Nature today | Peat algae’s surprising nightlife

In the largely empty tropical sea, coral reefs can still thrive. This is due to photosynthesis of algae and corals. Algae and corals use about half of the converted solar energy to grow and multiply. Some of the residual energy is stored in dissolved organic substances and is eventually released into the water. These precious resources are not lost. They are food for bacteria, which in turn are eaten by larger animals. This energy recycling process is called microbial loop named. In general, the release of dissolved organic substances is greatest during the day. At night, when no photosynthesis takes place because it is dark, the release drops sharply.

An unexpected discovery

“During my PhD research, I investigated the effect of light on the release of dissolved organic substances in various corals and algae. As expected, the release dropped sharply in the dark. In addition, more dissolved organic substances were released at high light intensity than at “moderate light intensity,” explains Benjamin Mueller. “But when he looked at peat algae, he made the surprising discovery that they had the highest release in the dark.” At first I was worried that we had made a mistake. We thought the samples had been contaminated or that something had gone wrong with the chemical analysis. “

What appeared to be a mistake turned out to be a very important discovery. It is now believed that the high nocturnal release in peat algae is likely caused by cyanobacteria, which can ferment in the dark and release organic compounds in the process. Cyanobacteria are a growing part of grassroots communities. A diverse team of researchers from the Netherlands, Germany and the United States gathered to investigate the phenomenon.

Quality over quantity

The team set up an experiment to measure the release of dissolved organic matter from peat algae during the day and night. The so-called day substances and night substances were then fed to heterotrophic bacteria to see how effectively they use the substances to grow. The results show that the bacteria absorb the nocturnal substances much faster, but that they are not able to use these substances effectively for growth. Instead, they exhale and CO2 handed over. The day materials are of higher quality. They are absorbed much more efficiently and converted into bacterial biomass, which benefits the reef. “These newly grown bacteria can then be eaten by filter feeders and other bacterial predators, bringing energy and nutrients – originally stored in dissolved organic matter – into the classic food chain,” explains Mueller.

Change over time

Curaçao’s coral reefs have changed dramatically in recent decades. At first they consisted mainly of corals and calcifying algae, now they consist more of fleshy algae and peat algae containing cyanobacteria. The team wanted to compare the day and night release of dissolved organic compounds in the present with the release a few decades ago. Mueller: “Our calculations show that in the past, about two-thirds of DOCs were released during the day and only one-third at night. Today, up to half of the DOCs can be released at night.” This change affects the entire reef system: the substances released at night by bacteria are exhaled and lose precious energy. These nocturnal substances therefore do not end up in the original food chain.

Future perspectives

Mueller predicts that the relative proportion of dissolved organic compounds released at night is likely to increase in the future. He expects the amount of mat-forming cyanobacteria and algae communities to increase as a result of climate change. Mueller and his team will continue to study the peat algae and map the chemical composition of the dissolved organic substances released during the night. They will also focus on the changes in the bacterial community and the effect on other important organisms that consume dissolved organic substances, such as fungi.

More information

Text: IBED
Photos: Benjamin Mueller (main photo: healthy coral reef with corals and calcifying algae)

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