Homologs of three cell cycle proteins, proliferating cell nuclear antigen (PCNA), p34cdc2, and cyclin B have been detected in marine phytoplankton. Their molecular masses were close to those observed previously in other organisms (PCNA=36 kiloDalton; p34cdc2=34 kDa; cyclin B=63kDa). In the species examined, they were all associated with active proliferation of the cell population, and decreased to undetectable levels in the stationary stage in the culture. The putative gene of PCNA was detected by northern and southern blotting in DUNALIELLA TERTIOLECTA and ISOCHRYSIS GALBANA with the molecular size of its mRNA being the same as PCNA mRNA in rat.

An immunofluorescence protocol was developed for marine phytoplankton. This protocol has been employed successfully for nuclear, chloroplastic, and cytoplasmic proteins as well as cell surface antigens in various species. It allows long-term sample storage at -20 degrees C and immunostaining at room temperature and thus is useful for both laboratory experiments and field investigations. Immunofluorescence using this protocol demonstrated that in the Chlorophytes D. TERTIOLECTA, and CHLORELLA AUTOTROPHICA, and diatom BACILLARIOPHYCEAE THALASSIOSIRA WEISSFLOGII, the PCNA homolog was located exclusively in the nucleus of the cell. In two other diatoms SKELETONEMA COSTATUM and ETHMODISCUS REX, the PCNA immunostaining was apparent in the chloroplasts as well as in the nucleus. Immunofluorescence also showed that PCNA in D. TERTIOLECTA was associated with the S phase of the cell cycle, a feature typical of PCNA in mammalian cells. In contrast, p34cdc2 was most abundant in G1/S and G2/M transitions, and least abundant when PCNA reached its peak.

Applying PCNA immunofluorescence to a growth rate model yielded estimates with an average overestimation of 33% for three species. When the model was modified, the accuracy of the estimation was significantly improved (<16% overestimation). Using the immunofluorescence method, the cell cycle was analyzed for a unculturable field-collected diatom E. REX, and a conservative estimate of growth rate was also obtained.

Attempts to purify PCNA from marine phytoplankton was partially successful on a small scale.