Influence of various redox conditions on the degradation of microalgal triacylglycerols and fatty acids in marine sediments

Sediment cakes, supplemented with microalgal cells (Nannochloropsis salina), were incubated for 35 days under permanently oxic, oscillating (5d:5d changeover oxic/anoxic) and strictly anoxic conditions of oxygenation in diffusively “open” sedimentary systems. Total lipids (TLip) and triacylglycerols (TG) concentrations were monitored by thin layer chromatography-flame ionisation detection, whereas the concentrations of the main extractable (free+ester-bound) individual fatty acids (C16:0, C16:1, C18:1) were followed using gas chromatography-mass spectrometry. Under the three conditions of oxygenation, TOC, TLip and TG showed a sharp decrease in concentration during the early days of incubation and seemed to stabilise thereafter, defining an apparent non degradable fraction (GNR). The GNR content was systematically higher in the anoxic incubation than under the oxic and oscillating conditions. The ratio of the main hydrolysis products of TG versus TG [(Free fatty acids+Monoacylglycerols+1,2-Diacylglycerols)/TG], used as an indicator of the hydrolysis of TG, showed that the presence of oxygen in the sediments (oxic and oscillating conditions) stimulates the hydrolysis of TG and the subsequent degradation of their metabolites. Unlike TOC, TLip and TG, individual fatty acids (FA) showed a continuous concentration decrease until the end of the experiment, which was fitted with a simple first order model [G(t)=G0e−kt] to yield apparent degradation rate constants. The values observed under oscillating conditions (kFA=0.019±0.001 d−1) were intermediate to those observed during oxic (kFA=0.029±0.003 d−1) and anoxic (kFA=0.011±0.001 d−1) incubations, and no significant difference between individual FA could be observed. The production of saturated and monounsaturated C16 (and to a lesser extent C18) alkanols under oscillating and anoxic redox conditions suggested that (a part of) the dominant FA were reduced to the corresponding alcohols under anoxic conditions, following their release from acylglycerols.