PHOTOACCLIMATION OF PHOTOSYNTHESIS IRRADIANCE RESPONSE CURVES AND PHOTOSYNTHETIC PIGMENTS IN MICROALGAE AND CYANOBACTERIA¹

Abstract

The photosynthesis-irradiance response (PE) curve, in which mass-specific photosynthetic rates are plotted versus irradiance, is commonly used to characterize photoacclimation. The interpretation of PE curves depends critically on the currency in which mass is expressed. Normalizing the light-limited rate to chl a yields the chl a -specific initial slope ( ␣ chl ). This is proportional to the light absorption coefficient (a chl ), the proportionality factor being the photon efficiency of photosynthesis ( m ). Thus, ␣ chl is the product of a chl and m . In microalgae ␣ chl typically shows little ( Ͻ 20%) phenotypic variability because declines of m under conditions of high-light stress are accompanied by increases of a chl . The variation of ␣ chl among species is dominated by changes in a chl due to differences in pigment complement and pigment packaging. In contrast to the microalgae, ␣ chl declines as irradiance increases in the cyanobacteria where phycobiliproteins dominate light absorption because of plasticity in the phycobiliprotein:chl a ratio. By definition, light-saturated photosynthesis (P m ) is limited by a factor other than the rate of light absorption. Normalizing P m to organic carbon concentration to obtain P m C allows a direct comparison with growth rates. Within species, P m C is independent of growth irradiance. Among species, P m C covaries with the resource-saturated growth rate. The chl a :C ratio is a key physiological variable because the appropriate currencies for normalizing light-limited and light-saturated photosynthetic rates are, respectively, chl a and carbon. Typically, chl a :C is reduced to about 40% of its maximum value at an irradiance that supports 50% of the species-specific maximum growth rate and light-harvesting accessory pigments show similar or greater declines. In the steady state, this down-regulation of pigment content prevents microalgae and cyanobacteria from maximizing photosynthetic rates throughout the light-limited region for growth. The reason for down-regulation of light harvesting, and therefore loss of potential photosynthetic gain at moderately limiting irradiances, is unknown. However, it is clear that maximizing the rate of photosynthetic carbon assimilation is not the only criterion governing photoacclimation.

Comparison of photosynthetic responses in diploid and haploid life-cycle phases of Emiliania huxleyi (Prymnesiophyceae)

“…Comparing the range of values of photosynthetic parameters for a species in the literature may provide some idea of the reliability of each set of results, but direct comparisons between the values reported in different studies can only be made when culture conditions are very similar. In balanced growth, phytoplankton tend to acclimate to light conditions by matching E k to the incident irradiance level by varying the values of α chl a and P chl a max (Sakshaug et al 1997, McIntyre et al 2002. The fact that E k was similar to the culture irradiance level in all of our experiments indicates that the cultures were well acclimated to the light conditions.…”

“…Accessory pigments may contribute to light harvesting or to photoprotective dissipation ('quenching') of excitation energy. An important short-term (on the scale of minutes) modulator of energy flow to the photosynthetic reaction centres involves an enzymatic de-epoxidation reaction of specific xanthophyll pigments, in the case of chromophytes the conversion of diadinoxanthin to diatoxanthin, the latter being the quenching form (McIntyre et al 2002). In addition, β-carotene may serve as an energy quencher, although it is not regulated in the same manner as the xanthophyll cycle (McIntyre et al 2002).…”

“…An important short-term (on the scale of minutes) modulator of energy flow to the photosynthetic reaction centres involves an enzymatic de-epoxidation reaction of specific xanthophyll pigments, in the case of chromophytes the conversion of diadinoxanthin to diatoxanthin, the latter being the quenching form (McIntyre et al 2002). In addition, β-carotene may serve as an energy quencher, although it is not regulated in the same manner as the xanthophyll cycle (McIntyre et al 2002). Our preliminary results, which -for the first time -compare the pigment composition of the 2 phases within the haplo-diploid life cycle of coccolithophores, indicate that there are no fundamental differences in pigment content between the 2 phases cultured under identical conditions.…”

Limits to productivity of the algaPleurochrysis carterae(Haptophyta) grown in outdoor raceway ponds

“…Photoacclimation in free-living phytoplankton has been demonstrated in several studies (Brown and Richardson, 1968;Meeson and Sweeney, 1982;Olaizola and Yamamoto, 1994;Prezelin, 1976), however these studies have only measured photoacclimation over periods of less than 3 weeks whereas our results show that outdoor cultures of P. carterae take 4 weeks to photoacclimate fully. Several mechanisms are known to be involved in photoacclimation (Raven and Geider, 2003): (1) changes in the composition and cellular abundance of light-harvesting pigment-protein complexes, (2) changes in the abundance and ratios of PSI and PSII reaction centres and of other catalysts within the electron transport chain, (3) changes in the activity of CO 2 -fixing enzymes such as Rubisco and/or electron transport systems (Rivkin et al, 1982;Sukenik et al, 1987), and (4) changes in the content of photoprotective pigments (Macintyre et al, 2002). The exact mechanism of photoacclimation in P. carterae remains to be determined, however the slow rate of acclimation suggests it may be mainly due to changes in enzyme systems such as Rubisco.…”

Photosynthetic electron turnover in the tropical and subtropical Atlantic Ocean

“…Phytoplankton communities adjust their photochemical constituents and pathways towards an acclimatory state that can be characterised by their rate of cellular light absorption, saturating light-intensity and maximum rate of cellular production (Allen, 2002;MacIntyre et al, 2002;Behrenfeld et al, 2004). This dynamic process of photoacclimation is described here by s PSII,478 and s max PSII;478 , E k and ETR max RCII .…”