Long-term picocyanobacteria development in an athalassohaline warm-monomictic Lake Alchichica: Virus- vs. ciliate-control?

Autotrophic picoplankton (APP) was monitored regularly in Lake Alchichica (Puebla/Veracruz; Mexico) for nearly 10 years. In the present study, the control of APP development by viruses is analysed. The maximum cyanobacteria abundance was localised within the water column using WET-Labs ECO Triplet fluorometer and oxygen production by 2µm fraction was measured at four depths using clear & dark bottles. Autofluorescence and DAPI staining combined with an image analysis (Image J) were applied to evaluate APP and bacterial biomass, respectively. The Sherrs´ fluorescently labelled bacteria-protocol was employed to evaluate a role of ciliates. Viruses were estimated microscopically as "virus like particles" according to SYBR Gold protocol, taking CCD camera images. APP were peaking (over 106 cells/mL) during transition between mixing- and stratification period, being responsible for important oxygen production only within the epilimnion; Synechococcus strains were molecularly identified in the isolates. Viruses (107 to 108 VLP/mL) peaked in the surface layer during mixing or in the metalimnion, being coupled apparently more tightly with APP than with bacteria numbers. Both elevated ciliates' APP elimination during mixing period, and the maximum APP elimination around the oxycline in the late stratification roughly coincided with VLP maxima.