Role of free-living and particle-attached bacteria in the recycling and export of organic material in the Hudson Bay system

This study investigates, for the first time, the role of free-living and particle-attached bacteria in the sinking export and recycling of organic matter in the Hudson Bay system (i.e. Hudson Bay, Hudson Strait, and Foxe Basin), a large subarctic estuarine system. During the late summers of 2005 and 2006, the abundance, cell size, nucleic acid content, and sinking velocity of free-living and particle-attached bacteria were studied simultaneously, using a new approach that combines the settling column method with flow cytometry. Biomass, production, and respiration of both types of bacteria were estimated using published models. Our results showed that particle-attached bacteria were, on average, twice as large as and contained 1.3 times more nucleic acid than free-living bacteria. Particle-attached bacteria also sank faster than predicted by Stoke's Law, with estimated sinking velocities comparable to those of chlorophyll a biomass and protist cells. Each individual cell of the particle-attached bacterial community had high carbon demand, but their low abundances (< 3% of total bacterial numbers) resulted in low total carbon demand. Therefore, the main contributors to POC recycling were found to be free-living bacteria using the non-sinking dissolved organic material, which is released from particles due to the hydrolytic activity of particle-attached bacteria.

Research highlights
► Attached bacteria were larger and contained more nucleic acid than free bacteria.
► Attached bacteria sink faster than bacteria free in the surrounding environment.
► Attached bacteria abundance was low resulting in low contribution to sinking export.
► Attached bacteria had a low carbon demand.
► Free bacteria were the main contributors of organic matter recycling.