Why are the lakes in scenic spots green? Is it the reflection of the trees? Actually, it’s thanks to this!

The guest road is beyond the green mountains, and the boat is in front of the green water.

Why are the lakes in many scenic spots green? Is it because of the green trees on the shore? Or is it because of the swaying water plants at the bottom of the lake? In fact, these are not the main reasons. This is also due to a type of plant that we cannot see - microalgae .

Image source: unsplash.com Photographer: Dirk Von Loen Wagner

What is Microalgae?

When we mention the word "algae", we always think of delicious ingredients such as kelp and laver. They look similar to green vegetables, but different parts taste the same, unlike the roots, stems and leaves of green vegetables, which each have their own merits.

This is because their cells are at a low level of differentiation, and there is almost no difference in the cell composition, structure and function in different parts . They can be understood as a "building" made of the same "building blocks".

Because they can be several meters long, they are collectively called "macroalgae".

In contrast, more algae choose to survive as single-cell microorganisms, collectively referred to as "microalgae", or microalgae for short.

Several common microalgae. Image source: Reference [1]

Although microalgae are usually only a few micrometers in size, many times thinner than a hair, and can only be seen clearly with the help of a high-power optical microscope, the microalgae family is very large and has a wide variety of species.

They have a strong ability to adapt to the environment, so they are widely distributed in nature, from the numerous rivers and lakes to the vast oceans, from the permafrost in Northern Europe to the deserts in North Africa, their footprints can be found everywhere.

The secret of the prosperity of microalgae is that they are photosynthetic microorganisms. Similar to terrestrial plants, as long as there is sunlight, air and water, they can synthesize organic matter through photosynthesis to maintain their own survival and reproduction.

At the same time, as single-celled organisms, they can exchange matter and energy with the environment very conveniently, and their photosynthesis rate is much higher than that of terrestrial plants.

According to statistics, half of the organic matter produced by photosynthesis in the world each year is contributed by microalgae. In other words, half of the air we breathe and the nutrients we take in every day are attributed to these invisible algae.

The past and present of microalgae

We humans have only a few million years of history, and the dinosaur era is far away from us, but microalgae were born 3.5 billion years ago, much earlier than the dinosaurs.

At that time, the Earth had just experienced a peak of impacts from foreign celestial bodies and had become devastated. There was nothing on the land except rocks, and there was not a trace of oxygen in the atmosphere.

Microorganisms from submarine volcanic vents spread to shallow seas and began to use sunlight shining into the water as a source of energy, thus evolving into the earliest oxygen-releasing photosynthetic microalgae - cyanobacteria.

Cyanobacteria form eukaryotic microalgae through endosymbiosis. Image source: Reference [2]

Cyanobacteria are prokaryotic cells without a formed nucleus. Through fusion with other cells, they produce eukaryotic cells with photosynthetic functions, which are eukaryotic microalgae.

Because these microalgae live in different light environments, the pigments responsible for absorbing light energy in their bodies are also different, making their colors different. Based on this, there are green algae, red algae and golden algae.

As we all know, the general direction of life evolution is from the ocean to the land, from single cells to multi-cells. Microalgae are no exception, especially green algae, which gradually occupied rivers and lakes on land and gave birth to the earliest land plants 900 million years ago.

From this moment on, the earth began to slowly become what we see today.

On the one hand, microalgae and terrestrial plants absorb carbon dioxide from the atmosphere and release oxygen through photosynthesis, regulating the earth's climate and creating basic conditions for life to move from anaerobic respiration to more efficient aerobic respiration, promoting the birth and evolution of large multicellular terrestrial animals such as dinosaurs.

On the other hand, microalgae and terrestrial plants, as bottom producers in the food chain, support numerous herbivores and higher-level carnivores , maintaining ecological balance and greatly enriching the species diversity of the Earth's biosphere.

Green algae came onto land and evolved into land plants. Image source: Reference [3]

Today, green plants dominate the land. When it comes to photosynthesis, they are the first thing we think of. However, microalgae still quietly participate in our daily lives in places we cannot see, such as:

Microalgae has a high protein content and can be served on our tables as a meat substitute or as a food additive to make our dishes more nutritious and flavorful. Even if we are not used to eating it, microalgae can also serve as animal feed to defend our freedom to eat meat.

Microalgae have good environmental purification functions . They can absorb heavy metal ions and excess nutrients in various types of wastewater, avoid eutrophication and poisoning of natural water bodies, and give us green water and blue sky.

Microalgae can also be used to refine biodiesel or ferment to produce ethanol and other renewable biomass energy , gradually replacing fossil energy, thereby reducing global carbon dioxide emissions and eliminating climate anomalies caused by greenhouse gases. How to improve the efficiency of microalgae photosynthesis?

