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Synechococcus clone WH7803 (DC2) was grown on a 12:12 h light-dark cycle of either blue-green fluorescent, whlte fluorescent or daylight-filtered tungsten light. Integrated irradiance for each culture was set at 15 p E~n m-' S-' Subsequent measurements of absorption, pigmentation and carbon action spectra were used to examine wavelength-dependence of photosynthetic quantum yield. Comparison of directly-measured and reconstructed absorption spectra suggests that 'pigment packaging' effects are minimal in Synechococcus. Spectral quality had a marked effect on pigmentation and quantum yield. Cellular concentrations of chlorophyll a, /i-carotene and phycoerythrin were all -2-fold lower in daylight-grown Synechococcus relative to blue-green and white light-grown cells; ratios of pcarotene-and phycoerythnn-to-chlorophyll a were markedly constant for all 3 illuminations. Blue-green light grown Synechococcus cells had a -2-fold higher zeaxanthin content than those grown under white light or daylight illuminations. These results indicate that cellular zeaxanthin content is not an implicit constant and its concentration is dependent on irradiance levels of blue-green light. Zeaxanthin probably serves an important function as a photoprotectant pigment in Synechococcus, and as such, can also produce significant decreases (20 to 40 Yo) In the apparent quantum yield for photosynthesis in the blue-green region of the vislble spectrum In contrast, highest quantum yields were routinely measured between 525 and 650 nm suggesting that light absorbed by phycobilins (phycoerythrin and phycocyanin) drive the majority of carbon fixation in DC2-like coccoid cyanobacteria. In calculating the spectral quantum yield for natural phytoplankton populations, it is suggested that (1) carbon action spectra b e determined under 'enhanced' conditions and (2) photosynthetically absorbed radiation for phytoplankton be estimated using spectral reconstruction techniques where absorption contributions by nonphotosynthetic chromophores are removed from whole cell absorption signatures.

Phytoplankton absorption spectra were determined for communities collected in the upper euphotic zone over a 250 km transect across a highly variable region of the Southern California Bight. The influence of the 'package effect' on phytoplankton absorption spectra was assessed by comparison of absorption coefficient spectra based on direct measurement with spectral reconstructions calculated from HPLC-determined pigment concentrations. Measurable package effect occurred in less than 25 % of samples, principally from samples taken in the subsurface chlorophyll a maxin~um layer and in association with populat~ons of large diatonls or dense pryn~nesiophyte concentrations. Estimates of the package effect in the field derived from these measurements were consistent wlth the majority of laboratory-determined data for chromophyte and chlorophyte algae. In the cases where reconstructed phytoplankton absorptlon spectra overestimated measured spectra, the majority of differences could be reconclled by the application of a n algorithm calculating the package effect. Where package effects were minlmal, reconstructed absorption spectra provided accurate estimates of phytoplankton photosynthetic light absorptlon without correction for package effects. Existing models for phytoplankton absorption properties will benefit from inclusion of information on the package effect, determined from dlrect absorption measure~nents or from information on the taxonomic composltlon of the phytoplankton community.

Measurements of photosynthetic p~g m e n t s , nutnents, spectral Irradiance and vanous physical parameters were performed in the western Sargasso Sea (35"N, 70°W) to ~n v e s t~g a t e the factors affecting phytoplankton blomass dlstribut~ons Algal plgment concentrations and c o m p o s~t~o n s measured during spring 1985 showed cons~derable time-dependent v a n a t~o n s which were cons~stent with those documented by dlrect mlcroscoplc observation D u r~n g early Apnl, 2-fold Increases In chlorophyll a and fucoxanthln were measured on a relatively short tlme scale of days The presence of a d~atom-dominated community malnly specles of the genera Rhlzosolen~a and Chaetoceros, suggested that we were witnessing a stage of the spring bloom Upon return to thls location 2 wk later, the diatom bloom was replaced by a considerably more diverse phytoplankton assemblage consisting of prymnes~ophytes, cyanobacter~a, d~noflagellates, green algae (~ncludlng praslnophytes) and d~a t o m s The vert~cal structures d~splayed by ~n d i v~d u a l accessory plgments dunng Apnl were markedly simllar and suggest that the major phytoplankton taxa were not un~formly distributed In the upper 200 m The phytoplankton were d~s t r~b u t e d as broadly overlapp~ng layers, 1~1th cyanobacterla and diatoms most abundant In the m~x e d layer, prymnesiophytes at lntermed~ate depths, and green algae (Including prasinophytes) deeper In the water column Results p r o v~d e descnptive evidence for a rapid succession of chromatically-adapted phytoplankton during springtime in the Sargasso Sea