Depth-dependent bacterial colonization on model chitin particles in the open ocean.

Sinking particles transport carbon from the surface to the deep ocean. Microbial colonization and remineralization are important ecosystem services constraining ocean biogeochemistry by recycling and redistributing nutrients from the surface to the deep ocean. Fragmentation of particles by zooplankton and the resulting colonization by microorganisms before ingestion, known as 'microbial gardening', allows for trophic upgrading and increased microbial biomass for detritivorous zooplankton. Using model chitin particles incubated with seawater collected from the surface, mesopelagic and bathypelagic depths in the Northeast Atlantic Ocean, we determined particle-attaching bacterial communities to identify general and depth-specific candidates of particle colonization. Comparison of particle-attached communities at the amplicon sequence variant level showed that bacteria found on surface particles were also colonizers in the bathypelagic, in line with sinking particles promoting vertical connectivity. Bathypelagic particle-attached communities were most diverse. We propose that some particle colonizers attach to the surface and sink out with the particle, whilst other colonizers are depth-specific. This suggests that candidates for particle colonization differ with depth, which may be important when considering the implications for the delivery of ecosystem services, including carbon cycling and the role they play for zooplankton grazers.