http://web.pdx.edu/~rueterj/algae/course_notes.htm
Productivity versus irradiance energy are widely used to visualize and help understand the relationships between phytoplankton and the parameters in their aquatic environment. The relationship of productivity (either O2 or 14C) and light represents the net contribution of many processes including, including; light dependent O2 production, carbon fixation at RuBisCO, dark respiration, O2 consumption at cytochrome oxidase, and Mehler O2 consumption, photorespiration (O2 consumption at RuBisCO). In addition, nitrogen nutrition can affect this relationship through the requirement for reductive power for nitrate and nitrite reduction or the competition for carbon skeletons for amino acid synthesis with carbon intermediates in the reductive pentose phosphate pathway (RPPP, Calvin Cycle). Because all of these above process interact in growing cells, it is impossible to attribute any particular characteristic of a P vs. E relationship to any one physiological process. It is possible, however, to use these P vs. E relationships to help understand the relative importance of these processes in a growing cell under different conditions.
The relationship between P vs. E can be modelled with equations. These models are most useful if the parameters that describe the curve can be related to physiological responses of the cell. We are using the model of Platt et al. to describe the relationship between Productivity (O2 production) and light energy. This model can be approximated with an equation that has four parameters. The goal of this type of model is to both match the shape of the response curve and to have paramters that are related to physiological sub-processes. For example, alpha is related to the efficiency of light harvesting, Pmax is related to the maximum enzymatic rate of carbon fixation and resp is an estimate of the light-independent respiration rate for cell maintainence. Beta is not as easily matched with an identifiable physiological function although there are some models that attempt to provide a more physiological interpretation of the decrease in photosynthesis (Sakshaug et al 1997)
P = Pmax*(1-exp(-alpha*E/Pmax))*exp(-beta*E/Pmax) - respiration
where:
P is the rate of O2 production
E is the irradiance
respiration - the uptake rate of O2 in the dark
alpha - the slope of the P vs. E relationship at low light
Pmax - the calculated highest productivity
beta - the slope of the P vs. E relationship at light levels greater than Pmax
The shape of the P vs. E curve at subsaturating light (i.e. where E < Pmax/alpha) should represent the saturation process of the light harvesting centers. The saturation process follows a Poisson distribution that can be approximated with the tanh function (Kirk ** does he address the poisson dist?**), or the 1-exp(X) function that is used above.
Although this model is widely used there are difficulties in bridging between the curve fitting paratmeters and the physiological responses of the cell. This gap is particularly evident in the interpretation of the decrease in photosynthesis at high light.
Although we usually think of photosynthesis as an oxygen producing process, the related oxygen consumption processes are crucial to understanding the net effect. O2 consuming processes include photorespiration, Mehler rxn and dark respiration (Geider 1992). Whether or not these processes depend on light or intracellular oxygen concentrations can affect the interpreation of the P vs. E relationship. In particular, there seems to be various descriptions of whether the measured dark respiration is constant in the light or whether dark respiration shuts off in the light (Kok effect). The Kok effect is when the O2 uptake through cytochrome oxidase ceases in the light because cyanobacteria are using cytb6f complex for light driven e- flow to NADPH. (Peschek ****) Dark respiration is consumption of O2 at cytochrome oxidase. It is assumed to a function of intracellular O2 concentration (Raven *****). Dark respiration is composed of maintenance and variable, the variable is associated with growth rate (Geider 1992). The respiration rate in the dark isn't constant, Beardall et al 1994 demonstrated an increased rate of respiration right after the lights are turned off. Respiration rates have been estimated to equal to about approx 25% of gross photosynthesis which is an important factor in P vs. E relationships (Falkowski & Owens 1978). Using O18 isotopes, Kana (1992) showed that the rate of O2 cycling in Trichodesmium can be 30% of the Pmax gross. These estimates from other studies demonstrate the potential importance of O2 uptake mechanisms in describing productivity of microalgae. Besides dark respiration, there are three other mechanisms that may contribute to the decrease in net productivity at high light; photoinhibition, photorespiration and the Mehler reaction.
Photoinhibition is defined as the light-dependent, reversible decrease in photosynthesis (Falkowski and Raven 1997). This process has been mechanistically modelled as a decrease in the alpha as a function of exposure to light due to the destruction of D1 and the time it takes to replace D1 in the thylakoid membranes (Marshall et al. 2000). Eilers and Peeters (1993) proposed that the photoinhibition mechanism is decreases both quantum efficiency and Pmax. The effect of photoinhibition on alga cultures has been dynamically modelled as a function of an inhibitory factor that accumulates with light exposure above E critical that decays with time (Pahl-Wostl & Imboden 1990).
Photorespiration is the consumption of O2 by the oxygenase in RuBisCO (Falkowski and Raven 1997). The rate of O2 consumption is is sensitive to CO2/O2. Because this is a competition for enzyme active sites and not destruction, any decrease in photosynthesis caused by photorespiration be reversed without new protein synthesis. Photorespiration has been described as becoming increasingly important in the net productivity budget at higher light intensitities (Mohr and Schopfer pg 230, Asada 1999). The rate of photorespiration should depend on intracellular O2 concentration and temperature because the KmHCO3- for RuBisCO increases with temperature. Photorespiration may be particularly high in cyanobacteria, the ratio of oxygenase (Vo) to carboxylase (Vc) is highest for cyanobacteria, Aphamizomenon Vo/Vc at air values = 0.412 which means that photorespiration would account for about 30% of total Rubisco activity (Raven 19**). The rate of photorespiration should increase with increased cellular oxygen that could be the result of either high ambient [O2] or the intracellular build up caused by rapid photosynthesis.
The Mehler reaction is the e- transport from donor side of PSII to reducing side of PSI (Falkowski and Raven 1997). Thus O2 competes with NADP+ for electrons from PSI (Polle 1996). If NADP+ is low, might increase the rate of Mehler rxn (Polle 1996), or where CO2-fixation is suppressed for some reason (Asada 1999). The value of the Mehler reaction is possibly to avoid damage from excess photons of what is required for the reduction of CO2, nitrogen and sulfate (Falkowski and Raven 1997). Mehler-peroxidase reaction can lead to down-regulation of electron flux (Polle 1996) and leads to a greater pH gradient across the thylakoid membrane ( becasue of the consumption of H+ by dismutation of superoxide and re-reduction of ascorbate in the stroma) also leads to increase formation of zeaxanthin in the xanophylll cycle. It is very important for the cells to keep superoxide low to keep the hydroxyl radical as low as possible because there is no enzymatic way to deal with this species (Asada 1999). Increasing the pH gradient will down-regulate PSII, lowering the quantum yield and dissipate photon energy as heat (Asada 1999)
Platt, T., C.L. Gallegos and W.G. Harrison 1979. Photoinhibition of photosynthesis in natural assemblages of marine phytoplankton. J. Mar. Res. 38: 687-701.
Raven 1984 book
Yunes 1995
Bednarz et al 1989
Marshall et al 2000
Pahl-Wostl & Imboden 1990
Eilers & Peeters, 1993
Falkowski and Raven 1997 - book
Raven **** chapter in book