Testing the conveyor-belt hypothesis
The study by Tang and his colleagues sought to test an alterative means of bacterial dispersaltheir "conveyor-belt" hypothesis that bacteria might also be able to move upward, against gravity and across stratified density boundaries, by hitching a ride with the many zooplankton species that migrate from the depths to the surface each day with the setting sun. Their idea requires that bacteria not only climb aboard the upwardly migrating zooplankton, but that they later disembark in surface waters. Another possibility is that zooplankton eat the bacteria, and later egest them unharmed.
Tang says that intermittent or permanent stratification due to differences in temperature and salinity is common in many water bodies, whether they be lakes, coastal bays, or the open ocean. Stratification occurs frequently in Chesapeake Bay and other estuaries when relatively fresh river water forms a layer that floats atop the saltier, denser seawater entering the estuary's mouth.
To test their hypothesis that bacteria might be moving between water masses by hitching a ride on "water fleas" like Daphnia, the researchers filled a series of graduated cylinders with fresh- and then salt-water to form a stably stratified water column, then added bacteria labeled with a green fluorescent protein to the lower layer and kept the top layer bacteria-free.
The researchers then added Daphnia to the cylinders and used a directed light source to manipulate their swimming direction. (The species of Daphnia they used instinctively move toward light.) By alternating the position of the light every 2 hours for 8 hours, the researchers were able to make the water flea swim up and down across the stratified water column several times, and followed the rate at which bacteria were transported upward. A cylinder with bacteria but without Daphnia served as the control. Afterward, the experiment
|Contact: Kam Tang|
Virginia Institute of Marine Science