Bacterial degradation of green microalgae: incubation of Chlorella emersonii and Chlorella vulgaris with Pseudomonas oleovorans and Flavobacterium aquatile

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Abstract The influence of cell wall composition on the bacterial degradation of various constituents of green microalgae was examined during prolonged incubation in the dark with two ubiquitous, aerobic, heterotrophic bacteria ( Pseudomonas oleovorans and Flavobacterium aquatile ). The algae belong to the same genus, Chlorella , and were killed by heat shock prior to incubation. The two species exhibit conspicuous differences in cell wall composition: presence of both a classical polysaccharide wall and of a trilaminar outer wall (TLS) composed of highly aliphatic, non-hydrolysable macromolecules (algaenan) in C. emersonii and lack of such a resistant outer wall in the case of C. vulgaris . The changes induced by the bacteria in the abundance and the distribution of the algal hydrocarbons, fatty acids (FA), triacylglycerols (TAG) and chlorophyll (Chl) were determined after 1 and 4 months of incubation. Transmission and scanning electron microscopy observations showed that the algal cell walls were not disrupted by the initial heat shock. A complete lack of bacterial attachment to (or penetration into) the incubated cells was also noted after four months, indicating that bacterial degradation was probably mediated by extracellular enzymes. Examination of the bacteria-free controls showed large decreases in the algal constituents, especially after 4 months. Such non-bacterial degradation could originate, in the case of the hydrocarbons, FA and TAG, from radical oxidations initiated by the formation of hydroperoxide derivatives of polyunsaturated FA, whereas another type of pathway appeared to be implicated in chlorophyll alteration. Important additional decreases in algal hydrocarbons, FA and TAG, reflecting bacterial attack, were noted in the case of the incubated algae. Due to a combination of non-bacterial and bacterial degradation processes, a sharp lowering in the abundance of all the tested compounds was always observed in the incubation experiments. Moreover, comparison of the TLS-containing and of the TLS-devoid algae did not reveal clear-cut differences in the extent of hydrocarbon, FA, TAG and Chl degradation. Accordingly, no specific protective influence appears to be associated with the presence of an algaenan-containing TLS in C. emersonii . It is well documented that TLS plays a major and direct role in the formation of a number of kerogens from source rocks and oil shales. The present results, in agreement with previous TEM observations on such kerogens, suggest that TLS would not play an additional indirect role, during fossilization, via the protection of diagenetically-sensitive constituents of algal cells.