Abstract:
Nutrient cycling in a subarctic oligotrophic lake was explored using current kinetic theory for organisms adapted to low nutrient environments with emphasis on bacterial contributions to system function. Techniques were refined which minimize sample disturbance and contamination for the purpose of accurately measuring bacterioplankton activity. Seasonal variations in DNA content, cell mass, species composition, specific affinity for amino acids and cell yield were observed. Quasi-steady state formulae describe bacteria as a flywheel in nutrient cycling; energy is conserved within a relatively constant biomass by varying bacterial activity with nutrient availability. The bacterial flywheel paradigm provides a bacteriocentric view of mineral cycling, linking kinetics to specific cytoarchitectural properties while maintaining links to substrate and grazing pressures. As an extention of the microbial loop paradigm, the flywheel becomes essential at high latitudes. In winter, low solar input interrupts the microbial loop so that the dissolved organic carbon (DOC) pool is cycled through bacteria only. This activity allows bacterioplankton to persist through winter and respond rapidly to springtime warming and nutrients. Microbial adaptations to seasonal variations in nutrient availability and temperatures were examined within the bacterial flywheel framework. Organisms are well-adapted to a narrow (17°C) in situ temperature range. Activation energies for small warming were low at the temperature extremes (20.6 kJ mol -1 at 0.5°C; -32 kJ mol-1 at 17°C) and high in spring (110 kJ mol-1 at 1.2°C). Nutrition varies by season, supplied in large part by amino acids in spring and summer. Winter growth rates are at least 0.013 day-1 whereas partial growth rate on amino acids for that season is only 2.8 x 10-5 day -1. It is proposed that winter organisms rely on diffusion transport and/or shift toward concurrent use of a large suite of substrate types for growth and maintenance.
