EFFECTS OF DARKNESS ON MULTI-EXCITATION IN-VIVO FLUORESCENCE AND SURVIVAL IN A MARINE DIATOM

Surveys of the California Current System in 1993 revealed high concent rations of photosynthetic pigment biomass at similar to 200-m depth, w ell below the euphotic zone. The deep fluorescence feature contained a n estimated 2.2 x 10(4) metric tons of carbon and contained similar to 2.5 times the amount of chlorophyll observed in surface waters direct ly above it. Deep phytoplankton assemblages may be a signature of wate r mass subduction, suggesting the possibility of using phytoplankton a s water mass tracers. These field observations led to a laboratory stu dy of the fluorescence characteristics of autotrophic cells as possibl e indices of acclimation to extended periods of darkness. In vivo mult i-excitation Chl a fluorescence of the diatom Thalassiosira weissflogi i was monitored for 2 months of total darkness. Numbers of living and dead cells were determined using the vital stain fluorescein diacetate (FDA). By the end of the dark incubation period, in vivo Chl a fluore scence and fluorescence per cell had leveled off to similar to 45% and 65% of initial values, respectively. The contribution of accessory pi gments to Chl a fluorescence, expressed as multi-excitation fluorescen ce ratios, was higher in the dark than prior to transfer to darkness b ut showed no significant changes during 2 months of darkness. The FDA assay indicated that similar to 85% of the cells were alive for at lea st the first 3 weeks during the first dark experiment and for the enti re 2 months of a second dark incubation. Cell numbers decreased to 65% of initial values and then grew exponentially upon reexposure to a li ght:dark photoperiod. Our results for T. weissflogii suggest that exte nded light limitation of photosynthesis does not preclude the survival of subducted phytoplankton assemblages and the consequent accumulatio n of Chl a at depths below the euphotic zone. If these results extend to natural assemblages, it is not possible to estimate advective time scales based on a maximum persistence time of pigment fluorescence bel ow the euphotic zone. Nevertheless, the deep phytoplankton assemblage we observed provides evidence for water mass subduction and suggests t hat large, intermittent pulses of phytoplankton carbon are a part of c ross-shelf exchange and vertical flux from surface waters to depth in this region.