Bacterial colonization and ectoenzymatic activity in phytoplankton-derived model particles. Part II. Cleavage and uptake of carbohydrates

The bacterial colonization and development of the ectoenzymatic glucosidase activity and glucose uptake were followed together with bacterial growth (measured as thymidine incorporation) in laboratory experiments, using phytoplankton-derived particles incubated in rolling tanks. Bacterial colonization of the particles was rapid. In the particles, bacterial turnover rates (production/biomass) were low (0.02 to 0.14 d^-1). In the ambient water, turnover rates increased from 0.1 d^-1 to 23.3 d^-1, until the end of the experiment. In the control, lacking any particles, turnover of bacteria ranged from 0.3 to 7.6 d^-1. Similarly, glucose uptake rates, per bacterium, were 1 to 2 orders of magnitude lower for particle-attached bacteria than for their free-living counterparts. Generally, K(m) values for glucosidase activity declined, over the incubation period, in particles and free-living bacteria until 168 h, and slightly increased, thereafter, to values of approximately 0.1 μM. Particle-attached bacteria exhibited significantly lower uptake rates of both thymidine and glucose, per bacterium, throughout the incubation. The per-cell ectoenzymatic activity was similar in particle-associated and free-living bacteria during the initial phase of the experiment, but was significantly higher after ≃200 h. Dissolved total (TCHO), as well as monomeric carbohydrates (MCHO), declined continuously in both particles and ambient water; they remained constant in the control; TCHO comprised about 50% of the dissolved organic carbon (DOC) in the particles. In ambient water TCHO contribution to DOC varied, with only one exception, between 25 and 45%; and in the control, between 20 and 50%. The shift detectable in the relation between ectoenzymatic activity and uptake of glucose between free-living and attached bacteria over the incubation period may reflect changes in the physiological status of the bacteria.