Pigment packaging effects in Thalassiosira pseudonana under light regulated steady-state growth

Transcription

1 Pigment packaging effects in Thalassiosira pseudonana under light regulated steady-state growth B. Greg Mitchell, Niu Du, Elliot Weiss and Maria Vernet Scripps Institution of Oceanography, UCSD Acknowledgements: Department of Energy

2 Introduction Photons are the fundamental currency of the algae bio-energy enterprise but are generally poorly quantified. The details of absorption in discrete packages, referred to by Duysens (1956) as the sieve effect are also referred to as the packaging effect (Morel and Bricau, 1981). Implications for absorption per unit pigment (e.g. chl-a) and considerations for modeling are reviewed. Definitions P/chl/time = l a ph * (l) E (l) (l) dl PAR= l E (l) dl Flux/chl = l a ph * (l) E (l) a ph * (l) chl-a specific absorption coeficient P production C/vol/time

3 Qa Q*a ρ (λ) = a cm (λ) d A particle (e.g. algal cell) can not absorb more than 100% of the light that intercepts the cell. As more pigments are added to the cell with acclimation the absorption efficiency approaches 100% in a non-linear way

4 From Mie Theory (Van de Hulst, 1957) Q a is defined as - Absorption efficiency fraction of light striking the geometrical cross section that is absorbed Q a (λ) = 1 + (2e -ρ (l) )/ρ (λ) + 2(e -ρ (l) 1)/ρ 2 (λ) ρ (λ) = a cm (λ) d d - cellular diameter (m) a cm (λ) - absorption coefficient of cell material (m -1 ) Since the particle can not absorb more than 100% of the light intecepting the cross-sectional area, for a fixed d, as a cm increases (e.g. with higher pigment per cell) the absorption per unit pigment (e.g. chl-a) decreases. This phenomenon is defined by the pigment packaging parameter (Morel and Bricaud, 1981): Q a * (λ) = (3/2) [Q a (λ)/ ρ (λ)] A value of was assigned to a* cm as established by Johnsen et al. (1994) and confirmed by Moisan and Mitchell (1998), and a cm was calculated using the equation: a cm (λ) = a* cm (λ) [chla/cellular volume]

5 An extreme example of pigment packaging effects for under ice bloom in the Arctic a*ph (m^2/mg chla) Station 56, sample 227 (bloom) HPLC_recon Aph Wavelength (nm) The HPLC pigment reconstruction to estimate total unpackaged absorption is a proxy for a cm and significantly over-estimates the true chl-a speicific absoprtoin coefficient, a*ph. This sort of extreme pigment packaging effect would be expected in high density commercial ponds if the cells were in the range of 5-10 um.

6 Modeling of the under ice bloom compared to a surface sample from well-lit open waters Qa Q*a Bloom Non-bloom Modeled a*ph(676) vs a*ph(676) y = 0.977x R 2 =

7 Table 1. Summary of growth, cellular, and photosynthetic characteristics for Thalassiosira pseudonana gown at 20 C under six irradiances during steady-state conditions. Irradiance (μmol quanta m -2 s -1 ) μ (d -1 ) Chl a:c ratio (w:w) C:N ratio (w:w) pg Chl a cell * a ph a ph (436 nm ) * (436:676 (676 nm) nm )

8 Chl-a/Cell y = x R² = mole Quanta m -2 sec -1

9 Chlorophyll-specific absorption spectra for T. pseudonanan for the experimental light gradient

10 Model of Production per unit chlorophyll plotted against PAR and against Flux/chl. Variations in the initial slope, plotted vs total PAR are due to changes in the pigment packaging effect, not changes in the quantum yield. The initial slope when plotted vs PAR is termed and often is referred to as the light efficiency parameter but it actually varies with both the true quantum yield and the chl-a specific absorption coefficient. PAR FLUX = initial slope P/chl/time = l a ph * (l) E (l) (l) dl PAR= l E (l) dl Flux/chl = l a ph * (l) E (l)

11 Conclusions Pigment packaging effects have very large influence on the absorption per unit pigment (e.g. per unit chl-a) This can lead to large misunderstanding about the actually efficiency of absorb quanta for light limited conditions Proper estimates of absorption are trivial but not generally implemented in most studies of algae physiological acclimation, growth and modeling Another major problem with many experimental designs observed at ABS 2013 is that cultures are grown to such high densities that the experiment becomes light limited and that is often confounds the interpretation of the data The algae industry enterprise needs to account for photons like bankers account for pennies

12 Thank You! Thank You!