The critical pressures of gas vesicles in Planktorhrix rubescens in relation tothe depth of winter mixing in Lake Zürich, Switzerland

Abstract

The vertical distribution of the cyanobacterium Planktothrir (Oscillazoria) rubescens in Lake Zürich was investigated from March 1993 to June 1995 by collecting filaments on filters and measuring them by epifluorescence microscopy and computer image analysis. The initial population, which began to stratify in April, decreased by up to 99% by June. During the summer, the population peaked at depths of 8-15 m; it reached a maximum areal filament-volume concentration of -60 cm −3 of lake surface in early September and was then entrained in the deepening surface layer. It became mixed progressively deeper, to the lake bottom in the cold winter of 1993-94, but less completely in the milder winter of 1994-95. Most of the filaments remained viable during the winter. At the end of the mild winter of 1994-5, 70% of filaments in the water column retained buoyancy, but after the cold winter of 1996-7 only 22% were buoyant. Few remained buoyant below 80 m, where the hydrostatic pressure caused gas vesicle collapse. The proportion that remain buoyant decreases with the depth and duration of winter mixing, and increases with the critical collapse pressure (Pc) of the gas vesicles, which provide buoyancy. Strains of P.rubescens isolated from Lake Zürich differed in mean (Pc) of their gas vesicles, from 0.9 to 1.1 MPa, the highest values in freshwater cyanobacteria. Allowing for a turgor pressure of 0.2 MPa. these strains would remain buoyant at depths down to 70 and 90 m, respectively. Natural selection for gas vesicles of high (Pc) will operate by increasing the proportion of filaments that remain buoyant in the upper parts of the water column after circulation to various depths during the winter because only buoyant filaments will form the inoculum for the following season

The daily integral of photosynthesis by Planktothrix rubescens during summer stratification and autumnal mixing in Lake Zürich

“…As in the two previous years (Walsby et al, 1998) the population of Planktothrix rubescens developed from a small remnant of the previous year's growth in the metalimnion (Fig. 3).…”

“…Thomas & Ma$ rki (1949) investigated the annual changes in the depth distribution of the population over 3 yr from January 1945 to December 1947 ; they showed that Planktothrix rubescens stratified in the metalimnion of Lake Zu$ rich during the summer, became entrained in the surface mixed layer during the autumn and was mixed to depths of 80 m or more during the winter. We showed a similar pattern during the period from March 1993 to June 1995, in which we measured the Planktothrix population by computer image analysis of filaments collected on membrane filters from samples taken at 10-20-m depth intervals each month (Walsby, Avery & Schanz, 1998). By integration of the filament concentration throughout depth in the water column, we demonstrated that the population fell to a minimum in May and June, reformed in the metalimnion, and grew throughout the summer to reach its maximum biomass in the autumn.…”

“…It is noted that the rate of increase in the population during this period, 0n38 d −" , exceeded the maximum growth rate of this organism in culture, 0n12 Walsby, unpublished). Part of the increase must therefore be due to recruitment of filaments from the overwintering population at greater depths, which, it has been argued, will continue for the several weeks that it takes for individual filaments to float up from depths of 90 m or more (Walsby et al, 1998). (2) The subsequent rise in biomass (to day 214) was exceeded by the photosynthetic activity, which reached its maximum over the same period.…”

Modelling the plankton groups of the deep, peri-alpine Lake Bourget

“…In Lake Bourget (Vinçon-Leite et al, 2002;Jacquet et al, 2014b;Anneville et al, 2015), as well as in other deep lakes like Lake Zurich (Walsby et al, 1998;Posch et al, 2012), Lake Garda (Salmaso, 2010) and Mondsee (Dokulil and Teubner, 2012), the abundance of P. rubescens by the end of winter is largely determined by meteorological conditions during winter. In such deep, warm monomictic lakes, cooling in mild winters may end up being insu cient to cause a full overturn, leading to a reduced dilution of the P. rubescens concentrations, as well more P. rubescens cells survive the winter (Walsby et al, 1998).…”

“…Gas vesicles of P. rubescens are known to collapse at high pressures (70-90 m in the strains found in Lake Zurich Walsby et al, 1998). In reality, this only a ects their buoyancy, and consecutively leads to an increased sedimentation of P. rubescens, but as we do not explicitly account for buoyancy in this study, we mimic this e ect by parameterizing the mortality rate of AP R as a sigmoidal function of depth, that has the in iction point at z=40 m, converging to l i=P R (=0.02 d −1 , as all other plankton, see Table3) at the surface, and 5*l i=P R (=0.1 d −1 ) towards 80 m:…”