At night, on a swaying deck thousands of kilometers from land, with the salt spray in one’s eyes, one truly understands the concept that all life on Earth – from the tiniest marine bacterium to humans – is afloat on the same small boat. Just ask some of the 30 scientists who recently participated in a one-month Atlantic research cruise to investigate the life cycles of the trillions of tiny, single-celled, plant-like creatures that float in the ocean’s waters. These creatures, called phytoplankton, are the basis of the marine food chain: Without them, there would be no life in the oceans and not enough oxygen to support life on our planet.
After setting out from Ponta Delgada in the Azores, the ship headed north and continued up the middle of the Atlantic to Reykjavik, Iceland. One of the five research teams aboard the ship, from the Weizmann Institute’s Plant Sciences Department, was led by Dr. Assaf Vardi
. Says Vardi, the voyage was more than just a research trip to investigate the role of phytoplankton in the environment – it was also a social experiment: “Take a few dozen people from different countries, pack them into a small space and stir; place them in the middle of the ocean and then add a bit of tension and a good dollop of scientific curiosity. And what do you get?”
Of algae and people
Phytoplankton can sometimes, for unknown reasons, multiply very rapidly. This causes algal blooms that can extend for hundreds, and even thousands, of kilometers – large enough to be seen from space. And then, all at once, the bloom collapses and disappears. Understanding how these blooms grow and collapse is critical: The oceans’ phytoplankton absorb as much carbon dioxide as the world’s rain forests, so they could be important allies in the effort to reduce global warming; and the sustenance they supply to the marine food chain is vital for keeping the world’s population fed.
Vardi and his colleagues had previously discovered that viruses are responsible for algal bloom collapse. Like viruses everywhere, these invaders insert their genes into their host’s cells. In the case of those that attack the phytoplankton, these virus genes produce enzymes that, in turn, produce fat molecules called sphingolipids. Interestingly enough, these sphingolipids are very similar to certain human ones that have been implicated in a number of diseases, but they had never been found to be produced by viruses. Vardi: “These enzymes and mechanisms have been conserved throughout evolution, and they function in an extremely wide variety of organisms.”
Vardi had discovered a cellular mechanism in the phytoplankton that senses the accumulation of sphingolipids and initiates a programmed cell death sequence called apoptosis. Apoptosis is well-known in multicellular organisms: Damaged or used-up cells take their own lives for the good of the whole. But what use is cell suicide to a single-celled organism? How do the cells withing the phytoplankton community communicate so that some survive, even as others lose their lives? How does this affect the ecology of the oceans as a whole? These are a few of the questions that the researchers on board have been attempting to clarify.
According to recent findings of Vardi and others, phytoplankton are actually not so keen on the idea of suicide, and they at first resist the invading tide of viruses with an entire arsenal. In a dramatic evolutionary arms race, they fight with chemical weapons, military maneuvers, head-on attacks and evasive end runs – all conducted on the molecular level within the phytoplankton cells. Multiplied by billions of cells, such molecular processes can determine the very course of the earth’s bio-geochemical pathways and, ultimately, the climate. And the fact that these pathways have been preserved throughout evolution means that researchers can study them in phytoplankton and apply them to humans. For instance, certain substances these marine organisms use to fight viral invasion might be effective against those that cause disease in humans, while research into the mechanisms of cell death in these simple creatures could lead to new ways of initiating cell death – in cancer cells, for instance.
On the briny, in search of a bloom
On the ship’s deck, the research teams brought up water samples from different depths. These were tested in on-board research labs. Vardi: “For two weeks, we sailed without seeing a single bloom. And then we found one. That was an unforgettable moment of relief, on the one hand, and a shot of new energy, on the other, to go back out and keep working 20-hour days to continue our research efforts.”
In addition to these studies, Vardi’s team was involved in a unique collaboration with a group in the Institute’s Environmental Sciences and Energy Research Department to investigate the feedback cycle that includes marine biology (especially algal blooms) and tiny particles called aerosols in the atmosphere that affect cloud formation. Though highly complex, this feedback cycle could play a significant role in climate change, and understanding it might clear up a few open questions in the field.
“This effort,” says Vardi, “was the first of its kind to focus on all levels simultaneously, from molecular processes within the cell to cellular processes and up to the behavior of whole populations spread over thousands of kilometers. Working right alongside the ocean’s surface, accompanied by whales – this was a one-time experience. Every day, there was a brand-new sunset, until we neared the Arctic Circle and experienced the midnight sun there. Those are things that none of us will ever forget.”
Meanwhile, back in his Weizmann lab, researchers have already begun experiments on the thousands of samples now stored there. In these samples, they are searching for clues to the mechanisms of life, to cycles in the environment, to the ways that tiny, single-celled creatures can drive the earth’s natural cycles.
Dr. Assaf Vardi’s research is supported by Charles Rothschild, Brazil; Roberto and Renata Ruhman, Brazil; Luis Stuhlberger, Brazil; and the European Research Council. Dr. Vardi is the incumbent of the Edith and Nathan Goldenberg Career Development Chair.