Scientists have found that an increase in water salinity in the cells of the marine diatom Nitzschia weakens the connections between the components of the photosynthetic apparatus and disrupts the formation of the cell shell. The authors were able to track these changes using a wide range of advanced photonic methods that provide comprehensive information about the condition and functional properties of diatoms. Thanks to their valuable silica shells, diatoms are widely used in the food and beverage industries, as well as in the treatment of drinking water and wastewater. Diatomite — the fossilized remains of the shells — is used as a natural sorbent in filtration systems. The research, supported by a grant from the Russian Science Foundation (RSF), was published in Scientific Reports.

An essential component of aquatic communities, the single-celled microscopic diatoms sequester about 20% of the planet’s carbon dioxide, form the core of marine food chains, and synthesize and accumulate various chemical compounds, primarily, silicon derivatives — the main component of diatom shells called frustules. Although the shape and structure of the frustule vary from one diatom species to another, the frustule itself is a rather complex and well-organized structure capable of withstanding heavy loads. This makes the diatom frustule a perfect model for high-strength nanostructured materials and components of sensors for medicine and microelectronics.

Researchers from the Skolkovo Institute of Science and Technology in Moscow and their colleagues from leading Russian universities and research centers have identified the effect of water salinity on the Nitzschia diatoms, which inhabit oceans, salt lakes, and freshwater reservoirs. In nature, however, the salinity varies widely among different species of diatoms, from 0 (diatoms can exist for some time even in distilled water) to more than 150‰, when salt deposition begins. The researchers studied how Nitzschia adapts to changes in salinity from 10 to 150‰. For comparison, the salinity of the Red Sea, the saltiest sea on Earth, is 41‰ and reaches 350‰ in some hypersaline environments. By choosing this range of salinities, the researchers were able to model stress conditions for the algae.

For the first time in the study of diatoms, the team used advanced methods, such as laser scanning microscopy, time-resolved fluorescence microscopy, photoacoustic imaging, and transmission electron microscopy, to obtain images of the cells and their subcellular structures, called organelles, with the required resolution and contrast.

Using laser scanning microscopy, the researchers discovered that larger lipid droplets form in the cells of the diatom when it is stressed by low or high salinity. Under these adverse conditions, the droplets store carbon and energy and deposit fatty acids for lipid synthesis. When the salinity is high, the lipids accumulated in the droplets help to keep the membrane intact and prevent it from rupturing due to pressure imbalance. The size of a lipid droplet, which was about 1 micron at 40‰, increased to 2.3 microns at 10‰ or 150‰. The pattern of silicon accumulation in the valves, and thus valve formation, also changed when the diatoms were exposed to stress. The largest anomalies in the frustule structure occurred at 60‰.

The team used a combination of time-resolved fluorescence microscopy and rapid fluorescence induction to study the effect of salinity on energy and electron transport in the cells. Looking at the way chlorophyll — a green pigment involved in photosynthesis — interacts with light, the researchers found that an increase in the cell salinity alters the processes of absorbed energy conversion. It turned out that at 80‰, the transfer of energy and electrons between the components of the photosynthetic system was the slowest, because fluorescence and heat losses accounted for a large proportion of the absorbed energy.

In addition, researchers at the Saratov National Research University discovered that as salinity increases, diatom pigments become more active in absorbing light and converting its energy into ultrasonic vibrations — an effect largely due to an increase in the concentration of chlorophyll a and other pigments.

Using transmission electron microscopy, the team found that salinity affects the structure of the diatom’s polysaccharide layer located between the frustule and the cell membrane. This organic shell protects the cell, while contributing to the integrity and formation of the frustule. In cells grown at 20‰, this shell is virtually invisible, while at 40 ppm it appears as a thin layer next to the valve, and at 60‰ it reaches its maximum size.

Overall, the team showed that diatom cells grow at about the same rate across a wide range of salinities, allowing diatom cells to live in different bodies of water.

“Understanding how water salinity affects diatoms will potentially help select optimal conditions for their growth in bioreactors used to produce biogenic nano- and microstructured silica, bioactive compounds, and biofuels. In addition, diatoms can serve as indicators of changes in water salinity and as a biological sensor for monitoring the effects of climate change on marine biodiversity,” says Dmitry Gorin, a professor at the Skoltech Photonics Center and the head of the project supported by an RSF grant.

Other organizations involved in the study include the Limnological Institute of the Siberian Branch of RAS in Irkutsk, the T.I. Vyazemsky Karadag Scientific Station in Feodosia, and the Lomonosov Moscow State University.

Figure: Time-resolved fluorescence microscopy image of diatoms at different salinities. Credit: Dmitry Gorin.