Plants between light and darkness

For research, plants are frequently grown under stable lighting, which does not reflect natural conditions. In a series of experiments with changing light conditions, simulating the natural interplay of light and shadow, researchers from the Max Planck Institute of Molecular Plant Physiology in Potsdam-Golm (Germany) and the College of Natural Science at Michigan State University (USA) reveal the importance of two key proteins for the dynamic control of photosynthesis.

Plants perform photosynthesis to grow. In this process they use energy from sunlight, release oxygen, and produce carbohydrates, which are the basic food resource for all humans and almost all animals on earth. Under natural conditions, light availability can change rapidly in a very short time. One of the main reasons are clouds which provide light and shadow as they pass in front of the sun. Plant leaves and branches can also temporarily provide shade when they are moved by the wind. Plants cannot move from shade to sun when light is limited, and conversely, cannot evade from sun to shade when exposed to too much sunlight. They have to respond to changing light conditions in other ways.

Just like for humans, too much sunlight is harmful to plants. In particular, a rapid change between faint and intense light is problematic. Like the retina in our eyes, plants use molecules in their leaves to capture light particles. When light is low, these light traps are very efficient at catching as much of the low light as possible. If light conditions suddenly change, too much light energy might reach the plant. This energy can overload or damage the sensitive photosynthetic apparatus inside the plant cells. Accordingly, plants have to constantly adapt their photosynthetic activity to their environmental conditions in order to obtain maximum light yield on the one hand, but avoid being harmed by too much light on the other hand.

To date, plants in greenhouses and laboratories are grown almost exclusively under stable and uniform light conditions. Therefore, our understanding of how adaptation to changing light conditions works is very limited. In the worst case, this can lead to plants that are growing well in laboratories and greenhouses but suddenly perform much worse than expected when cultured in the field.

Regulation of photosynthesis under changing light conditions

The researchers around Ute Armbruster from the Max Planck Institute of Molecular Plant Physiology in Potsdam-Golm and David Kramer from the College of Natural Science at Michigan State University (USA) examined the model plant Arabidopsis thaliana for their study. Plants were grown under a wide variety of conditions including static, fluctuating and natural light. The study focused on two ion transport proteins called VCCN1 and KEA3 which play a key role in dynamically adjusting photosynthetic performance. It is known from earlier studies that VCCN1 activates sun protection if the light suddenly becomes too strong. When the light intensity decreases, the second protein KEA3 quickly breaks down this sun protection so that the plant can catch more light again. However, the two proteins VCCN1 and KEA3 have never been examined under realistic light conditions.

The researchers used an innovative new approach to measure photosynthesis in combination with a targeted use of gene knockouts -- i.e. plants whose genes for VCCN1 and KEA3 have been switched off. They show that the activities of the proteins VCCN1 and KEA3 depend on the light conditions the plants were raised in. Following suggestions by the head of the Plant Cultivation Infrastructure Group, Dr. Karin Köhl, the researchers focused on two growth-related light factors in the analysis and were able to show that both the amount of light a plant receives, and the frequency of light fluctuations have a strong influence on the function of the two ion transporters. The protective function of VCCN1 is only important in plants previously grown under low light. On the other hand, KEA3 which abolishes protection, was even active in high light periods when the plants were grown under conditions with elevated light intensities.

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Microbes produce oxygen in the dark

It is common knowledge that there would be no oxygen on Earth were it not for sunlight; the key component in photosynthesis. Now researchers have made the surprising discovery that oxygen is also produced without sunlight, possibly deep below the ocean surface. There is more going on in the deep, dark ocean waters than you may think: Uncountable numbers of invisible microorganisms go about their daily lives in the water columns, and now researchers have discovered that some of them produce oxygen in an unexpected way.

The study is led by Beate Kraft and Donald E. Canfield from University of Southern Denmark and published in the journal Science. Contributing authors are Nico Jehmlich, Morten Larsen, Laura Bristow, Martin Könneke and Bo Thamdrup. Beate Kraft is an assistant professor at Department of Biology. Her focus is on microbial physiology and biochemistry, and her research is supported by a Villum Young Investigator Grant. Don E. Canfield is Professor of Ecology at the Department of Biology and Danish Institute for Andvance Study chair of Biology.

Oxygen is vital for life on Earth, and is mainly produced by plants, algae and cyanobacteria via photosynthesis. A few microbes are known to make oxygen without sunlight, but so far they have only been discovered in very limited quantities and in very specific habitats.

Enter the ocean living microbe Nitrosopumilus maritimus and its cousins, called ammonia oxidizing archaea.

Ghost organisms hanging out in the dark

"These guys are really abundant in the oceans, where they play an important role in the nitrogen cycle. For this they need oxygen, so it has been a long standing puzzle why they are also very abundant in waters where there is no oxygen," says biologist Beate Kraft, adding:

-"We thought; They just hang out there with no function; they must be some kind of ghost cells."

But there was something puzzling to this;

"These microbes are socommon, that every 5th cell in a bucket of sea water is one of them," adds Don Canfield, co-author of the paper.

So the researchers became curious; could they have a function in the oxygen depleted water after all?

They make their own oxygen

Beate Kraft decided to test them in the lab;

"We wanted to see what would happen if they ran out of oxygen -- like they do when they move from the oxygen rich waters to oxygen depleted waters. Would they survive?"

"We saw how they used up all the oxygen in the water, and then to our surprise, within minutes, oxygen levels started increasing again. That was very exciting!," Don Canfield recalls.

Enough for me and my friends

Nitrosopumilus maritimus turned out to be able to make oxygen in a dark environment. Not much -- not at all so much that it would influence oxygen levels on Earth, but enough to keep itself going.

"If they produce a little more oxygen than they need themselves, it will quickly be taken by other organisms in their neighborhood, so this oxygen would never leave the ocean," Beate Kraft explains.

But what effect do they have on the environment they live in, these extremely abundant oxygen-producing microbes?

New ocean expedition

Researchers already knew that the ammonia oxidizing archaea are microorganisms, that keep the global nitrogen cycle going, but they were not aware of the full extent of their capabilities.

In the newly discovered pathway, Nitrosopumilus maritimus couples the oxygen production to the production of gasous nitrogen. By doing so they remove bioavailable nitrogen from the environment.

"If this lifestyle is widespread in the oceans, it certainly forces us to rethink our current understanding of the marine nitrogen cycle," adds Beate Kraft.

"My next step is to investigate the phenomenon we saw in our lab cultures in oxygen depleted waters in various ocean spots around the world," she adds.

Read more at Science Daily