OPTICAL CHARACTERIZATION OF THE OCEANIC UNICELLULAR CYANOBACTERIUM SYNECHOCOCCUS GROWN UNDER A DAY-NIGHT CYCLE IN NATURAL IRRADIANCE

The optical properties of the oceanic cyanobacterium Synechococcus(clo ne WH8103) were examined in a nutrient-replete laboratory culture grow n under a day-night cycle in natural irradiance. Measurements of the s pectral absorption and beam attenuation coefficients, the size distrib ution of cells in suspension, and microscopic analysis of samples were made at intervals of 2-4 hours for 2 days. These measurements were us ed to calculate the optical properties at the level of a single ''mean '' cell representative of the actual population, specifically, the opt ical cross sections for spectral absorption sigma(a)(lambda), scatteri ng sigma(b)(lambda), and attenuation sigma(c) (lambda). In addition, c oncurrent determinations of chlorophyll a and particulate organic carb on allowed calculation of the Chl a- and C-specific optical coefficien ts. The refractive index of cells was derived from the observed data u sing a theory of light absorption and scattering by homogeneous sphere s. Low irradiance because of cloudy skies resulted in slow division ra tes of cells in the culture. The percentage of dividing cells was unus ually high (>30%) throughout the experiment. The optical cross section s varied greatly over a day-night cycle, with a minimum near dawn or m idmorning and maximum near dusk. During daylight hours, sigma(b) and s igma(c) can increase more than twofold and sigma(a) by as much as 45%. The real part of the refractive index n increased during the day; cha nges in n had equal or greater effect than the varying size distributi on on changes in sigma(c) and sigma(b). The contribution of changes in n to the increase of sigma(c)(660) during daylight hours was 65.7% an d 45.1% on day 1 and 2, respectively. During the dark period, when sig ma(c)(660) decreased by a factor of 2.9, the effect of decreasing n wa s c dominant (86.3%). With the exception of a few hours during the sec ond light period, the imaginary part of the refractive index n' showed little variation over a day-night cycle, and sigma(a), was largely co ntrolled by variations in cell size. The real part of the refractive i ndex at lambda = 660 nm was correlated with the intracellular C concen tration and the imaginary part at lambda = 678 nm with the intracellul ar Chl a concentration. The C-specific attenuation coefficient showed significant diel variability, which has implications for the estimatio n of oceanic primary production from measurements of diel variability in beam attenuation. This study provides strong evidence that diel var iability is an important component of the optical characterization of marine phytoplankton.