Sources and transformations of chlorophylls and carotenoids in a monomictic sulphate-rich karstic lake environment

A study of the pigment composition in the water column particulates and bottom sediment column of an anoxic lake, Lake Ciso (Catalonia, Spain), has been carried out. The depth-dependencies of the carotenoid and chlorophyll distributions in the water column during stratification and holomixis, as well as during the diurnal cycles, have been determined. In the sediment, the composition of the pigments has been studied over the top 25 cm. The analyses were carried out by high performance liquid chromatography (HPLC) and HPLC coupled to mass spectrometry. The distributions of carotenoids, bacteriochlorophylls and chlorophylls in the water particulates closely follow the population dynamics of the photosynthetic organisms during stratification and holomixis. Among these, only chlorophyll a transformation products are found in the water column, namely phaeophytin a, pyrophaeophytin a and pyrophaeophorbide a. These derivatives also occur in the sediment, where they are found together with bacteriophaeophytin a and pyrobacteriophaeophytin a. This compartment-dependent occurrence corresponds to a higher degree of preservation of the bacterial vs algal pigments. The former occur in the anoxic hypolimnion or mixed water column during holomixis, and the latter in the oxic epilimnion. The presence of these compounds and the very low abundance of phaeophorbide a and absence of bacteriophaeophorbide a point to a transformation pathway where, in the absence of zooplankton grazing or high abundances of the enzyme chlorophyllase, the loss of the phytyl chain occurs predominantly after loss of the C-13² carbomethoxyl group. Okenone, alloxanthin, α-carotene and zeaxanthin/lutein are the major water column carotenoids. Their relative abundances contrast with their distribution in the sediments where okenone is the predominant compound and zeaxanthin/lutein are below detection limit. This effect may be in part a consequence of a more uniform okenone concentration in the water particulates over the year, leading to a higher annual sedimentation flux. Again, it may also be influenced by the higher degree of preservation of the bacterial vs algal carotenoids. A similar situation is observed for the relative abundances of chlorophyll a and bacteriochlorophyll a in water particulates and sediments.