Marine snow viscosity regulates microbial degradation and the ocean carbon sink.

Bacterial degradation of marine snow aggregates is a major component of global carbon cycling. Whether aggregate carbon is released in the upper ocean or is sequestered at depth depends on its sinking speed and degradation rate. However, little is known about the physical constraints of bacterial colonization and degradation of individual aggregates. Using molecular rotors to measure the nanoscale viscosity field of natural aggregates, we show that aggregates are highly structured microhabitats with less-viscous regions that are accessible for colonization and more-viscous regions that correlate to a barrier to bacterial infiltration. By quantifying the viscosity degradation rate, we demonstrate that more viscous aggregates take longer to degrade, sink farther, and contribute more to carbon sequestration, revealing a microscale physical constraint of the global ocean carbon pump.