Biology Chemistry Methodology Microbiology Physiology

Protist Factories: A Potential Way to Make Gold and Silver

References: Kalidasan, Kaliyamoorthy; Asmathunisha, Nabikhan; Gomathi, Venugopal; Dufossé, Laurent; Kathiresan, Kandasamy. (2021). Isolation and Optimization of Culture Conditions of Thraustochytrium kinnei for Biomass Production, Nanoparticle Synthesis, Antioxidant and Antimicrobial Activities. J. Mar. Sci. Eng. 9, 678-696.


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Picture this: you have built yourself a nice backyard garden. It has all of your favorite herbs and vegetables: zucchini, tomatoes, basil, peppers, kale, parsley, mint, and etc. You are proud of it, not only because of how good the food tastes but also in your hard work to set it all up. It is no laughing matter to set up a successful garden! All of the plants are well adapted to your backyard’s weather and the amount of sunshine they get. Well, one Summer your backyard gets very little rain and all of your plants dry up from the Sun. That’s just no good! Just like vegetables and herbs, animals need a set of optimal conditions in their environment to survive. Each type of organism has their own set of ideal conditions, these include properties like humidity, heat, nutrients, shelter, and food. Maintaining these conditions are important for any organism to live, and (in regard to our example above) make edible produce. So, how do we know what organisms like what conditions? How can we maximize the productivity of the plant or animal with their environment? How can we best run these living factories?

A picture of Thraustochytrium kinnei. Image Credit: Kalisadan et. al.

A multi-national team of scientists from Annamalai University in Parangipettai, India and the Université de La Réunion in Saint-Denis, France investigated the ideal environmental conditions of one species of water-living protist, named Thraustochytrium kinnei. This single-celled organism is usually found decomposing dead plant matter in mangroves. They have very particular cell membranes filled with special types of polyunsaturated fatty acid molecules like omega-3 fatty acids. These types of molecules are very valuable for their usage in medicinal contexts. Furthermore, these types of protists can synthesize silver and gold in the form of nanoparticles, which are very small aggregates of these metal atoms. These particles are valuable as well because they are needed to build technological and medical devices. In the same way we try our best to make our backyard gardens produce a lot of fruits and vegetables, the group attempted to optimize the environmental conditions of the protists. With healthy protists, the team could maximize the production of these omega-3 fatty acids and nanoparticles. Today, the team examined the best conditions for Thraustochytrium kinnei to thrive and measured their production of these important products.

What did they find?

The team found that Thraustochytrium kinnei grown with glucose, peptone, yeast extract, and agar (all types of foods and nutrients) in a solution of water yielded up to 13.53 grams of Thraustochytrium kinnei per liter of water. For comparison, this is about the same as 36 and a half bars of soap in a full bathtub! After further investigation, the protists generally needed water with a neutral pH close to 7, 6-14 days of incubation in the solution, and a varied amount of additives like bread crumbs and peptone. Generally, the pH of the water solution, the number of incubation days, and the amount of peptone influenced the protists’ productivity the most. Of this 13.53g of biomass (otherwise known as cell material), 41.33% of the mass were lipid molecules. 39.16% of these lipids molecules was a specific molecule called docosahexaenoic acid (DHA), an important omega-3 fatty acid for medicinal purposes.

A microscopic picture of silver (left) and gold (right) nanoparticles. Image Credit: Kalidasan et. al.

As mentioned above, the Thraustochytrium kinnei also produced silver and gold nanoparticles! Specifically, the silver particles were around 5-90 nanometers and the gold particles were around 10-85 nanometers. For some comparison, that is about ten million times smaller than a line drawn by a pencil! While small, this method of collecting silver and gold is much more environmentally friendly than other traditional chemical methods. The team decided to further investigate the properties of these nanoparticles, specifically how they interact with infectious bacteria. Both the silver and gold particles expressed a significant ability to kill multiple bacteria species! All of these findings exhibit the potential of Thraustochytrium kinnei as a truly exciting organism!

How did they do it?

First, the team isolated the protist from decomposing leaves in mangroves on the southeast coast of India. From there, the group experimented with the environmental conditions (or the chemicals in the culturing media) while they grew colonies of these protists. They found the optimal environment by measuring the weight of the biomass produced. In addition, the group grew this protist in separate containers where the molecule silver nitrate was added. The protists would create these silver nanoparticles and the team measured them using a special technique called Dynamic Light Scattering (DLS). To synthesize gold nanoparticles, chloroauric acid was added to the protist growing containers and the resulting particles were also measured by DLS. Last, these nanoparticles were examined for their antibacterial properties by exposing colonies of multiple bacteria species to the particles.

Why does it matter?

This investigation informs us of both the ability of Thraustochytrium kinnei to defend itself from bacteria as well as the significant potential of these protists as productive factories. As these types of products become either harder to find or more environmentally damaging to produce, using alternative methods like culturing protists can become quite important. With this knowledge, we can find better ways to live and thrive in an environmentally sustainable future.

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