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 (α c h l ). This is proportional to the light absorption coefficient (a c h l ), the proportionality factor being the photon efficiency of photosynthesis (Φ m ). Thus, α c h l is the product of a c h l and Φ m . In microalgae α c h l typically shows little (<20%) phenotypic variability because declines of Φ m under conditions of high-light stress are accompanied by increases of a c h l , The variation of α c h l among species is dominated by changes in a c h l due to differences in pigment complement and pigment packaging. In contrast to the microalgae, α c h l 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.

Photoacclimation of photosynthesis irradiance response curves and photosynthetic pigments in microalgae and cyanobacteria1

http://www.nt.ntnu.no/users/skoge/prost/proceedings/dycops-2010/Papers_DYCOPS2010/WePlenaryT5-01.pdf

Hurdles and challenges for modelling and control of microalgae for CO2 mitigation and biofuel production

We examined the effects of increased temperature, PCO(2), and irradiance on a calcifying strain of the marine coccolithophore Emiliania huxleyi in semi-continuous laboratory cultures. Emiliania huxleyi CCMP 371 was cultured in four temperature and PCO(2) treatments at both low and high irradiance (50 and 400 mu mol photons m(-2) s(-1)): (i) 20 degrees C and 375 ppm CO(2) (ambient control); (ii) 20 degrees C and 750 ppm CO(2) (high PCOA (iii) 24 degrees C and 375 ppm CO(2) (high temperature); and (iv) 24 degrees C and 750 ppm CO(2) ('greenhouse'). The growth of E. huxleyi was greatly accelerated by elevated temperature at low irradiance. Photosynthesis was significantly promoted by increases in both PCO(2) and temperature at both irradiances. Higher cellular C/P ratios were found in the higher CO(2) treatments at high irradiance, indicating a reduced requirement for P. The PIC/POC (particulate inorganic to organic carbon) ratio remained constant at low light, regardless of CO(2) or temperature conditions. However, both the cellular PIC content and PIC/POC ratio were greatly decreased by elevated irradiance, and were further decreased by increased PCO(2) only at high light, indicating a combined effect of CO(2) and light on calcification. These results suggest that future trends of CO(2) enrichment, sea-surface warming and exposure to higher mean irradiances from intensified stratification will have a large influence on the growth of Emiliania huxleyi, and potentially on the PIC/POC 'rain ratio'. Our study demonstrates that it is possible to obtain a more complete picture of global change impacts on marine phytoplankton by designing experiments that consider multiple global change variables and their mutual interactions.

https://www.tandfonline.com/doi/pdf/10.1080/09670260701664674

Interactive effects of increased pCO 2 , temperature and irradiance on the marine coccolithophore Emiliania huxleyi (Prymnesiophyceae)

To consider the relationship between chlorophyll a (Chl a) content and phytoplankton growth and nutrient status, four phytoplankton species were grown in nitrogen (N)-limited [and, for one species, phosphorus (P)-limited] culture and measurements were made of CNP biomass, in vivo and in vitro Chl a content, the ratio of variable to maximum fluorescence (FV/FM) and the performance index for photosynthesis, PIABS (a derivative of the O-J-I-P analysis of photosystem II functionality). Interspecies differences plus the development of intraspecies differences during nutrient stress produced c. 10-fold variations in Chl : C. Estimates of C from in vivo Chl content were better than those from extracted Chl content, as the decline in Chl : C during nutrient stress was offset in part by increased Chl fluorescence. FV/FM was not a robust indicator of nutrient status or relative growth rate. Responses of FV/FM in cells re-fed the limiting nutrient showed no consistent pattern with which to gauge nutrient status. PIABS showed some promise as an indicator of nutrient status and relative growth rate. Chl a content and fluorescence parameters do not deserve the unquestioned status they usually enjoy as indicators of biomass and physiological status.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.2005.01601.x

Chlorophyll content and fluorescence responses cannot be used to gauge reliably phytoplankton biomass, nutrient status or growth rate

