Long-term pigment dynamics and diatom survival in dark sediment

In order to investigate survival of diatoms and long-term pigment dynamics in dark sediment, we incubated samples of homogenized, sieved, tidal-flat sediment for 1 yr in darkness. Microscopic observations revealed that some diatoms survived the full year in darkness and retained their pigments. Concentrations of the diatom-specific phospholipid-derived fatty acid (PLFA) 20:5 omega 3 indicate that these survivors represented only a small fraction of the original diatom community, probably no more than a few percent. Consequently, the effect of pigment retention in living diatoms on the overall pigment dynamics was negligible. The photosynthetic potential of the surviving diatoms was further investigated by re-exposing the sediment to light in the presence of (13)C-bicarbonate. Rapid (13)C fixation into algal PLFAs at a level similar to that in sediment freshly collected from the field indicates that surviving diatoms fully retained their photosynthetic capacity. Concentration dynamics of photosynthetic pigments generally reflected degradation of two different pools with clearly different loss rates. Losses during the first 1-2 months of the experiment (half lives of 6-22 d) reflected degradation of fresh algal material, while losses during the rest of the experiment reflected much slower degradation of more refractory pigment pools (half lives of >= 1 yr). Individual pigments showed distinctly different labilities, with losses ranging from 0% to 86% over the full year. The order of overall loss was fucoxanthin. chlorophyll c > chlorophyll a > diadinoxanthin > pheophorbide > pheophytin > diatoxanthin.