Scientists Discover New Source of Natural Fertilizer in Oceans

New findings suggest that the deep ocean is teeming with organisms that produce essential natural fertilizers. A research team including David Karl and Andrew Hansen of the Department of Oceanography at the University of Hawaiʻi has discovered a previously unknown type of photosynthetic bacteria that fixes nitrogen, converting nitrogen from the atmosphere into a form other organisms can use. The researchers, led by Jonathan P. Zehr of the University of California, Santa Cruz, are publishing their findings in the August 9 issue of the journal Nature.

"This is a previously unknown group of bacteria that are capable of manufacturing the nitrogen that they need for the synthesis of proteins and nucleic acids from nitrogen gas in the atmosphere around them — literally growth ʻout of thin air‘," said Karl, who led the UH team of researchers, which included a graduate student, on this study.

Although nitrogen accounts for nearly 80 percent of the Earth‘s atmosphere, most organisms can use it only when it is "fixed" to other elements, to make compounds like ammonia or nitrate. As a component of proteins, nitrogen is essential to all known forms of life.

The research team found the nitrogen-fixing bacteria, which they have grown in the laboratory, in water samples collected from the Pacific Ocean at the Hawaiʻi Ocean Time-series Station ALOHA, the HOT‘s deep-water station located about 100 km north of Oʻahu. The organisms appear to belong to the genus Synechocystis, a group of cyanobacteria (photosynthetic bacteria formerly known as blue-green algae) that includes both marine and freshwater species. The newly discovered nitrogen fixers appear to be active at greater depths and over longer time periods than other marine cyanobacteria known to fix nitrogen in the open ocean.

On land, nitrogen-fixing bacteria are a known quantity, residing in the roots of legumes like peas and beans. But in the ocean, they are something of a mystery. While many nitrogen-fixing cyanobacteria have been found in coastal waters, very few are known to occur in the open ocean. In both environments however, these nitrogen-fixing microorganisms can influence the fertility of the environments where they are found with implications for all the major cycles of bioelements on earth, especially the carbon cycle of the sea, according to Karl.

"Just as in agricultural applications where farmers fertilize their fields with fixed nitrogen to make crop growth more efficient especially with higher yields, the presence of these microorganisms in the sea can influence the food web with implications all the way to fishery yields," said Karl.

It also has implications for global warming because nitrogen stimulates the growth of marine algae, which absorb carbon dioxide from the atmosphere.
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Over the past 12 years, Zehr has uncovered evidence of dozens of nitrogen-fixing bacteria by looking not for the organisms themselves but for their DNA fingerprints—specifically, for a gene encoding the protein responsible for nitrogen fixation.

The nitrogen-fixing marine Synechocystis is the first of these organisms Zehr‘s team has succeeded in cultivating in the laboratory. It was isolated from samples collected at Station ALOHA.

Karl now thinks that their relative importance may be increasing near Hawaiʻi over the past decade due to habitat changes that are imposed by the major climate cycles and their interactions with the sea near Hawaiʻi.

"Rather than relegating this key ecological process to a few well known, relatively large microbes (millimeters in length), we have discovered that this process may be commonplace in the smallest of the marine plankton, microbes only a few micrometers in length," said Karl. "Because of their small size, they have gone unnoticed and unappreciated for the critical role that they apparently play in ocean fertility and productivity."

The new bacteria are abundant as deep as 100 to 200 meters below the surface, compared to 50 meters for the most productive known nitrogen fixer. And unlike the previously known nitrogen fixers, which are only active in warm seasons, the new bacteria have shown activity in February, Zehr said. "They are likely to be there, fixing nitrogen, over wider scales than was previously thought," he said.

This is the third major discovery of a fundamentally new group of microorganisms at the HOT Station ALOHA in just the past year. Previous research results from this ongoing 13-year program have also appeared in Nature.