Cellular Processes behind Oxygen Production in Phytoplankton Revealed by Scientists

take a deep breath. Now take another nine. According to new research, the amount of oxygen in one of these 10 breaths is made possible by a newly identified cellular mechanism that promotes photosynthesis in marine phytoplankton.

Described as “groundbreaking” by a team of researchers at UC San Diego’s Scripps Institution of Oceanography, this previously unknown process accounts for 7% to 25% of all produced oxygen and fixed carbon in the ocean. When also considering the process of photosynthesis that occurs on Earth, the researchers estimated that this mechanism could be responsible for generating up to 12% of the oxygen on the entire planet.

Scientists have long recognized the importance of phytoplankton—microscopic organisms that drift in aquatic environments—due to their ability to photosynthesize. These tiny oceanic algae form the base of the aquatic food web and are estimated to produce about 50% of the oxygen on Earth.

The new study published in Current Biologyoutlines how the proton pumping enzyme (known as VHA) aids in global oxygen production and carbon fixation by phytoplankton.

“This study represents a breakthrough in our understanding of marine phytoplankton,” said lead author Daniel Ye, who conducted the research while earning his Ph.D. Student at Scripps University of Oceanography and currently working as a joint postdoctoral researcher at the European Laboratory of Molecular Biology and the University of Grenoble-Alpes in France.

“Over millions of years of evolution, these tiny cells in the ocean carry out delicate chemical reactions, in particular to produce this mechanism that promotes photosynthesis, which has shaped the course of life on this planet.”

Working closely with Scripps physiologist Martín Tresguerres, one of his associate advisors, and other collaborators at Scripps and Lawrence Livermore National Laboratory, Yee revealed the complex inner workings of a particular group of phytoplankton known as diatoms, which are famous for their single-celled algae for ornamental walls made of silica.

Understanding the “proton pump” enzyme

Previous research in Tresguerres’ lab has worked to determine how VHA is used by a variety of organisms in processes critical to life in the oceans. This enzyme is found in almost all forms of life, from humans to unicellular algae, and its primary role is to modulate the pH level of the surrounding environment.

“We imagine proteins as Lego blocks,” explains Tresguerres, co-author of the study. “Proteins always do the same thing, but depending on what other proteins they’re paired with, they can fulfill a completely different function.”

In humans, the enzyme helps the kidneys regulate blood and urine functions. Giant clams use the enzyme to dissolve corals, where they secrete acid that digs holes in the corals to take shelter.

Corals use the enzyme to boost photosynthesis through their symbiotic algae, while deep-sea worms known as Osedax use it to dissolve the bones of marine mammals, such as whales, so they can consume them. The enzyme is also found in the gills of sharks and rays, where it is part of a mechanism that regulates blood chemistry. And in the eyes of the fish, the proton pump helps deliver oxygen, which enhances vision.

Given this earlier research, Ye wondered how the VHA enzyme might be used in phytoplankton. He set out to answer that question by combining high-tech microscopy techniques in the Tresguerres lab with genetic tools developed in the Scripps lab, the late scientist Mark Hildebrand, who was a leading expert on diatoms and one of Yee’s co-consultants.

Using these tools, he was able to mark the proton pump with a green fluorescent marker and precisely locate it around the chloroplasts, which are known as “organelles,” or specialized structures within diatom cells. The chloroplasts of diatoms are extra membrane-enclosed compared to other algae, enveloping the space where carbon dioxide and light energy are converted into organic compounds and released as oxygen.

“We were able to generate these images that show the protein of interest and where it is located inside a cell with many membranes,” Ye said. Combined with detailed experiments to quantify photosynthesis, we found that this protein actually enhances photosynthesis by providing more carbon dioxide, which is what chloroplasts use to produce more complex carbon molecules, such as sugars, while also producing more oxygen as a measure. secondary. product.”

Evolution connection

Once the basic mechanism was established, the team was able to relate it to multiple aspects of development. Diatoms were derived from a symbiotic event between a protozoan and an algae about 250 million years ago that culminated in the fusion of the two organisms into one, known as a symbiosis.

The authors highlight that the process of one cell consuming another, known as phagocytosis, is pervasive in nature. Phagocytosis relies on the proton pump to digest the cell, which serves as a source of food. However, in the case of diatoms, something special happened as the ingested cell was not fully digested.

“Instead of one cell digesting the other, acidification by the predator’s proton pump enhanced photosynthesis by ingested prey,” said Trisgers. “Over evolutionary time, these two separate organisms merged into one, which we now call diatoms.”

Not all algae possess this mechanism, so the authors believe this proton pump gave diatoms an advantage in photosynthesis. They also note that when diatoms arose 250 million years ago, there was a huge increase in oxygen in the atmosphere, and a newly discovered mechanism in the algae may have played a role.

The majority of fossil fuels extracted from the Earth are believed to have originated from the transformation of biomass that sank to the ocean floor, including diatoms, over millions of years, creating oil reserves.

The researchers hope their study will provide inspiration for biotechnological approaches to improve photosynthesis, carbon sequestration, and biodiesel production. In addition, they believe it will contribute to a better understanding of global geochemical cycles, environmental interactions, and the effects of environmental fluctuations, such as climate change.

“This is one of the most exciting studies of symbiosis in the past decades and will have a huge impact on future research around the world,” said Trisgers.