Degradation of alkenones and related compounds during oxic and anoxic incubation of the marine haptophyte Emiliania huxleyi with bacterial consortia isolated from microbial mats from the Camargue, France

To investigate the effects of bacterial degradation on the distribution of alkenones and related compounds, the marine haptophyte Emiliania huxleyi strain CS-57 was incubated under different conditions with bacterial consortia isolated from cyanobacterial microbial mats from the Camargue region of France. Extensive degradation occurred under aerobic and denitrifying conditions, but the effects on U37k′ values were variable, ranging from slight to moderate (0-0.1). Under oxic conditions, C37 alkenes, C37 and C38 alkenones and the C36 methyl alkenoate were rapidly degraded. The degradation of C37 alkenones by the aerobic consortium appeared to be selective in some flasks, resulting in a moderate increase in U37k′ values by up to 0.1 (corresponding to an inferred temperature difference of 3 °C). This variation in response is attributed to the numerous possibilities of initial attack on the alkenone structures in the presence of oxygen, plus the well known diversity of aerobes which may use different degradative pathways. Under denitrifying conditions, C37 alkenes appeared to be degraded to a much lesser extent than the alkenones and alkenoates. Although extensive alkenone degradation occurred under these anoxic conditions, the U37k′ index remained practically constant. Under anoxic non-denitrifying conditions, the production of unusual isomeric alkenes and alkenones was observed. The double bond positions in these compounds were identical to those in the original compounds, so they were tentatively assigned as cis/trans or trans/cis alkene and alkenone isomers. Their formation is attributed to the involvement of extracellular bacterial cis/trans isomerases. Owing to its coelution with the C37:3 alkenone, the presence of one or more isomeric C37:2 alkenone(s) leads to a simultaneous overestimation of the amount of C37:3 alkenone and underestimation of the C37:2 alkenone, and could significantly affect the measured U37k′ value (an underestimate of up to 20% was found for some flasks). Clearly, the environmental conditions have a major influence on alkenone degradation and in some cases could produce changes in U37k′ values that would lead to a bias of several degrees in reconstructed palaeotemperatures.