Much remains unknown about the deep sea, but scientists do know that it’s important to the global carbon cycle. Just as forests capture carbon dioxide and store it in the soil as dead trees, or biomass, carbon can persist in the deep sea for thousands of years, delaying its return to the atmosphere. This carbon—in the form of biomass from fish, plankton, and other dead creatures—drifts slowly down from the sunlit surface until it reaches the abyssal depths as a microscopic, ashy rain known as marine snow.
As marine snow falls, microbes consume the tastier and easier-to-eat compounds first. The leftover carbon molecules that reach the deep sea consist of a tough scaffolding called recalcitrant carbon, which takes more energy and specialized tools for microbes to break down. Yet scientists in the past have shown that deep-sea microbes grow extremely slowly. Therefore, researchers don’t know whether microbes leave this recalcitrant carbon alone because it’s difficult to digest, or because they lack the energy to even try.
But what if you could feed these microbes a boost of energy by giving them a tasty carbon source like sugar, providing a proverbial all-you-can-eat buffet? Would this bonus allow them to consume more carbon in the deep sea?
That’s exactly what researchers from McGill University and Rutgers University did to test a hypothesis called the priming effect. Scientists have shown that in soils, adding easily digestible carbon increases the microbes’ appetite for difficult-to-degrade biomass, so they wondered if the same would hold for microbes in the deep sea. The researchers collected water from a depth of 2,500 meters (or approximately 8,200 feet). This water was approximately the temperature of your freezer and had been in the deep ocean for approximately 1,400 years, so any carbon it contained was made of recalcitrant marine snow.
The researchers added carbon-, nitrogen-, and phosphorous-containing priming compounds, including glucose, amino acids, and glucose-6-phosphate, to some of the samples. They determined which nutrients helped the microbes grow the most by comparing these samples to control samples with no priming compounds. They measured the concentration of naturally available carbon in the water before and during the experiment to track whether the microbes ate more of it after their initial snack. They also tracked how the microbial communities changed throughout the experiment by counting the number of cells and analyzing their DNA sequences.
The team found that adding the priming compounds helped the microbes grow, but it did not encourage them to consume recalcitrant carbon already present in the seawater. By the end of the experiment, the amount of natural carbon in samples with added nutrients was roughly the same as in control samples without added nutrients, suggesting that no priming effect had occurred.
They also observed that mixed amino acids were consumed faster than other priming compounds, with about 7 times more microbial growth occurring in samples with added amino acids. Amino acids contain nitrogen, so the researchers interpreted this to mean that the microbes were more starved of nitrogen than carbon or phosphorous. They were surprised by this result, since the concentration of phosphorous in the deep sea is far lower than the concentration of nitrogen, so they expected the microbes to be more phosphorus-starved.
The researchers also found fewer different types of microbes in samples where priming compounds were added. They suggested the microbes that consumed priming compounds fastest became dominant, pushing the population towards a few big winners rather than many diverse inhabitants.
The team proposed several possible explanations for why giving the microbes an energy boost didn’t help them eat any deep-sea carbon. First, the microbes could have been more competitive with each other as they fought over the added nutrients, focusing on fast growth rather than fiddling around with recalcitrant carbon.
Alternatively, these carbon compounds could be extremely resistant to degradation, leaving the microbes simply unable to eat them. Over 100,000 different forms of carbon are present in the deep-sea, so even if a microbe specialized in eating one kind, it still might not make a difference to the total amount of carbon.
The researchers concluded that deep-sea microbes’ reluctance to eat recalcitrant carbon is not due to their lack of energy. This suggests that even if more carbon were added to the deep sea–which some people have proposed as a way to combat climate change–it wouldn’t kick-start any additional microbial munchies. That suggests the deep ocean is a stable place, which is good news, since it stores carbon equivalent to the amount of carbon dioxide in Earth’s atmosphere!
