Effects of growth stages, respiration, and microbial degradation of phytoplankton on cellular lipids and their compound-specific stable carbon isotopic compositions

A series of laboratory experiments were conducted to examine effects of biochemical processes on phytoplankton lipids and their stable carbon isotopic compositions. Thalassiosira weissflogii and Emiliania huxleyi were axenically cultivated in batch cultures through exponential growth (phase I) and stationary period (phase II), followed by dark respiration (phase III) and subsequent microbial degradation (phase IV). The results indicated that the bulk parameters, lipid contents and associated isotopic compositions of two phytoplankton species varied differently during different cell growth stages, cell respiration and microbial degradation processes. T. weissflogii continuously produced neutral lipids and fatty acids at similar rates between phase I and phase II. By contrast, E. huxleyi produced neutral lipids at faster rates in phase II than in phase I although the production rates of fatty acids were still similar between the two phases. The lipid isotopic compositions of these two phytoplankton species varied in a similar pattern through cell growth: remarkably positive shift in phase I but relatively small negative shift or little change in phase II. During cell respiration (phase III), two phytoplanktons lost their lipids by varying percentages but the isotopic compositions of all compounds remained relatively constant. With degradation of lipids in phase IV, their isotopic compositions varied in three different ways: positive shift for most fatty acids; little change for sterols and alkenones; and a negative shift for 20:5 fatty acid of T. weissflogii. This experimental study implies: (1) isotopic compositions of phytoplankton lipids are heterogeneously generated through different cell growth phases, likely dependent on cell physiological states, medium conditions, and relative proportions of intracellular components; and (2) microbial degradation plays a more important role in altering isotopic compositions of lipid compounds than cell respiration, probably related to different labilities of lipids between different cellular components.