Bacteria Discovered as Primary Driver of Calcium Carbonate Dissolution in Marine Snow
New research has identified bacteria as the primary mechanism responsible for the dissolution of calcium carbonate within marine snow particles in the upper ocean. This discovery clarifies previously unexplained observations of calcium carbonate breakdown in shallow waters, challenging long-held assumptions about its stability.
The findings suggest that bacterial activity, by eroding this crucial mineral ballast, can slow the sinking rate of marine snow, potentially reducing the efficiency of the ocean's carbon sequestration and necessitating adjustments to climate models.
Understanding Marine Snow and Carbon Sequestration
Marine snow consists of aggregates of organic material, including dead plankton, animal waste, and decaying organisms, that drift through ocean waters. This process is integral to the ocean's "biological carbon pump," a natural mechanism that transports atmospheric carbon dioxide from the surface to deeper ocean layers.
Phytoplankton at the ocean's surface absorb atmospheric carbon dioxide, converting it into biomass and, in some cases, calcium carbonate shells. Upon the death of these organisms, their remnants, including calcium carbonate, become part of marine snow and descend through the water column.
If these particles reach the deep ocean, the carbon they carry can be stored for hundreds to thousands of years, a process known as carbon sequestration. Calcium carbonate plays a role as a dense mineral ballast, contributing to the faster sinking of these particles.
Resolving the Calcium Carbonate Dissolution Anomaly
Historically, scientists had assumed that calcium carbonate would largely remain solid in the upper ocean layers, based on general temperature and pH conditions. However, oceanographers have frequently observed signs of dissolved calcium carbonate in shallow waters, indicating an unknown dissolution process occurring at shallower depths than expected.
Bacterial Role in Dissolution Identified
A study conducted by researchers from Rutgers University-New Brunswick, the Massachusetts Institute of Technology (MIT), and Woods Hole Oceanographic Institution has identified bacteria as the primary factor driving this dissolution. The research indicates that significant microbial activity occurs within marine snow particles.
As bacteria consume the organic material within these particles, they release acidic waste products through respiration. These waste products create localized acidic microenvironments around the particles, which then accelerate the dissolution of inorganic calcium carbonate.
Benedict Borer, an assistant professor of marine and coastal sciences at the Rutgers School of Environmental and Biological Sciences and lead author of the study, stated that what happens in microscopic particles can control bulk seawater chemistry, with consequences for the ocean's carbon dioxide sequestration capacity.
Experimental Confirmation
To investigate this process, the research team constructed a three-layer microfluidic chip designed to simulate a sinking marine snow particle and its microscale interactions. The chip incorporated marine particles with calcite (a form of calcium carbonate) and marine bacteria in a middle layer, with artificial seawater flowing through a narrow channel to replicate particle descent.
By manipulating gas pressure, temperature, oxygen levels, and bacterial abundance, the team recreated conditions within a sinking particle. Experiments revealed that particles hosting bacteria rapidly lost some calcium carbonate, which dissolved into the surrounding seawater.
Sinking Speed: A "Sweet Spot" for Dissolution
The study also found that the rate of calcium carbonate dissolution was dependent on the particle's sinking speed:
- Slow Sinking: Limited oxygen supply reduced bacterial activity, consequently limiting dissolution.
- Fast Sinking: While providing oxygen for bacteria, the rapid water flow flushed away acidic waste products before they could significantly dissolve the calcium carbonate.
- Intermediate Sinking: This speed provided sufficient oxygen for bacteria and allowed their waste products to accumulate around the particle, leading to efficient dissolution of calcium carbonate. This was identified as a "sweet spot" for dissolution.
Implications for Carbon Cycling and Climate Models
The dissolution of calcium carbonate, which acts as a ballast, reduces the density of marine snow particles, causing them to sink more slowly. This slower descent increases the time particles spend in shallow waters, raising the likelihood that they are respired, which would release carbon dioxide back into the shallow ocean and potentially the atmosphere. This process may decrease the overall efficiency of carbon sequestration in the deep ocean.
Andrew Babbin, a study co-author and associate professor at MIT, highlighted the necessity of considering these natural microbial mechanisms in climate solutions. The findings suggest that carbon may not sink as deep or as fast as previously anticipated.
Researchers emphasize that understanding these microbially driven alterations in marine snow is crucial for accurately predicting ecosystem responses to marine carbon dioxide removal efforts and future climate scenarios. The study indicates that the role of bacteria must be integrated into future climate models and considered in strategies aimed at enhancing the ocean's biological pump.
The research was published in the Proceedings of the National Academy of Sciences.