For decades, marine chemists have tried to solve the “ marine methane paradox”: The surface waters of the world's oceans are supersaturated with methane, but where is it coming from? Well, this riddle may have finally been cracked.
According to geochemist Dan Repeta from Woods Hole Oceanographic Institution, the answer may lie in how bacteria break down dissolved organic matter, a cocktail of substances excreted into seawater by living organisms.
Much of the ocean's dissolved organic matter comprises novel polysaccharides–long chains of sugar molecules created by photosynthetic bacteria in the upper ocean. Bacteria begin to break these polysaccharides, tearing out pairs of carbon and phosphorus atoms (called C-P bonds) from their molecular structure. In the process, methane, ethylene and propylene gasses are created as byproducts.
"All the pieces of this puzzle were there, but they were in different parts, with different people, in different labs, at different times," said Repeta. "This paper unifies a lot of those observations."
Prior to this study, Repeta had suspected that microbes were involved in creating methane in the ocean, but were unable to identify the exact ones responsible.
In 2009, one of Repeta's co-authors, David Karl added a manmade chemical called methylphosphonate, which is rich in C-P bonds, to seawater samples. As he did, bacteria in the samples started making methane. Since methylphosphonate had never been detected in the ocean, the bacteria in the wild must be utilising another natural source of C-P bonds. Exactly what that source was remained elusive.
An analysis of the samples of dissolved organic matter from surface waters in the northern Pacific showed that the polysaccharides within it possessed C-P bonds identical to the ones in methylphosphonate; if bacteria could break down those molecules, they might be able to access the phosphorus within it.
"That made us think it's a two-part system. You have one species that makes C-P bonds but can't use them, and another species that can use them but not make them," said Repeta.
Repeta and co-author Edward DeLong, a microbial oceanographer at the University of Hawaii, then explored how bacteria metabolised dissolved organic matter. Using metagenomics, DeLong catalogued the genes he found in a seawater sample. He then found genes responsible for breaking apart C-P bonds, which would allow bacteria to extract phosphorus from carbon atoms. If these genes were active, they would give an organism access to an important but rare nutrient in seawater.
"The middle of the ocean is a nutrient-limited system," said Repeta. "To make DNA, RNA, and proteins, you need nitrogen and phosphorus, but in the open ocean, those nutrients are at such low concentrations that they're almost immeasurable." DeLong's study showed that the microbes are able to crack into nitrogen and phosphorus hidden deep inside organic molecules.
Although it is possible for bacteria to break apart C-P bonds, it's not easy to do so, with phosphorus tied up in organic molecules. So if they can find other sources of the nutrient, they will use those first.
Repeta elaborated on this, using the analogy of a buffet: "If you're a microbe, inorganic nutrients are like fruits and meats and all the tasty stuff that you reach for immediately. Organic nutrients are more like leftover liver. You don't really want to eat it, but if you're hungry enough, you will. It takes years for bacteria to get around to eating the organic phosphorus in the upper ocean. We don't exactly know why, but there's another really interesting story there if we can figure it out."