No abstract

The elemental composition and the cell cycle stages of the marine diatom Thalassiosira pseudonana Hasle and Heimdal were studied in continuous cultures over a range of different light‐ (E), nitrogen‐ (N), and phosphorus‐ (P) limited growth rates. In all growth conditions investigated, the decrease in the growth rate was linked with a higher relative contribution of the G2+M phase. The other phases of the cell cycle, G1 and S, showed different patterns, depending on the type of limitation. All experiments showed a highly significant increase in the amount of biogenic silica per cell and per cell surface with decreasing growth rates. At low growth rates, the G2+M elongation allowed an increase of the silicification of the cells. This pattern could be explained by the major uptake of silicon during the G2+M phase and by the independence of this process on the requirements of the other elements. This was illustrated by the elemental ratios Si/C and Si/N that increased from 2‐ to 6‐fold, depending of the type of limitation, whereas the C/N ratio decreased by 10% (E limitation) or increased by 50% (P limitation). The variations of the ratios clearly demonstrate the uncoupling of the Si metabolism compared with the C and N metabolisms. This uncoupling enabled us to explain that in any of the growth condition investigated, the silicification of the cells increased at low growth rates, whereas carbon and nitrogen cellular content are differently regulated, depending of the growth conditions.

Different stages of the automortality in phytoplankton have been studied applying flow cytometry. These stages are, in order of expression : (1) compromised cell membranes, (2) degradation of the photosynthetic pigments and reduction of the photosynthetic activity, (3) fragmentation of the genomic DNA. The integrity test of the cell membranes is based on the inability of the DNA-specific stain SYTOX Green to pass into cells with intact plasma membranes. The reduction in photosynthetic activity was examined by sorting "%C-labelled phytoplankton cells differing in viability. Finally, DNA fragmentation was traced by measuring changes in genomic DNA. The different phytoplankton species tested showed a great variety in response when grown under the same conditions, but there was also considerable intraspecific variation. Unstained cells, fully stained cells (equivalent to full staining of genomic DNA in fixed cells) and cells with intermediate fluorescence signal occurred together within the same culture. The photosynthetic activity in cells with a reduced viability dropped by as much as 60 % relative to that of the viable cells. In the subsequent stage, when photosynthetic pigments were fully degraded, this value dropped further to around 10 %. Cells in this stage also showed subdiploidy as a result of genome fragmentation. Field tests using samples of phytoplankton collected in the North Atlantic Ocean (40m N, 23m W) during spring showed staining properties similar to those found in cultures grown at suboptimal growth conditions. The percentage of non-viable cells varied considerably (ranging from 5 % to 60 %) between the various phytoplankton groups present. The lowest value was observed for Synechococcus, but some pico-eukaryotes showed percentages as high as 60 %. Moreover, the viability varied with depth (light level) and over a light-dark cycle. The present findings suggest the existence of a (genetically based) uniform process of automortality in phytoplankton. Non-viable cells are a substantial component of the oceanic phytoplankton, affecting the food-web structure and species succession.

Estimates of gross primary production (GPP) based on fast repetition rate fluorometer (FRRF) measurements were compared with independent 14 C and O 2 at three stations during a spring bloom in the North Atlantic. A photosynthesis versus irradiance (P-E) curve was constructed for each station from the observations of in situ photon efficiency of photosynthesis. This composite P-E curve was compared with P-E curves determined for discrete samples from 14 C assimilation. Estimates of ␣ Chl and P m Chl from the 14 C-uptake method were 1.5-2.5-fold lower than those estimated from the FRRF data. Much of this discrepancy can be accounted for if 14 C assimilation approximates net phytoplankton photosynthesis with use of a photosynthetic quotient of 1.4 mol O 2 (mol CO 2 )Ϫ1 . Photosynthetic oxygen consumption may have also contributed to the difference. In situ GPP was calculated from incident irradiance, light attenuation, light absorption by phytoplankton, and the light dependence of the in situ photon efficiency. This estimate of GPP was two times greater than net community photosynthesis determined from diel changes of in situ oxygen concentration. Thus, the in situ net O 2 and in vitro 14 C techniques yielded similar estimates of phytoplankton photosynthesis that were about twofold lower than the estimates of GPP provided by FRRF. Uncertainties in the FRRF technique associated with choosing an appropriate value of photosynthetic unit size and fitting a P-E curve to the in situ measurements are discussed. Despite these uncertainties, the FRRF results were consistent with the independent estimates of phytoplankton productivity.

We separated chlorophylls c1 c2, and c3 of marine phytoplankton together with other pigments by a modification of the commonly applied reversed‐phase‐C18‐high‐performance liquid chromatography (RP‐C18‐HPLC) method. However, the chlorophyll c‐like pigment 2,4, Mg‐divinylpheoporphyrin as monomethyl ester, co‐eluted with chlorophyll c1. The method involves optimization of the mobile phase by using a very high ion strength solvent in combination with a high carbon loaded RP‐C18 column. Fingerprints of the various taxonomic groups of algae can thus be developed in a single run, including separation of the carotenoids lutein and zeaxanthin.