e. 1 mole of photons It is convenient to work with Ein since photochemistry is a quantum process (if the quantum yield is 1, then one mole of photons causes 1 mole of molecules to react) Light photon flux – often given as μEin cm-2 s-1. Photosynthetically active radiation (PAR) is often given in these units; A bright summer day has a solar photon flux of about 2500 μEin cm-2 s-1 (= 2500 μmol photons cm-2 s-1) The rate of light Energy delivery is often given in Watts 1 Watt (W) = 1 Joule second-1 Light energy flux is given in W m-2 Wavelength-specific energy is often specified. That is, the energy at a certain λ (or from a range of λ i.e. PAR (400-700 nm))

5. Energy per photon at specific wavelengths UV-R Visible InfraRed Photosynthetically-Active Radiation (PAR) is ~400-700 nM 400 nm Short wavelength light (i.e. UV) has higher energy per photon!

6. No UV-C reaches Earth’s surface UV-R Visible light Most incident solar energy is in the visible band! From Whitehead et al, 2000

7. Light penetration into the ocean Light energy is absorbed in seawater such that total light energy (irradiance) at a given wavelength decreases exponentially with depth into the water Total light energy Iz = irradiance at depth z Io = irradiance at surface Kd = attenuation coefficient (m-1) Depth (z) Kd is the fraction absorbed per meter There will be a different Kd for each wavelength! Iz = Io e-Kd z What happens to this light energy? Absorbed Energy

8. Iz/Io (fraction of surface irradiance) Large Kd rapid extinction & shallow penetration λ1 λ2 Small Kd - slow extinction with depth & deep penetration λ3 Depth Light absorption (i.e. Kd,λ) will be affected by several factors – more later….

9. Within the visible bands, red wavelengths are absorbed rapidly with depth. Blue wavelengths generally penetrate the deepest. The penetration of visible light (PAR) depends on the characteristics of the water, including phytoplankton abundance.

10. Spectral irradiance at depth in the ocean is measured by spectral radiometers

11. Spectral radiometer data for optically-clear water from the central Gulf of Mexico Low UV wavelengths are attenuated rapidly with depth. Greater than 90% of UV-B (< 320 nm) is absorbed above 15-20 m. Total energy in 400-700 nm band The 1% PAR depth at this site was ~120 m

12. UV absorption properties vary among water masses 305 This is equal to the 10% irradiance depth From Whitehead et al, 2000 1% light depth for any wavelength is given by 4.6/Kd That is Iz/Io = e-Kz or ln(Iz/Io) = -Kdz ln(0.01) = -Kdz or 4.6/Kd= z1%

13. DOM is the main chromophore (absorber of light) in the ocean. More correctly, it is specific constituents of the DOM that are the chromophores. Together these organic chromophores constitute the Colored Dissolved Organic Matter (CDOM) (also called chromophoric DOM) CDOM (at high concentration) can give water a yellow color (Gelbstoffe) and a high optical absorbance, particularly in the UV part of the spectrum. Tea colored, black-water rivers are very high in CDOM!

14. Total 0.8 DOM 0.6 Absorption coefficient (m-1) 0.4 0.2 particles Water 0.0 300 500 550 350 400 450 Wavelength (nm) Light absorption by seawater is mainly by DOM! Absorption spectra for whole water, and the DOM, particulate matter, and pure water fractions for a coastal seawater sample from the mid Atlantic Bight. From DeGrandpre et al, 1996

15. Differences in light absorption/attenuation in different water masses is governed mainly by: • Particles – organic and inorganic • DOM - quantity and quality Influenced by primary production and proximity to rivers and sediments River water The optical absorbance of water is usually directly related to the DOC (and DOM) concentration – but this relationship varies from one water mass to another. Abs350 nm Shelf water Ocean water With exposure to UV-R, CDOM becomes bleached and it losses its absorbance, thereby changing the A350 vs. DOC relationship DOC conc.

16. Seawater CDOM absorption coefficient for 370 nm light as a function of Chl a concentration in those same waters Chlorophyll a (mg m-3) This study found that seawater absorption coefficients in these hyper-oligotrophic waters were lower than published values for pure water!

