Encasing algae in microdroplets increases photosynthesis efficiency by 250%

November 7, 2021. By Kolemann Lutz

Encasing cyanobacteria and algae in tiny liquid crystal (LC) droplets and optical WGM microcavities boosts the natural energy harvesting, photocurrent, and photosynthesis efficiency by 250%.

Considering 600-700 nm is one of the most efficient light wavelengths for algae photosynthesis, algae cultures should receive at least 10 hours of sunlight per day (17 hrs max), depending on the species. Artificial photosynthesis technologies currently run at efficiencies around 4-5%, which is relatively poor efficiency. In the field of bioenergy, extracting energy from light-absorbing phycobiliproteins in cyanobacteria algae holds great potential.

A new study published in ACS Applied Materials & Interfaces from the Singapore's Nanyang Technological University (NTU) outlines a breakthrough method to supercharge water-soluble phycobiliproteins ability to turn captured light into bioavailable energy by encapsulating red algae in tiny liquid crystal (LC) droplets (20 to 40 microns in size).

As photons reach the droplet, the curved edges induce a whispering-gallery mode (WGM), when light travels around the perimeter and is trapped inside the droplet for longer, which leads to more photosynthesis time for the three major fluorescent phycobiliproteins [R-phycoerythrin (RPE), C-phycocyanin (CPC), and allophycocyanin (APC)] to harvest and pass on light to chlorophylls.

“The droplet behaves like a resonator that confines a lot of light,” mentioned by lead author Professor Chen Yu-Cheng. With prolonged photon lifetime inside the WGM optical cavity, the electrons generated from the optical cavity can then be collected with electrodes.

Moreover, WGM microcavities have been extensively explored in biosensing, imaging, solar cells, and photocatalytic applications.

Figure 1. Illustration of the WGM-enhanced photoelectricity from light-harvesting protein microcavity assemblies. Phycobiliproteins absorb photons and then produce photons and electrons.

“By exploiting microdroplets as a carrier for light-harvesting biomaterials, strong local electric field enhancement and photon confinement at the cavity interface resulted in significantly higher electricity generation.”

Next, the scientists then thought to develop, test, and apply a coating to the outside of the microdroplet with the algae RLC phycobiliproteins. With the existence of the

WGM microcavity, an extremely large photocurrent of 693 nA/cm2 was achieved upon ultraviolet (UV) illumination, while only 242 nA/cm2 was obtained without the cavity, or a remarkable 250% photocurrent improvement in the presence of a cavity.

"How well we can passively provide food for mars could have significance use here on Earth", says Daniel Tompkins, Faculty for Agriculture, MarsU.

This method could be viewed as converting bio-trash into bio-power. Another possibility lies in harnessing this technology to boost the performance of organic solar cells. We looked at one interesting example of this back in 2017, where scientists showed how incorporating a type of algae called a diatom could improve the efficiency of a solar cell, by trapping and scattering the light for more effective harvesting.

On Mars, the maximum solar irradiance on Mars is about 590 W/m2 compared to around 1000 W/m2 at the Earth's surface, depending on the latitude. As the increasing distance from a host star limits bioavailable photons and vitamin D production, light harvesting coatings could significantly prolong photon lifetime and exposure for a wide range of species with chlorophylls to passively provide food for cyanobacteria, humans, and organisms on Mars, inner belt, and beyond.

"Light could be less of a limiting factor for photosynthetic organisms for space exploration. Optimising WGM microdroplets to deliver customised photon flux with high accuracy and reliability could even help isolate the effects of alternative gravity", mentions Kole Lutz, Cofounder at MarsU

Further experiments and research will help quantify the effects and performance of liquid crystal droplets on hardy cyanobacteria Mars candidates such as Shewanella oneidensis, Anabaena, Synechococcus, Spirulina, Azotobacter, lichens, and more.

Microdroplets could even be scaled up to larger droplets for larger forms to encase algae growing in bodies of water, which could enable floating power generators and cyanobacteria algae mats on Mars and extreme space environments with less solar irradiance. Vast pools of cyanobacteria encapsulated microdroplets could provide abundant food, biomass, and biofuel to sustain humans and organisms on distant worlds.


Light-Harvesting in Biophotonic Optofluidic Microcavities via Whispering-Gallery Modes

Zhiyi Yuan, Xin Cheng, Tsungyu Li, Yunke Zhou, Yifan Zhang, Xuerui Gong, Guo-En Chang, Muhammad D. Birowosuto, Cuong Dang, and Yu-Cheng Chen. ACS Applied Materials & Interfaces, 2021, https://doi.org/10.1021/acsami.1c09845

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