Changes in the composition of bacterial populations in the chemocline of Lake Cadagno were analyzed since 1993 using various molecular methods (26,30,31,52,170,171). These studies have shown that the chemocline harbors a dense population of phototrophic sulfur bacteria composed of heterogeneous PSB from the genera Chromatium, Lamprocystis, Thiocystis and Thiodictyon, as well as two GSB: Chlorobium phaeobacteroides and one proposed “clonal population” of Chlorobium clathratiforme. Quantitative analyses have shown for instance that this latter species increased the total abundance of phototrophic sulfur bacteria by a factor of three since the year 2000 (28). Ultimately, C. clathratiforme represented 95% of total bacterial abundance in the chemocline, however, due to its small cell size (i.e. cellular volume of ca. 1.2 µm3), this species represented only 15% of the total microbial biovolume in the chemocline (53). In contrast, the less abundant but large-celled PSB C. okenii (cellular volume approx. 270 µm3) contributed up to 83 % of total biovolume in the chemocline (53). Since the total number of PSB did not vary significantly through the years, the blooming of C. clathratiforme did not occur to the expense of C. okenii and other PSB populations (28). These patterns suggest that either GSB and PSB do not compete for the same resources, or that limiting resources were abundant enough to sustain population growth of C. clathratiforme. The simultaneous presence of both species of phototrophic sulfur bacteria in the same water layer was also observed in other meromictic lakes, although with a distinct spatial distribution of GSB just below PSB because of its more effective light-harvesting system (16).
The data presented in my thesis revealed a marked change in the community composition of phototrophic sulfur bacteria in the chemocline of Lake Cadagno. In season 2016, the average cell density of the bacterial community in the chemocline amounted to 106 cells mL-1 (compared to 107 cells mL-1 in 2007 (28)). The relative abundance of C. clathratiforme amounted just to 40%
and that of C. okenii to 8% (compared to 95% and 0.3%, respectively in the year 2008 (53)).
This shift in bacterial community composition, resulting from a reduction of C. clathratiforme abundance, suggests a gradual recovery to the situation present before the dramatic change occurred in year 2000 (25,27), when particular meteorological events resulted in a complete mixing of the lake (25).
The reasons for the drastic reduction in the abundance of the GSB community remain unclear.
Previous studies revealed the low inorganic carbon fixation rates of C. clathratiforme (53,55), especially in contrast to the potential capacity to perform glycogen fermentation in the dark part of the water column (56). Consequently, substantial reduction of adequate carbon substrates in the chemocline for glycogen production might have contributed to the decrease of C.
clathratiforme abundance. In fact, in the same period, the cantonal authority introduced new regulations for organic waste disposal by farmers in the Piora Valley. These “environmental friendly” regulations reduced or even stopped the spill of dairy waste rich in labile carbon (e.g.
cheese whey) which may have altered the resource availability for the microbial community in the water column of Lake Cadagno.
Shifts in microbial community composition might not be an unusual phenomenon and has been observed in other stratified lakes, although mostly linked to the occurance of complete mixing (holomixis) events. For example during the meromictic periods in Lake Rogoznica, anoxygenic phototrophic GSB dominated in both the chemocline and in the hypolimnion whereas after a complete mixing event, the anoxygenic phototrophic GSB disappeared and the homogeneously anoxic water column was dominated by a bloom of gammaproteobacterial sulfur oxidizers named SUP05 (from SUiyo seamount plume) related to the Gammaproteobacterial sulfur oxidizers (GSOs) (17). Such a shift in the microbial community after holomixis has also been observed in other stratified lakes (120).
Variations in the abundance of the anoxygenic phototrophic sulfur bacteria community may occur over long timescales (discussed above). However, substantial variations have also been shown to occur seasonally, suggesting that the community is adapted to changing environmental conditions (31). In Lake Cadagno a shift in dominance from C. okenii in the spring and early summer to small-celled PSB Lamprocystis purpurea in the late summer and autumn was observed by Bosshard et al. (26). However another seasonal study (52) suggested opposite patterns with high abundance of C. okenii in late autumn, and high abundance of small-celled PSB in spring and summer.
My data suggest that in 2016 the seasonal occurrence of phototrophic sulfur bacteria was similar to the one observed by Bosshard and colleagues (26), with large-celled PSB C. okenii abundant during summer (July and August) and small-celled PSB (especially Lamprocystis and Thiodictyon) abundant during late autumn (October). This observation requires explanations of the coexistence of diverse, but closely related, populations of PSB in the apparently same ecological niche (5). Moreover, the proliferation of Chlorobium spp. in late autumn (Chapter 1 and Chapter 2) also suggests that competition may occur between these two phylogenetically distinct but metabolically similar taxa. Additionally, a parasitic or opportunistic behavior of C.
clathratiforme against C. okenii might be hypothesized. For instance, in a study using single-cell NanoSIMS (53) C. okenii had significantly lower ammonium uptake rates if they had epibiontic microorganisms attached to their surface. These epibiontic cells might grow on dissolved organic matter excreted by C. okenii, or on organic matter released by damaged C. okenii cells. Although the identification of these epibionts is unknown, further analysis would be necessary to determine the possible involvement of C. clathratiforme.
While theoretically PSB and GSB occupy diverse ecological niches due to the different light harvesting complexes (39), the long-term changes in microbial community composition
observed in the chemocline of Lake Cadagno spark questions regarding evolutionary mechanisms or behavioral strategies adopted for the coexistence of these species.
Sulfide (H2S), as principal electron donor in the photosynthetic reaction of anoxygenic sulfur bacteria, might be the limiting factor inducing competition between PSB and GSB.
Chlorobiaceae deposit S0 extracellularly and exhibit higher affinities for sulfide than Chromatiaceae, which store S0 inside the cell (16,139). In Chapter 3, sulfide oxidation rates of C. okenii at different light intensities was investigated. This data suggests that C. okenii might be favored due to its flagella-mediated motility which allows it to settle at the depth with light intensity optimal for sulfide oxidation. Therefore, the active movement of C. okenii may serve to expand its ecological niche and to compete with small PSB and GSB.
1.3 Application of flow cytometry for studying the ecophysiology of phototrophic