Light scattering by coccoliths detached from Emiliania huxleyi

Abstract

We used in situ radiance/irradiance profiles to retrieve profiles of the spectral backscattering coefficient for all particles in an E. huxleyi coccolithophore bloom off the coast of Plymouth, UK. At high detached coccolith concentrations the spectra of backscattering all showed a minimum near approximately 550 to 600 nm. Using flow cytometry estimates of the detached coccolith concentration, and assuming all of the backscattering (over and above the backscattering by the water itself) was due to detached coccoliths, we determined the upper limit of the backscattering cross section (sigma(b)) of individual coccoliths to be 0.123+/-0.039 microm(2)/coccolith at 500 nm. Physical models of detached coccoliths were then developed and the discrete dipole approximation was used to compute their average backscattering cross section in random orientation. The result was 0.092 microm(2) at 500 nm, with the computed sigma(b) displaying a spectral shape similar to the measurements, but with less apparent increase in backscattering toward the red. When sigma(b) is computed on a per mole of calcite, rather than a per coccolith basis, it agreed reasonably well with that determined for acid-labile backscattering at 632 nm averaged over several species of cultured calcifying algae. Intact coccolithophore cells were taken into account by arguing that coccoliths attached to coccolithophore cells (forming a "coccosphere") backscatter in a manner similar to free coccoliths in random orientation. Estimating the number of coccoliths per coccosphere and using the observed number of coccolithophore cells resulted is an apparent backscattering cross section at 500 nm of 0.114+/-0.013 microm(2)/coccolith, in satisfactory agreement with the measured backscattering.

Impact of calcification state on the inherent optical properties of Emiliania huxleyi coccoliths and coccolithophores

“…All the quantities are sensitive to the calcification state because the particle volumes are quite different. According to Gordon et al [12], an upper limit of the backscattering cross section estimated from in situ measurements is 0.12370.039 μm 2 at the wavelength of 0.5 μm. The backscattering cross sections shown for the diameter of 4.5 μm (the upper limit value of the size) is consistent with the estimation in a previous study [12].…”

“…Incremental progress has been achieved in the optical properties of coccoliths and coccolithophores. The particles assumed for optical modeling reported in the literature include spheres e.g., [8]; three-layer spheres [9,10]; simple nonspherical particles such as disks, hollow disks, and fishing reels [11][12][13][14]; and, more realistic morphologies defined by Zhai et al [15]. Using nonspherical particles instead of spheres for modeling coccoliths is necessary from both quantitative and qualitative perspectives, but diverse nonspherical models may lead to noticeable or large differences in the simulations, causing biases in downstream applications, particularly, oceanic remote sensing data interpretation.…”

Biometry of detached Emiliania huxleyi coccoliths along the Patagonian Shelf

“…Considerable variability also exists in the calcite content of coccoliths of different coccolithophore species (Young & Ziveri 2000), which has important implications for their relative contribution to deepsea CaCO 3 fluxes (Baumann 2004, Ziveri et al 2007). Such differences in size will also affect coccolith optical properties (Gordon & Du 2001, Gordon et al 2009). The morphotypes of Emiliania huxleyi also have different coccolith quotas (Young & Ziveri 2000), although how these may equate to differences in contributions to pelagic calcite production or export is unclear.…”

Photoacclimation to abrupt changes in light intensity by Phaeodactylum tricornutum and Emiliania huxleyi: the role of calcification

“…As for the group-specific characteristics, it has been hypothesized that the coccoliths (in E. huxleyi) could increase light scattering (e.g. Gordon et al 2009), but their organization has also been thought to amplify the light reaching the chloroplasts in species occurring in deep waters (Young 1994) and consequently allow for lower cellular chl a contents.…”