17. Photochemistry

18. Photochemistry affects: • Photosynthesis & Bacterial growth (photobiology) • Biological reactivity of DOM (both increasing and decreasing its lability) • Molecular weight distribution of DOM • Mineralization (loss) of DOM • Production of CO2(aq) from DOM • Metal cycling and availability – via photoreduction etc. • Pollutant degradation

19. When photon (light) energy is absorbed by molecules, a variety of things can happen. • Electrons transiently jump to higher orbitals, then spontaneously fall to their original position (times scales of nanoseconds). This results in fluorescence with emission being longer than the  of the photon absorbed (the excitation photon) • Molecule becomes “excited” and more reactive A --> A* • Molecule becomes oxidized (loses electron to a receptor) • Molecule becomes reduced (steals electron from a donor)

20. For a primary process, compound A absorbs light energy directly and is converted into terminal products: A  B + C hν For a secondary reaction, A absorbs light energy and becomes excited – but it then transfers the energy to a receiving molecule B, forming excited-state B*. This can go on and on … A  A* + B  B* + A’  chain reactions In this case, A functions as a photosensitizer - it absorbs light energy and then causes something else to react. hν Where A* and B* are excited state species Primary vs. Secondary Photochemistry Example: DMS does not absorb light directly so no primary photolysis. DMS oxidation in the light, occurs via a photosensitizer (e.g. DOM or NO3-).

21. Molecular oxygen is a major reactant in photochemical reactions (though it doesn’t absorb light directly). If O2 reacts with excited molecule, highly reactive singlet O2 (1O2) can form If a photoactive molecule absorbs a photon and donates e- to O2, it yields superoxide anion (O2-) a reduced form of oxygen that is highly reactive. Superoxide can be converted to hydrogen peroxide (H2O2), either chemically or enzymatically (superoxide dismutase does this). H2O2 is also a strong oxidant and reactive form of oxygen. H2O2 can undergo direct photolysis (with UV-R) or can react with Fe(II) to form OH radicals (OH), one of the most potent oxidants known. All these reactive oxygen species are formed in seawater via photochemical reactions!

22. OH radicals are about the most potent oxidants known! Polar seas have very high nitrate concentrations! This has implications for photochemisty & biology Concentrations in Antarctic surface waters Nitrate Nitrite 15-30 µM 0.1 – 0.2 µM Inorganic constituents in seawater are not generally photoreactive. Several notable exceptions include: Nitrate NO3- + H2O + light  NO2- + •OH + OH- Nitrite NO2- + H2O + light  NO + •OH + OH- The specific photo-reactivity (per mole) of nitrite is much greater than for nitrate 319 - 333 nm 325 - 380 nm Nitrate and nitrite can range from 1-15 µM in temperate waters, mainly in winter and spring. Even higher concentrations can be found in coastal waters & river plumes

23. Transition metals such as Fe, Mn & Cu have primary photochemistry Fe(III)  Fe(II) hν More labile and biologically-available Surface irradiance Depth integrated in water column Percent contribution of different wavelength bands of solar radiation to photoreduction of Fe(III) colloids in Antarctic waters UV-A (320-400 nm) >60% 55% Visible (400-700 nm) 30% 40% UV-B (290-320 nm) 3.5 – 6.5% 1.8 – 3.0% From Rijkenberg et al. 2005. GRL Photoreduction (direct & photosensitized) is important in maintaining metals in surface waters and keeping some of the metal pool available to phytoplankton

24. Mainly driven by UV-R • DOM (i.e. CDOM) is the main absorber of UV-R in seawater • UV-R absorption by CDOM causes alteration of DOM Influence of photochemistry on organic compounds CDOM Altered CDOM hν + photoproducts CO2 CO COS H2O2 Low molecular weight organic compounds e.g. formaldehyde, glyoxylate, etc. (i.e. labile to bacteria!) Photodegradation also bleaches CDOM, decreasing its absorption and its photoreactivity • After • Kieber • Mopper • Miller • Moran etc

25. Moran and Zepp, 1997, L&O

26. From Mopper and Kieber, 2000

27. Photooxidation as a major sink for refractory DOM in the sea Photochemical Blast Zone - some DOM oxidized NADW formation. Labile DOM is utilized in relatively short time - leaving old refractory carbon to make another circuit Upwelling of refractory, old DOM Deep water transit (= 1000 y) Little alteration of old, refractory carbon If ultra-refractory DOM has average age of 6000 years, and if ocean circulation time is 1000 y, then on average 16.7% of this old carbon will be lost each circulation cycle.

29. Optical Buoy - In situ incubations in natural light field Quartz tubes Incubated water experiences natural light field

34. Comparison of Photochemical and Biological DMS loss processes – Ross Sea Polynya, Terra Nova Bay - January 13, 2005 In situ irradiation Photolysis MLD Bio consumption from CTD samples Conditions: Shallow mixing, calm winds, cloudy am, sunny pm.

