Incorporating a molecular antenna in diatom microalgae cells enhances photosynthesis

Abstract

Diatom microalgae have great industrial potential as next-generation sources of biomaterials and biofuels. Effective scale-up of their production can be pursued by enhancing the efficiency of their photosynthetic process in a way that increases the solar-to-biomass conversion yield. A proof-of-concept demonstration is given of the possibility of enhancing the light absorption of algae and of increasing their efficiency in photosynthesis by in vivo incorporation of an organic dye which acts as an antenna and enhances cells' growth and biomass production without resorting to genetic modification. A molecular dye (Cy5) is incorporated in Thalassiosira weissflogii diatom cells by simply adding it to the culture medium and thus filling the orange gap that limits their absorption of sunlight. Cy5 enhances diatoms' photosynthetic oxygen production and cell density by 49% and 40%, respectively. Cy5 incorporation also increases by 12% the algal lipid free fatty acid (FFA) production versus the pristine cell culture, thus representing a suitable way to enhance biofuel generation from algal species. Time-resolved spectroscopy reveals Förster Resonance Energy Transfer (FRET) from Cy5 to algal chlorophyll. The present approach lays the basis for non-genetic tailoring of diatoms' spectral response to light harvesting, opening up new ways for their industrial valorization.

Figure 1

Effects of in vivo incorporation of the Cy5 antenna dye in Thalassiosira weisflogii diatoms. Scale bar of the confocal diatoms micrograph: 10 µm.

Figure 2

Absorption spectra of Cy5 (blue continuous line) and pigments (red line) extracted from Thalassiosira weissflogii diatoms and emission spectrum of Cy5 (dashed blue line) in seawater.

Figure 3

(a) Curves for diatom growth and (b) biomass recorded, under normal lighting conditions, in the presence (blue line) and absence (red line) of Cy5. Curves for diatom growth recorded under (c) darkness and (d) blue light excitation.

Figure 4

Light response curves. (a) Algae oxygen evolution rates at different actinic lights in the presence (blue line) and absence (red line) of Cy5. (b) Pmax: Experimental data were fitted with hyperbolic function y = Pmax * x/(KI + x); (c) α-parameter related to the photosynthetic efficiency at limiting light intensity; (d) Oxygen consumption by dark respiration.

Figure 5

(a) Confocal microscopy image of diatom control (Ctrl), and diatoms grown with Cy5 1 μM after 45 min and 8 days’ incubation in normal lighting conditions (light: dark 16: 8 h). Red and blue colors were arbitrarily assigned for chloroplast and Cy5 emission, respectively. (Size bar 10 μm); (b) 3D reconstruction of diatoms incubated with Cy5 after 45 min (Size bar 10 μm).

Figure 6

(a) Temporal evolution of Cy5 alone and inside diatoms at different incubation times. (b) Evolution of the fluorescence lifetime decay of Cy5 alone and incorporated into diatoms after different incubation times.

Figure 7

Immunoblotting quantification of photosynthetic subunits. The bands appearing upon western blot analysis were quantified by densitometry and normalized to the control case to determine the protein content in the Cy5 treated sample.