# Hydrological optics and light capture in the aquatic enviroment.

This essay shall discuss hydrological optics, how light behaves in water, and light capture in particular the package effect and how it influences the absorption spectra. The radiation we commonly call light lies between 400-700 nm and coincidently is the same wave length for both photosynthesis and human sight, this gives us an easily comparable spectrum to deal with as I may compare to both colour of the light (ie. Red or blue) or the wavelength. Light is greatly influenced by the medium in which it travels through so the behaviour in water is significantly different to the behaviour in the atmosphere however we must consider the basic behaviours of light in the atmosphere before we approach such topics as absorption and scattering.

Photons travel at a speed of 3*10ms[-1] and on a summer day in direct sunlight 1m will receive 10quanta s[-1]. Although light often behaves as a particle it also as wave like properties, the wavelength of light can be found by c/v or the speed of light divided by the frequency.

The energy of light is not constant but changes with the frequency of the waves and can be found by E=hv where h= 6.63*10[-34] (planks constant) consequently red light only has 57% of the energy as blue light (700nm/400nm respectively) (Falkowski and Raven 1997). Radiant flux, Φ, is the time rate of the flow of radiant energy (expressed as W(Js[-1]) or quanta s[-1]). If it is appropriate to indicate radiant flux as a function of ᴓ or ɸ then it can be expressed by L(ᴓ,ɸ) (where ᴓis the zenith angle and ɸ is the azimuth angle). The radiant intensity, I, is the measure of radiant flux per unit of angle in a specific direction and is given by the equation: I=dɸ/dW. Irridance, E, is the radiant flux per unit area of a surface (W (or quanta s[-1]) m[-2]) and is given by the equation E=dΦ/ds. Ed = ∫_2π▒〖L(ᴓ,Φ)cos⁡〖ᴓ dw〗 〗 is the irridance due to downwelling light. Eu=∫_(-2π)▒〖L(ᴓ,Φ)cos⁡ᴓdw 〗 is the irridance due to upwelling light (with allowance made for ᴓ{90,180})(Kirk 1986).

If we move aside from theory then the radiation fields have varying irridance and scalar irridance values with the photosynthetic range, this range effects the extent to which photosynthesis can take place, this is expressed as the variation of irridance or scalar irridance per unit spectral distance across the spectrum (Wm[-2]nm[-1]). It is possible to understand the radiance distribution over all angles at any point in a medium by simply understanding the angular structure of the light on the point. Even this comes with its problems as if we were to use only 5͘͘͘͘͘͘͘· intervals this would represent 1369 separate radiance values, as this quantity is impractical we use 3 average cosines: upwelling light/ downwelling light/ total light, and the irridance reflectance to as a more approachable technique. Photons are either absorbed or they scatter when entering the aquatic medium, the absorption and scattering properties of the given aquatic medium are given by the: absorption coefficient, scattering coefficient and the volume scattering coefficient- these are inherent optical properties as there magnitude only depends on the specific substances comprising the medium.Some of the incident light from the beam is absorbed by the medium, the fraction of incident flux which is absorbed divided by the thickness of the thin layer of medium is the absorbtion coefficient, a. Some of the incident light is scattered by the medium, the fraction of incident flux scattered by the thin layer of medium divided by the thickness of the medium is the scattering coefficient, b. expressing these two ideas quantitatively we can obtain two equations: A=Φa/Φo or B=Φb/Φo- where Φo is the radiant flux incident of the beam, Φa is the radiant flux absorbed by the medium, Φb is the radiant flux scattered by the medium(Kirk 1986).

Now the light has reached the organism, the light must be captured for photosynthesis, this light must be in the specific absorption spectrum. The absorption spectrum may be expressed by absorptance, A, percent absorption, absorbance, D (D=-log⁡〖(1-A)〗) – the parameter of light absorption depends on the purpose of the absorption. The absorbance spectrum depends on the chlorophyll/ carotenoids/ billiproteins composition in the thylakoids- the fundamental light harvesting system. The size or shape of the chloroplasts, whether there is a single cell or colonies as well as pigment composition all influence the specific absorption coefficient per unit pigment. To determine the absorbance spectrum of a single thylakoid is to indirectly calculate it by dispersing chloroplasts into particles which eliminate size and shape influencing the absorbance spectrum. Absorbance spectrum of a cell or colony, suspension or segment of a thalllus or leaf will differ significantly from the dispersed thylakoid fragments thus the peaks are less pronounced and have specific absorption per unit pigment. This phenomenon is due to the package effect which is where pigment molecules are contained within discrete packages within chloroplasts, this lessons the effectiveness of which the light is collected from the field. The package effect is greatest when absorption is greatest(Kirk 1986).

Due to the nature of the package effect you can conclude the identity: Dsus/Dsol<1 or the absorbance of the suspension is less than the absorbance of the solution. If there are n particles per ml and the medium is illuminated by a parallel beam of light the j[th] particle (in solitude) absorbs proportionally α[j]A[j], where α[j] is the projected area of particle in the direction of the beam and A[j] is the particle absorption (fraction of light incident on it which is absorbed). The standard natural logarithm of absorbance is: ln⁡〖1/(1-A)〗 which becomes: ln⁡〖1/(1-α[j]A[j])〗 with the j[th] particle the subject. If you apply beers law for 1 cm path length the equation: ∑_(j=1)^n▒〖α[j]A[j]〗= nᾱĀ is reached where ᾱĀ is the average absorption cross-section. The base-10 logarithm of the suspension is given by Dsus=0.434nᾱĀ. If the pigment was in a solution then the concentration would be: nCṼmg ml[-1] where Ṽ is the average volume of the particles and C is the pigment concentration within the particles. The absorbance due to the dispersed pigment is: Dsol=0.434nCṼy where y is the specific absorption coefficient (absorption coefficient due to the pigment at concentration 1mg ml[-1]). Combining the equations we can see that Dsus/Dsol=ᾱĀ/CṼy is true so therefore can conclude that ᾱĀ/CṼy<1. If the particles only absorb weakly then Dsus≈Dsol and ᾱĀ≈CṼy. If we keep the values of α and Ṽ constant but increase either C, by increasing the pigment concentration in the particles, or y, by changing the wavelength to a more intensely absorbed wavelength, the value of CṼy will increase. As C or y increases so does A however A cannot increase in direct proportion as when A reaches a value nearing 1 there is less leeway to increase in value. This causes the obvious discrepancy between spectrum of the particle suspension and the corresponding solution as absorbtion increases by individual particles increases(Kirk 1986).

This essay has discussed the behaviour of light in and out of the water medium and how the particular water medium will influence the intensity of light before reaching any organism. I have briefly covered the pigments and thylakoids which capture the light but have looked in more detail at the way the pigments are arranged influences the absorption spectrum with some basic calculations to show how changing the concentrations of pigments and the specific wavelength can influence the specific absorption graph.

Refrences:

Kirk, J.T.O. (1986). Light and photosynthesis in aquatic ecosystems

Falkowski, P.G., Raven, J.A. (1997). Aquatic photosynthesis

7 years ago