Simulation Of Microalgal Biomarkers Degradation In Sediment-seawater Interface In Seasonal Hypoxia Environments

Series of laboratory incubation experiments were conducted to simulatedegradation of organic matter in sediment-seawater interface. Under four differentredox conditions (oxygen saturation:100%,50%,25%and0%), degradations ofseveal biomarkers originated from Skeletonema costatum, a typical red tide algaealong China coastal area were tracked. By analyzing concentrations of the majorfatty acids and the typical neutral lipid biomarkers obtained at various sampling time,the degradation behavior of these lipid biomarkers was investigated and discussed indifferent degrees of hypoxia environment. The results showed that:1) Twelve fatty acids and six typical neutral lipids were identified in incubationexperiments. The concentrations of different lipids varied significantly in algalspiked sediment samples and bulk sediment samples. And the concentrations invarious forms (free and bound) of different lipids were also different.2) Degradation of different fatty acids varied dramatically in various incubationsystems. There were several concentration variation trends of the fatty acids in theincubation systems under four oxygen saturations.①Concentrations of fatty acids insediment decreased fast at the initial incubation and then decreased slowly. Variationtrends of these fatty acids could be described by G-model. For free fatty acids inalgal spiked sediment, degradation of14:0,16:0,16:1(7),20:5and22:6were fit forthis kind of variation pattern. Their degradation rate constants range from0.12to0.24, from0.079to0.17, from0.12to0.19, from0.56to0.84and from0.25to0.80.For bound fatty acids, only degration of14:0matched G-model. Its degradation rateconstants ranged from0.05-0.13d-1. There was significant difference of Kavofvarious fatty acids under different oxygen satuarations.②Concentrations of fattyacids in the sediment increased first and then decreased, indicating accumunation of these fatty acids at the beginning of incubation. Concentrations of free fatty acids16:3,16:1(9),18:1and18:0increased after3-7day incubation, then their concentrationsdeclined dramatically. After that, they were degraed slowly in the systems.Concentrations of some bound fatty acids17:0and15:0increased at the initialincubation, then varied flutuctly.③Concentrations of fatty acids varied flutuctly,such as free fatty acids15:0,17:0and18:1(7). Their concentrations reflected bacterialbiomass in some degree.④Concentrations of fatty acids showed increasing trends.For example, concentration of bound fatty acids15:0and17:0increased significantlyduring incubation period.3) Time-dependent variations of concentrations of different netural lipids variedsignificantly with various oxygen saturations. Gererally, in the four incubationsystems, variation trends of concentrations of netural lipids could be considered asfour types:①The concentrations of the neutral lipids in the sediments decreasedrapidly at the beginning of incubation and then decreased slowly. This trend can bedescribed by G-model. Based on G-model, the kav of three free alcohols in the fourincubation system ranged from0.035d-1to0.39d-1. Under100%oxygen saturation,kavof phytol、cholesterol and brassicasterol were0.089d-1,0.0091d-1and0.019d-1，respectively. Degradation rate constants of different neutral lipids varied significantlyunder different oxygen saturations.②The concentrations of the neutral lipidsdecreased at the beginning of incubation and then increased, indicaing a kind offluctuating variation trend. In hypoxia seawater, time-dependent concentrations ofphytol、cholesterol and brassicasterol followed this trend.③Concentrations ofbound netural lipids varied fluctuatly with increasing trend as a whole.4) In sediment, total concentration of fatty acids was much higher than that ofnetural lipids. Generally, for each fatty acids and neutral lipids, the ratio between itsconcentrations in bound form to that in free forms (Rb/f) showed increasing trend withincreasing incubation time in all incubation systems.The results of this research indicated that besides oxygen, structure and featuresof lipids, environmental microorganism, and sediment absorption and desorption alsostrongly affected organic matter degradation in sediment-seawater interface in seasonal hypoxia environments along coastal area.