the photic zone in sediments is less extensive compared to that in the water, in fact, while in the latter the penetration depth of light is many meters, sediment particles strongly reduce the amount of light entering, and this is the reason why phototrophic bacteria in sediments often form thinner layers within the uppermost millimeters. Sulfate present in the sediments is metabolised to sulfide by sulfate-reducing bacteria and the sulfide thus produced diffuses upwards into the water column. Then, sulfide forms a vertical gradient with an opposite trend to that of the light, which on the contrary decreases going downwards. This is the reason why these bacteria can find their optimal growth conditions in small areas of overlap between the countercurrent gradients of the above-mentioned factors. The bacteria inhabiting meromictic lakes generally grow at a quite fixed depth and in a relatively stable way over time, as in these lakes there is a permanent stratification due to the higher salinity of the bottom water layers; in particular, photosynthetic purple sulfur bacteria are located at the level of the chemocline, where they can benefit from both the sunlight from above and the sulfide produced by the underlying anaerobic bacteria. Since the chemocline is usually relatively deep (from a few centimeters up to several meters, depending on the lake), the blooms of these bacteria tend not to be visible at the water surface. Regarding the holomictic lakes, here occurs a seasonal, thermal stratification maintained by temperature differences (especially during summer), which also provides a stable enough layering for purple sulfur bacteria growth; in this case, they cluster at the level of the anoxic and sulfidic hypolimnion, that is to say the dense, bottom water layer of the lake. Ectothiorhodospiraceae and Chlorobiaceae are the only other families of phototrophic bacteria which thrive under similar environmental conditions: despite their dissimilarities from a genetic and evolutionary point of view, both purple and green sulfur bacteria depend on reduced inorganic sulfur compounds for their growth and it allows them to play similar ecological roles. The metabolic flexibility of the versatile species belonging to the Chromatiaceae family provides a better adaptation to the environmental changes, such as those occurring in the intermittently oxygenated areas; anyway, the growth they obtain through photosynthesis is greater than that obtained through any other metabolic strategy, regardless of the fact that using forms of metabolism different from photosynthesis means having to compete