Lake Water and Sediment IV. Radiophosphorus Equilibrium with Mud, Plants, and Bacteria under Oxidized and Reduced Conditions

The phosphorus equilibration pattern and rate bctwecn mud and water was the same in natural Jcnkin sampler cores, in artificial cores, and in bottles in which dredged surface mud was packed by centrifuge. Thus any specific natural physico-chemical or bacteriological layering of the surface muds of lakes is relatively unimportant in phosphorus exchange, Phytoplankton or bacterial cells equilibrate within a few minutes after addition. When antibiotics are used the I’32 remains as inorganic PO.4 and is rapidly taken up by higher plants, or in the absence of plants, there is a rapid loss of I’s2 to the mud. With one exccption, in over 100 artificial systems tested, the amount of I? remaining in the water at cquilibrium was greater in the presence of bacteria than where antibiotic had been added. This was true whether the system was treated with nitrogen or air, i.e., was aerobic or anaerobic. After a week less than 10% of the I’32 remained in the water of an antibiotic treated sample, while two thirds remained in the control. The remarkable ability of bacteria to hold phosphorus in the water might be accomplished in two ways : 1) By an acceleration of the rate of I’32 return from the sediment to the water by bacteria in the mud. In all experiments while the turnover time of water was generally of the same order of magnitude for all systems, the turnover lime for mud was much shorter in the controls than in the antibiotic treated systems. 2) By the rapid uptake of radiophosphate by water bacteria and their ability to hold the radiophosphorus from the chemical or colloidal adsorption mechanism of the mud, which would be accomplished by incorporating the phosphate into non-participating organic compounds. An afFinity, or holding back by water bacteria of I’32 would be indistinguishable from an accelerated return to the water from the mud. Dead plankton deposited on the mud decay and greatly increase the removal of I’32 from the water. This reaction also is blocked by antibiotics. In bottle experiments there is a natural fallout of bacterial cells of about hfy$, per day. Neither the redox state of the system nor the level of lake productivity could bc shown to influence either living or inorganic exchanges. The events following addition of radiophosphorus can be described as a modified first order consecutive reaction in which PO4 yields organic P in the bodies of bacteria which in turn yields organic soluble phosphorus to the water. The rate of exchange is measured as turnover time, which is the time required for the appearance or disappearance of as much phosphorus as is present in the test material, say phytoplankton or water or mud. Some turnover times arc : water of a whole lake, one week; water in a bottle over mud, 0.5 week; return from lake sediment in nature including rooted aqllatics, 1 month; from mud in a bottle, 0.5 week; from bottle mud without bacteria, 2 weeks. Equilibration of PO4 between water and the inside of bacterial or phytoplankton cells is almost8 immediate, say 5 minutes, but conversion to the organic state is slower with average turnover time 0.3 days. Rooted aquatics, probably cannot take up organic P; with inorganic P their time is 0.5 week. Zooplankton arc opposite in behavior, unable to use phosphorus until bacteria have made it organic; their time is then 1 day.