Bacterioplankton productivity in lakes of the Taylor Valley, Antarctica, during the polar night transition

Abstract

Research on the lakes of the {McMurdo} Dry Valleys, Antarctica, is typically conducted during the period of 24 h sunlight (October to January) when logistical support is readily available. As part of the International Polar Year initiative, we obtained logistical support to study microbial dynamics in the permanently ice-covered lakes of the Taylor Valley during the transition from 24 h of sunlight to the complete darkness of the polar night (mid-April). Our study focused on the perennially ice-covered lakes Fryxell ({FRX)}, East Lobe Bonney ({ELB)}, and West Lobe Bonney ({WLB)}, all of which are chemically stratified and have food webs dominated by microorganisms. Depth-integrated bacterioplankton productivity ({BP;} leucine incorporation [Leu] and thymidine incorporation [{TdR])} in the lakes ranged from 1.2 to 3.4 mg C m(-2) d(-1). Overall, summer was characterized by relatively high rates of {BP} and photoautotrophic primary productivity. Rapid decreases in photosynthetically active radiation marked a subsequent transition period, which was characterized by variable cell counts and decreasing Leu: {TdR} ratios (ratios {\textgreater}1 signify a physiological shift from growth to maintenance mode). Finally, cell counts decreased and Leu: {TdR} increased by as much as 280% during the fall, revealing a distinct change in the physiological state of the bacterioplankton as light-mediated primary productivity ceased. Our data reveal that the shift in physiological state may result from a switch from contemporary phytoplankton-excreted carbon to other sources of dissolved organic carbon, which can support the bacterioplankton populations through the winter.