Degradation and dissolution properties of photosynthetically-produced phytoplankton lipid materials in early diagenesis

Natural phytoplankton populations were incubated with NaH13CO3 under light for 1 day, followed by incubation in the dark for 29 days to examine the degradation and dissolution of photosynthetically-produced organic carbon and lipid materials during early diagenesis. Organic matter obtained in this study was divided into two particulate fractions (0.2-0.7 μm and > 0.7 μm) and a dissolved fraction (< 0.2 μm) by filtration. The changes in the concentration of photosynthetically-produced organic carbon (P-OC), lipid carbon (P-LC) and carbon of lipid classes fractionated by their polarity in each size fraction were followed by a mass spectrometer combined with an elemental analyzer. In the particulate fraction (> 0.7 μm), changes in the concentration of the carbon of lipid classes showed that phospholipid (PL) degraded more slowly than triacylglyceride (TG) through the dark incubation period. At the end of incubation, 3.5% of the maximum concentration of PL remained, and the remaining percentage of PL was more than ten times higher than that of TG (0.3%). This result suggests that structural lipid remains more stable than storage lipid during early diagenesis. The increase of dissolved P-LC contribution to total P-LC (from 0.7% to 6.5%) suggested an accumulation of relatively stable lipids in the dissolved fraction during early diagenesis. However, a considerable fraction (more than 90%) of P-LC remained as a particulate form at the end of incubation, whereas relatively stable P-OC tends to accumulate into a dissolved fraction during incubation; the contribution of P-OC in dissolved fraction to total P-OC eventually reached 49%. The size composition in the P-OC approximated that of organic carbon in the ambient seawater through the early diagenetic process, demonstrating the size change in phytoplankton OM toward DOM in natural environments. The contribution of the dissolved fraction in P-LC to total P-LC on Day 30, however, was obviously lower than that of lipid carbon which originally existed in the sample seawater (Lipid carbon; LC), indicating that dissolved and refractory lipid materials exist in natural seawater. Lipid class composition of P-LC and LC in the dissolved fraction indicated that the stable LC dissolved in natural seawater is derived from both allochthonous fossil materials and photosynthetically-produced materials.