The JGOFS program and NASA ocean-color satellites have provided a wealth of data that can be used to test and validate models of ocean biogeochemistry. A coupled three-dimensional general circulation, biogeochemical, and radiative model of the global oceans was validated using these in situ data sources and satellite data sets. Biogeochemical processes in the model were determined from the influences of circulation and turbulence dynamics, irradiance availability, and the interactions among four phytoplankton functional groups (diatoms, chlorophytes, cyanobacteria, and coccolithophores) and four nutrients (nitrate, ammonium, silica, and dissolved iron). Annual mean log-transformed dissolved iron concentrations in the model were statistically positively correlated on basin scale with observations ðPo0:05Þ over the eight (out of 12) major oceanographic basins where data were available. The model tended to overestimate in situ observations, except in the Antarctic where a large underestimate occurred. Inadequate scavenging and excessive remineralization and/or regeneration were possible reasons for the overestimation. Basin scale model chlorophyll seasonal distributions were positively correlated with SeaWiFS chlorophyll in each of the 12 oceanographic basins ðPo0:05Þ: The global mean difference was 3.9% (model higher than SeaWiFS). The four phytoplankton groups were initialized as homogeneous and equal distributions throughout the model domain. After 26 years of simulation, they arrived at reasonable distributions throughout the global oceans: diatoms predominated high latitudes, coastal, and equatorial upwelling areas, cyanobacteria predominated the mid-ocean gyres, and chlorophytes and coccolithophores represented transitional assemblages. Seasonal patterns exhibited a range of relative responses: from a seasonal succession in the North Atlantic with coccolithophores replacing diatoms as the dominant group in mid-summer, to successional patterns with cyanobacteria replacing diatoms in mid-summer in the central North Pacific. Diatoms were associated with regions where nutrient availability was high. Cyanobacteria predominated in quiescent regions with low nutrients. While the overall patterns of phytoplankton functional group distributions exhibited broad qualitative agreement with in situ data, quantitative comparisons were mixed. Three of the four phytoplankton groups exhibited statistically significant correspondence across basins. Diatoms did not. Some basins exhibited excellent correspondence, while most showed moderate agreement, with two functional groups in agreement with data and the other two in disagreement.

/pdf/phytoplankton-and-iron-validation-of-a-global-three-1mnbnbnjee.pdf

Phytoplankton and iron: validation of a global three-dimensional ocean biogeochemical model

Photoacclimation was examined in the marine diatom Skeletonema costatum, which was subjected to reciprocal shifts between irradiances of 50 (low-light) and 1,200 (high-light) mmol photons m 22 s 21 . Cell chlorophyll a and fucoxanthin contents were higher but diadinoxanthin and diatoxanthin contents lower in cells grown at 50 mmol photons m 22 s 21 than in cells shifted to 1200 mmol photons m 22 s 21 . Cell carbon contents measured at the start of the light period were similar in both high-light and low-light treatments. However, b y 6 h into the light period, the carbon contents in the high-light cells were about twofold higher than in the low-light cells. Dark respiration rates, dark Chl a synthesis rates, and dark cell-division rates were greater in the high-light acclimated cells than in the low-light cells. Thus, there was a greater uncoupling of carbon assimilation from cell division during the day in the high-light cells, but pigment synthesis and cell division continued in darkness. Cell-specific, light saturated photosynthesis rates, and chlorophyll a specific light-limited photosynthesis rates were constant during reciprocal shifts between growth irradiances of 50 and 1200 mmol photons m 22 s 21 . Thus, differences of photosynthesis versus irradiance curves between cells acclimated to high-light versus low-light could be accounted for largely in terms of changes in cell chlorophyll a contents. Although the chlorophyll a-specific initial slope, a chl , was constant, the chlorophyll a-specific light absorbtion coeffecient, a chl , increased and the maximum quantum efficiency of photosynthesis (fm) declined following the shift to high light. The increase of a chl was most likely due to a decreased package effect. The decline of fm was most likely due to accumulation of xanthophyll cycle pigments. Carbonspecific, light-saturated photosynthesis rates were lower in high-light than in low-light cells; this observation may indicate that control of light-saturated photosynthesis shifts from enzymes of the carbon dioxide reduction cycle (Calvin cycle) in low-light cells to the photosynthetic electron transfer chain in high-light cells.

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.2000.45.8.1807

Sandra M. Pratt

Papers

Photoacclimation in the marine diatom Skeletonema costatum