As climate change pushes heat into deeper layers of ocean water, the scientific community has been concerned about upsetting the delicate balance of ocean life. But a new study suggests that a key deep-sea microbe—the marine nitrifying bacterium Nitrosopumilus maritimus—may have been quietly adapting to this warmer, nutrient-poor environment.
Under the combined effects of global warming and frequent marine heat waves, warming is no longer limited to the surface of the ocean. Seawater as deep as a thousand meters is also warming, raising concerns about disturbances to the ocean's chemical environment and ecosystems. However, a research team from the University of Illinois at Urbana-Champaign and other institutions pointed out that archaea such as Nitrosopumilus maritimus, which relies on iron and lives by oxidizing ammonia, seem to be adjusting their physiological strategies to adapt to the dual pressures of higher temperatures and lower metal supplies. Researchers believe that in an ocean that continues to warm, they are likely to play an increasingly important role in reshaping ocean nutrient distribution patterns.
Relevant results were recently published in the Proceedings of the National Academy of Sciences (PNAS). Nitrosopumilus maritimus and its related microorganisms account for about 30% of the total marine microbial plankton and are widely regarded as key players in maintaining the chemical balance of the ocean. By oxidizing ammonia in seawater, these archaea are involved in converting nitrogen into different chemical forms, thereby affecting the growth of entire microbial planktonic communities, which are the basis of marine food webs and are critical to marine biodiversity.
Wei Qin, corresponding author of the study and a professor of microbiology at the University of Illinois at Urbana-Champaign, said that in the past, it was generally believed that seawater below 1,000 meters was largely "isolated" from the effects of surface warming, but it is now increasingly clear that deep-sea warming is changing the way these abundant archaea use iron. Iron is a metal element on which they are highly dependent for their metabolic processes, and this change may further affect the availability of trace metals in the deep sea, with consequences for broader marine biogeochemical processes.
In order to be as close as possible to the real marine environment, the research team conducted a series of temperature and iron concentration gradient experiments on a pure culture of Nitrosopumilus maritimus under strictly controlled experimental conditions for metal contamination. The results showed that when iron supply was limited, rising temperatures not only did not weaken the survival ability of this type of microorganisms, but instead prompted them to reduce their demand for iron and improve their iron utilization efficiency. This shows that as seawater warms and available iron decreases, Nitrosopumilus maritimus has a certain ability to "self-regulate" and can maintain or even optimize metabolic activities in a more resource-constrained deep-sea environment.
On the basis of experiments, the team teamed up with Alessandro Tagliabue, an expert in marine biogeochemical modeling at the University of Liverpool, to incorporate these physiological data into the global ocean biogeochemical model for simulations. Simulation results show that in vast iron-limited sea areas, deep-sea archaeal communities may not only not "retreat" under future warming scenarios, but will have the ability to maintain or even strengthen their role in ocean nitrogen cycle and primary production support. In other words, in many deep-sea areas that were originally considered fragile, these tiny creatures may become an "adaptive force" that maintains ocean functions.
To test whether the laboratory findings also apply to real ocean systems, Qin Wei and David Hutchins, a professor of global change biology at the University of Southern California, will co-lead an offshore scientific expedition this summer. They will board the research vessel Sikuliaq, departing from Seattle, passing through the Gulf of Alaska, and then sailing to the subtropical gyre area, stopping in Honolulu, Hawaii. Twenty scientists from multiple institutions will carry out on-site observations and sampling on board the ship, focusing on assessing how archaeal communities in the natural environment respond and adjust under different combinations of temperature and metal restrictions.
The research team emphasized that this work is not only an examination of the adaptability of a single species, but also related to the entire ocean nitrogen cycle, trace metal cycle, and the "resilience" of deep-sea ecosystems in the context of climate change. If key microorganisms like Nitrosopumilus maritimus are indeed able to increase iron utilization efficiency and remain active in warmer deep-sea environments, then to some extent they may buffer some of the chemical imbalances caused by warming, buying precious time for marine ecosystems to adjust. But scientists also remind that this does not mean that the risks brought by climate change can be ignored. Human control of greenhouse gas emissions is still the fundamental means to protect the health of oceans and global ecosystems.
According to reports, the research was jointly funded by the National Science Foundation, the Simons Foundation, the National Natural Science Foundation of China, the University of Illinois at Urbana-Champaign and the University of Oklahoma. One of the study leaders, Qin Wei, is also affiliated with the Carl Wuth Institute for Genomic Biology. The relevant team stated that it plans to conduct long-term observations in more sea areas and different seasons in the future to further clarify the evolution path of the role of deep-sea archaea in the warming ocean.