It is precisely because of the photosynthesis inherited by microalgae that we have the material and energy basis for the survival of almost all living things on Earth, including humans. Photosynthesis is also recognized as the most important chemical reaction on Earth.

In the more than 100 years since the Nobel Prize was established, research related to photosynthesis has won the prize as many as eight times, which shows how much attention it receives and its important role.

At present, the photosynthetic efficiency of microalgae and green plants is relatively low, which makes biomass energy more expensive than fossil energy and new energy. Therefore, simply and efficiently improving their photosynthetic carbon fixation efficiency is the key to enhancing the market competitiveness of biomass energy.

Although scientists still do not understand the specific mechanism of photosynthesis, they can roughly outline its reaction process. For eukaryotic microalgae and green plants, photosynthesis is carried out in chloroplasts and can be divided into two parts: light reaction and dark reaction.

Light reaction and dark reaction in chloroplasts. Image source: Reference [4]

In the light reaction, chloroplasts absorb sunlight to decompose water, release oxygen and synthesize high-energy active substances; in the dark reaction, with the help of high-energy active substances produced by the light reaction and the catalytic action of a series of carbon-fixing enzymes, carbon dioxide is reduced to organic matter such as sugars.

Since the light reaction and the dark reaction are carried out in series, the rate of photosynthesis depends on the slower of the two, similar to the barrel effect. The dark reaction is the shortcoming, and there are two reasons for this: one is the insufficient supply of carbon dioxide, and the other is that the catalytic activity of Rubisco, a key enzyme for carbon fixation, is too low.

In response to the second reason, nature adopts the strategy of "making up for lack of efficiency with quantity" and synthesizes large quantities of Rubisco, a key enzyme for carbon fixation, making it the most abundant protein on earth.

But for the first reason, nature is helpless, because the concentration of carbon dioxide in the atmosphere is 0.04%. Microalgae mainly live in water, and the concentration of dissolved carbon dioxide is much lower than that in the atmosphere. As the saying goes, a good cook cannot cook without rice. Without the raw material of carbon dioxide, the synthesis rate of biomass will naturally not be fast.

In 2023, a paper published in the academic journal Nature Communications reported a new strategy for artificially enhancing photosynthetic carbon fixation in microalgae .
By self-assembling a layer of artificial material that can enrich carbon dioxide in water on the surface of microalgae, carbon dioxide can be effectively concentrated on the surface of microalgae, and then transported into the chloroplasts through the inherent carbon dioxide transport mechanism of microalgae, significantly accelerating the photosynthetic carbon fixation efficiency of microalgae.

Schematic diagram of the principle of artificially enhancing microalgae photosynthetic carbon fixation strategy. Image source: Reference [5]

This is like a person standing at the door with his mouth open on a rainy day, he can't drink much water. If he uses a water collection device to fill a tank with water and put it at the door, he will not lack water for a long time.

This strategy cleverly uses artificial materials assembled on the surface of microalgae to enrich carbon dioxide, breaking through the bottleneck that microalgae can only use carbon dioxide dissolved in water , thereby nearly doubling the photosynthetic carbon fixation rate of microalgae. Not only does it produce more biomass for us to use in the same amount of time, it also consumes more carbon dioxide in the atmosphere, helping us achieve the grand goal of carbon neutrality as soon as possible.

Conclusion

Microalgae have played a key role in the evolution of the earth, and they still support the well-being of human beings in many aspects. However, due to their low overall photosynthetic efficiency, the further development of microalgae-related industries has been restricted.

To this end, we must not only conduct in-depth research on the specific regulatory mechanisms of natural photosynthesis and strive to "understand nature", but also liberate our minds and seek ways to improve photosynthesis across disciplines and fields, so that the development of our human society can go hand in hand with the tranquility and peace of nature.

References

[1] BENEMANN J. Microalgae for Biofuels and Animal Feeds. Energies 2013, 217: 5869-5886.

[2] CLARK DP & PAZDERNIK N J. in Molecular Biology (Second Edition). 2013, 812-853.

[3] DE VRIES J & ARCHIBALD J M. Plant evolution: landmarks on the path to terrestrial life[J]. 2018, 217(4): 1428-1434.

[4] GAN P, LIU F, LI R, et al. Chloroplasts— Beyond Energy Capture and Carbon Fixation: Tuning of Photosynthesis in Response to Chilling Stress. International Journal of Molecular Sciences 2019, 20: 5046.

[5] LI D, DONG H, CAO X, et al. Enhancing photosynthetic CO2 fixation by assembling metal-organic frameworks on Chlorella pyrenoidosa[J]. Nature Communications 2023, 14(1): 5337.

Planning and production

Author: Li Dingyi and Li Xueyang Dalian Institute of Chemical Physics, Chinese Academy of Sciences

Producer丨China Science Expo

Editor: Yinuo