Simulation Of Microalgal Biomarkers Degradation In Hypoxia Seawater

In last several decades, seasonal hypoxia in bottom seawater become morefrequently in some coastal areas and threatens the costal and shelf sea ecosystems.Eventually, seasonal hypoxia has become hot research area in biogeochemistry andenvironmental sciences. In China, seasonal hypoxia often occurs in some estuaries,such as in estuaries of the Yangtze River and the Zhujiang River. Co-occurrence ofred-tide and seasonal hypoxia seriously affected local ecosystem. Studyingdegradation of organic matter in these hypoxia environments is very significant forunderstanding coastal carbon cycling and marine environmental protection.In this study, series of laboratory incubation experiments were connducted tosimulate degradation of organic matter in seawater in hypoxia environments alongChina coastal area by tracking degradation of Skeletonema costatum,a typical red-tidealga along China coastal area. By mixing oxygen and nitrogen with different volumerations, four hypoxia microcosms with oxygen saturation100%,50%,25%and0%were established and S. costatum collected in late expontial phase were incubated inthese microcosms. Time-dependent microalgae samples were collected at differentincubation time and neutral lipids and fatty acids in the algae were extracted.Concentrations of lipids were analyzed bby GC-MS and time-dependent variations ofconcentrations of these lipids were tracked. The results showed that the effects ofoxygen on organic matter degration were critical in seawaters.(1)In different environmental system, lipids were degraded faster and morecompletely under higher oxygen saturation. At the end of incubation (T=90d),in anoxic microcosm, some lipids were still reserved in anoxic microcosm,and higher percentage of sterol were reserved compared that of fatty acids. Inoxic microcosm, most lipids were degraded. Some lipids were even degradedcompletely.(2)By appling Berner`s classic multi G-model, degradation of different lipids in the algae were described and the average degradation rate constants werecalculated. The results showed that in90d incubation period, degradation rateconstants of neutral lipid ranged from0.019d-1to0.31d-1, and degradationrate constants of fatty acids ranged from0.15d-1to0.46d-1. For almost allneutral lipids and fatty acids, positive linear relationships between Kavandoxygen saturation were estabilished, indicating critical role of oxygensaturation on organic matter degradation in seawater.(3) By comparing the ratios (E) of degradation rate constants of each lipidbetween three oxic incubation systems and anoxic incubation system, theresults indicated that the E values increased with increasing oxygenconcentration. In100%oxygen saturation incubation system, the E values ofhexadecanol and brassicasterol were high, up to6.31and7.21, respectively.Similar variation trends of the E values of fatty acids were also detected, buttheir values were significantly lower than those of neutral lipids under thesame oxygen saturation (e.g., the maxima E value of fatty acids in100%saturation system is2.85). The results indicated that effect of oxygen onorganic compounds varied obviously. In the same environment, structuraldifference of organic compounds also affected their degradation.(4)Under the same oxygen saturation, fatty acids were degraded fast than neutrallipids. This difference was attributed to structure of organic compound andselected utilization of microorganisms. Gererally, higher degradation rateconstants were found in higher oxygen saturation system.