35. How are the biological processes affected by exposure depth (UV radiation)? In situ irradiation

36. Hypothetical UV-R penetration Deep Mixing 0 MLD = 50m 25 50 75 Deep mixing gives lower dosage to surface plankton - allows recovery/repair from UV damage t 100 Mixing Depth governs exposure of surface plankton to PAR and UV-R Mixing depth also affects surface nutrient regime and distribution of key phytoplankton e.g. N-fixers Shallow Mixing MLD = 25m UV Shade Depth (m) Shallow mixing results in higher UV dosage for surface plankton, with less recovery time t

37. Using Solar UV to disinfect drinking water National Geographic, April, 2010

38. Our changing atmosphere – stratospheric ozone depletion and the increase of UV-radiation at the Earth’s surface

39. Stratospheric ozone depletion is causing UV-R to going up everywhere on Earth • Highest total solar energy and highest total UV energy are at the equator. • Largest seasonal variability in UV-R occurs at high latitudes- like the Arctic and Antarctic.

40. The ozone hole over Antarctica enhances flux of UV-B at the sea surface Spectral shift in energy under ozone hole conditions Ozone in the atmosphere is measured in Dobson Units (DU). Data from Palmer Station, 1993 Marine organisms are very sensitive to UV-B radiation! Dave Kieber

41. How does UV-R affect biology (and hence chemistry) in the surface ocean? • Inhibition of photosynthesis • Inhibition of bacterial production and growth • Selection force for UV-resistant organisms, and those able to adapt by production of UV screens (i.e. Mycosporine amino acids) • Possible mutagen driving evolution? • Factor affecting viability of eggs and larvae of macroorganisms that reside in surface waters? • Possible synergism with pollutants (e.g. PAH’s)

42. End

44. Metals (e.g. Fe, Mn) held in organic complexes including • Humics • EDTA • Siderophores • are photolabile – resulting in photoreduction of metal and oxidation of the organic molecule. • This type of photosensitized metal photoreduction is more important than primary photo-reduction of the metals. Photoreduction (direct & photosensitized) is important in maintaining metals in surface waters and keeping some of the metal pool available to phytoplankton

45. The absorption of light is a quantum process The rate of a photochemical reaction of A is given by: Specific absorption of actinic radiation – light absorbed per unit volume of water per unit time Actinic = chemically-active radiation Quantum yield Φ = quantum yield = # of reactions / # of photons absorbed If the concentration of the actual chromophore is not known, the apparent quantum yield (ΦA ) is reported. The ΦA for seawater reactions is usually << 1 (few reactions per photon absorbed) Only when light is absorbed can photochemical reactions occur – if compounds are transparent to light, then no photo reactions occur

46. The quantum yield for marine photochemical reactions is often much less than 1. For example, the quantum yields of H2O2 photoproduction range from 0.00003 to 0.001. Quantum yields also tend to decrease at longer wavelengths (less energy per photon). Open symbols are seawater Figure from Moran and Zepp, 1997

47. reflection  Absorption 5-7 % backscatter K = aw + ap + ao+ ai + Sw + Sp Absorbance by inorganic molecules Total attenuation Absorbance by water molecules Absorbance by particles Absorbance by organics Scattering by water Absorption of light in the sea > 95% of light energy reaching the ocean surface enters the water if  > 20o. 5-7% of light that enters ocean is lost due to backscatter (water leaving radiance) About 50% of absorbed radiation is infrared which heats the water. Total attenuation of light in aquatic systems Scattering by particles Absorbance means light energy is absorbed by chemicals in the system. Molecules that absorb light are called chromophores

48. Practical considerations for UV-research Optical properties of various lab materials • Optical quartz – transparent to virtually all wavelengths • Borosilicate glass (e.g. Pyrex, Kimax, Duran) - (variable – can cut off < 340 nm) • Teflon (FEP) – Transparent to most UV- but some scattering • Polycarbonate (cuts off < 340 nm) • Whirlpak polyethylene bags - Transparent to UV-R – convenient to use, but must check for contamination • Acrylics – different optical properties depending on type (see next slide)

49. Selective filtration of light wavelengths UF-3 (or UV-O) acrylic – cuts off < 400 nm (i.e. all UV-R) Plexiglas-G – cuts off < 370 nm Mylar-D – cuts off < 315 nm (i.e. UV-B)

50. The ozone hole over Antarctica increased through 2005 – letting in more ultraviolet B radiation 2004 Total Ozone Mapping Satelite (TOMS) ftp://jwocky.gsfc.nasa.gov/pub/eptoms/images/spole/Y2004/