Heterotrophic capability of a metalimnetic plankton population in saline Lake Shira (Siberia, Khakasia)

The heterotrophic potential of a deep (12 m) phytoplankton community layer in Lake Shira (Siberia), dominated by several taxa of cyanobacteria (Aphanocapsa, Lyngbya contorta, and other unidentified species) was investigated. The plankton community was fractionated by size, allowing separation between the bacterioplankton and the phytoplankton, and ¹³C-labelled organic compounds were used as tracers. The uptake of ¹³C-labelled glucose and of ¹³C-labelled glycine was maximal in the bacterioplankton-enriched fraction (¹³ C = 557 and 323, respectively), but was also high in the cyanobacterial fraction (¹³ C=138 and 80, respectively). An inverse relationship between the uptake of organic compounds and the light intensity when the whole community was exposed to different irradiances was also investigated. These results suggest that the photosynthetic microorganisms from the investigated community are able to assimilate organic compounds and thus supplement their carbon and energy requirements. This heterotrophic capability appears to be favoured by the high in situ concentrations of dissolved organic carbon (>15 mg C l^–1), and may offset the effects of severe light limitation on the phytoplankton in this deep, highly shaded environment.