“The unseen microbial world underpins fisheries, climate, and the very function of ocean ecosystems, and we want to know how those ecosystems are going to shift in the future”
In the world’s oceans, microbes capture solar energy, catalyze key biogeochemical transformations of important elements, produce and consume greenhouse gases, and comprise the base of the marine food web. Microbial ecologists working in Sonya Dyhrman’s laboratory at the Lamont-Doherty Earth Observatory strive to understand the oceans’ ecosystem processes by studying a multitude of creatures, most too small for the human eye to see. These are the tiny microbes called phytoplankton that live beneath the ocean’s surface, taking in carbon dioxide, sunlight, and nutrients to produce oxygen. That oxygen is essential to human survival.
“One of our lab’s mottos is ‘take a breath, thank a phytoplankton,’” said researcher Matthew Harke. Diatoms, a type of microscopic phytoplankton, are estimated to produce one-fifth of the oxygen we breathe.
“The unseen microbial world underpins fisheries, climate, and the very function of ocean ecosystems, and we want to know how those ecosystems are going to shift in the future,” explains Dyhrman.
Among the questions driving Dyhrman’s team is how these all important diatoms survive and thrive under difficult conditions. A study authored by Harke and members of the Dyhrman team published in August in The ISME Journal sheds new light.
The discovery characterizes a symbiotic relationship between diatoms and bacteria.
“We had a feeling this friendly relationship was critical to the diatom’s success, but their relationship was largely uncharacterized,” said Harke.
He and his team describe a function similar to that of human digestion. As the human microbiome is critically important to our health, this research demonstrates that these bacterial partnerships are equally important to phytoplankton survival. The microscopic organisms the team chose to study were a diatom and its symbiotic nitrogen-fixing cyanobacteria partner. Because these organisms are very difficult to culture in the lab, the Dyhrman team had to sail to the middle of the Pacific Ocean, hundreds of miles off the coast of Hawaii, to find them. There, a large region of ocean trapped by currents is known as the North Pacific Subtropical Gyre (NPSG).
This sequestered water forms an ecosystem that tends to be low in resources, such as nitrogen and phosphorous. It is an ocean desert compared with coastal ecosystems that are nutrient rich. The Dyhrman lab has been taking part in an ongoing, collaborative investigation of the NPSG known as the Simons Collaboration on Ocean Processes and Ecology, or SCOPE. The study, funded by a multimillion-dollar grant from the Simons Foundation, engages scientific partners around the world. The team sifted through thousands of liters of seawater, which they collected over four days in July 2015.
“Because the organisms are microscopic, we used molecular tools to explore their interactions. From the filtered water, we sequenced all of the genes that were turned on and off by this partnership and used a supercomputer to reconstruct how their metabolisms were intertwined over day-night cycles,” explained Harke.
Importantly, the research suggests that these bacterial “friends” can help diatoms survive the harsh conditions of the open oceans where plant nutrients are scarce. For instance, the research witnessed shared gene expression patterns indicative of sharing of resources needed for growth. The bacterium in this partnership is able to capture nitrogen gas and convert it into a useable form, feeding the diatom much-needed nitrogen in exchange for protection (diatoms have a glass-like shell) and carbon. The research also found genetic evidence for how the two organisms stay together and reproduce.
“With this study, we have provided a new view of how this partnership works, providing insight into a baseline that we’ll need to study to predict what to expect in a future ocean,” said Harke. The NPSG contains one of the largest biomes on Earth. “As climate continues to change, these oligotrophic (nutrient starved) oceans are predicted to expand, likely making these partnerships more important.”