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 concentrations of photosynthetic pigment biomass at -200-m depth, well below the euphotic zone. The deep fluorescence feature contained an estimated 2.2 X 10J metric tons of carbon and contained -2.5 times the amount of chloroph,yll observed in surface waters directly above it. Deep phytoplankton assemblages may be a signature of water mass subduction, suggesting the possibility of using phytoplankton as water mass tracers. These field observations led to a laboratory study of the fluorescence characteristics of autotrophic cells as possible indices of acclimation to extended periods of darkness. In vivo multiexcitation Chl a fluorescence of the diatom Thalassiosira weissflogii was mlsnitored for 2 months of total darkness. Numbers of living and dead cells were determined using the vital stain flucarescein diacetate (FDA). By the end of the dark incubation period, in vivo Chl a fluorescence and fluorescence per cell had leveled off to -45% and 65% of initial values, respectively. The contribution of accessory pigments to Chl a fluorescence, expressed as multiexcitation fluorescence ratios, was higher in the dark than prior to transfer to darkness but showed no significant changes during 2 months of darkness. The FDA assay indicated that -85% of the cells were alive for at least the first 3 weeks during the first dark experiment and for the entire 2 months of 1 second dark incubation. Cell numbers decreased to 65% of initial values and then grew exponentially upon reexplsure to a light: dark photoperiod. Our results for T. weissjlogii suggest that extended light limitation of photosynthesis does not preclude the survival of subducted phytoplankton assemblages and the consequent accumulation 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 below the euphotic zone. Nevertheless, the deep phytoplankton assemblage we observed provides evidence for water mass subduction and suggests that large, intermittent pulses of phytoplankton carbon are a part of cross-shelf exchange and vertical flux from surface waters to depth in this region. Although photoautotrophic growth in the ocean is restricted to the euphotic zone, the vertical distribution of phytoplankton sometimes extends well below the euphotic zone. The presence of significant concentrations of phytoplankton below the euphotic zone suggests that the autotrophic cells have sunk to the observed depth, or that the euphotic zone water containing the cells has been physically displaced. For example, observations of deep phytoplankton assemblages have been presented as evidence for water mass subduction in ‘the California Current System (Hood et al. 1991; Kadko et al. 1991; Washburn et al. 1991), which suggests the possibility of using phytoplankton as water mass tracers. Additional observations of the California Current System in 1993 revealed photosynthetic pigment biomass at depths well below the euphotic zone (Cowles et al. 1994). The survey region, located -200 km offshore of northern California and seaward of the continental shelf, enclosed a portion OF a meandering jet and an adjacent cyclonic eddy. Surveys Acknowledgments We thank Ricardo Letelier for careful review of this manuscript. We thank Russ Desiderio for technical support with the multi-excitation lluorometer. HPLC analyses were performed by Robert Bidigare, Mike1 Latasa, and Kristi Hanson at the University of Hawaii. Sandy Moore analyzed the nutrient samples, and Margaret Sparrow and Bruce Eversmeyer assisted in the particulate organic carbon analyses. This work was supported by a grant from the Office of Naval Research (ONR N00014-92J1535) as part of the Eastern Boundary Currents/Accelerated Research Initiative. with an in situ fluorometer on a SeaSoar vehicle revealed a deep region of chlorophyll a (Chl a) fluorescence that was centered at 150-200 m (Fig. 1) with a horizontal extent of -50 X 60 km. This feature was physically separated from a “deep chlorophyll maximum” (Steele and Yentsch 1960; Cullen and Eppley 1981) centered at -50 m in the pycnocline. The depth of the 1% light level, determined from a photosynthetically available radiation (PAR) sensor (Biospherical Instruments), ranged from 45 to 55 m in the region where the deep feature was sampled. Chl a concentration in the core of the deep feature was -1.5-2 pg liter-’ (determined by calibration with extracted Chl a measured from a CTD cast) and the total chlorophyll content was -2.5 times that of the eupf otic zone directly above it. Assuming a carbon: chlorophyll value of 50 (e.g. Landry and Lorenzen 1989), the deeF feature contained -2.2 X 1 O4 metric tons (2.2 X 1O10 g) of carbon, representing a mean concentration of -60 pg C liter I. This concentration is comparable to reported values for nearshore euphotic zone waters in the California Current (Ohman 1988) and is 4-40 times greater than those reported for subeuphotic zone (I 00-1,000-m depth) waters in the north-central Pacific (Druffel et al.