Determining the Abundance and Diversity of Bacteria on Marine Particles of Different Size Classes

Microorganisms live in and on marine particles, where they metabolize those particles thereby regulating the biological pump and other elemental cycling processes. Particles in turn create micro-habitats that select for microbial communities that differ from those found in the surrounding water and that vary between particle types. This study examined the relationship between the size of particles and the abundance and diversity of microbes living on them. We combined particle size fractionation with quantitative PCR and amplicon sequencing approaches to quantify total bacterial abundance, and species-specific bacterial abundance per particle and per particle mass.

Particles were obtained from water samples from the Choptank River in Dorchester County, Maryland. We size fractionated particles using filters of decreasing size (500µm, 180µm, 53µm, 20µm, 5µm, and 1.2µm, and 0.2µm) and collected free-living microorganisms. Each size of filter was back rinsed to remove particles. Back-rinse from the filters was split, with a portion collected onto glass fiber filters for measurement of particle mass. A second portion was collected onto supore filters for DNA extraction. Quantitative PCR was used to determine the number of microbial cells per milligram of particulate organic matter. The 1.2µm size class had the highest concentration, of about 10¹⁰ cells per mg POM, while the 180µm class had the lowest concentration, of 10⁶ cells per mg POM. The number of cells per particle was also determined, which ranged between 3 cells per particle in the 1.2µm class and 270 cells per particle in the 53µm class. While it was hypothesized that cell number would increase linearly with particle size, it was instead observed that the abundance of cells per particle was variable between size classes. The DNA samples were also sequenced to identify specific abundances of genus-level taxa. Most of the bacterial taxa associated with small particles of <5µm or were predominantly free-living. However, several key taxa associated primarily with larger particles.