Cyanobacteria of the genus Synechococcus is a dominant component of microbial community in the world's oceans, and is a major contributor to marine primary productivity and thus plays an important role in carbon cycling in the oceans. Besides the ecological importance, the ultivability also made Synechococcus a very special group of marine microorganisms, which has attracted great attentions from oceanographers and biologists. Great progress in the physiology, biochemistry and phylogeny of Synechococcus has been made since its discovery. We here review the current status of molecular ecology of marine Synechococcus and give a perspective into the future based on our understanding of the literature and our own work.

Measurements of ammonium and nitrte uptakes bynatural phytoplankton assemblages from Jiaozhou Bay at various combinations of amencnium and nitrate concentations with 15N tracef techniques showed that uptake rate of cither nitrogen was influenced by the presence of the other but that the influence of ammonium on nitrate uptake was much greater that thatof nitrate om ammonmium uptake. The influence mechanism of ammonimum on iitrate uptake marifested as competition at lower concemtrations and as inhibition at higher concentrations (ammocium concentration>0.6umol/L). but no total inhibtion appeared within the concentration ragen of the eperiments (0-10 uml/L). The influence of nitrate on ammonoum uptzake seems to be a results of competiotion for uptake sites on the cell surface. In view of the in situ nutrient concentration in the given marine emironment, it is believed that both nitrogen souers are utilzed by phytoplankton. Nitrate uptake in the presence of ammorium and ammonoum uptake in the pressence of nitrate are exprssed by a non-lincurly and a linearly calibrated mMichaelis -Menten equation respectively. In addition, A non-linear fratio model is given withic may be useful in practicl studies on newproduction in Jiaozhou Bay.

A simplified protocol for determination of bacterial membrane potential (MP) by flow cytometry with 3,3'-dihexyloxacarbocyanine iodide (DiOC6(3)) as the MP probe was established, and was applied to an aerobic anoxygenic phototrophic bacterium (AAPB strain, Roseobacter clade) with an Escherichia coli strain as a reference. It was shown to be capable of precise indication of the physiological states of the cells as well as cellular responses to environmental factors. Based on the experimental results, a MP curve model was proposed for characterization of species-specific features, which offers parameters including the maximum MP of the curve (MPmax), the time to reach the MPmax (Tmax), the slope of the exponential phase of the MP curve (KMP), and the minimum (threshold) concentration of a particular substrate required for the organism to reach its MPmax (Smax). These parameters are functions of substrates and incubation conditions for a single species, and can be used for comparison between different species under the same conditions. The MPmax, Tmax, and KMP of the AAPB strain were quite different form those of the E. coli strain, and the AAPB strain was shown to be less sensitive to temperature and pH changes than the E. coli strain.

The structure and diversity of the Archaea collected from prawn farm sediment were investigated for the first time. A partial 16S ribosomal DNA library was constructed with Archaea-specific primers. Subsequently, 80 randomly selected archaeal clones from the library were analyzed by restriction fragment length polymorphism (RFLP), and resulted in 50 different RFLP patterns. Sequence analysis of representatives from each unique RFLP type revealed high diversity in the archaeal populations, and the majority of archaeal clones were either members of novel lineages or mostclosely related to uncultured clones. In the phylogenetic analysis, the archaeal clones could be grouped into discrete phylogenetic lineages within the two kingdoms Crenarchaeota and Euryarchaeota. Euryarchaeota dominated in our archaeal library, with up to 72.2% of the total clones, and Crenarchaeota represented 27.8%. Of all the Euryarchaeota clones, three clones (5.6%) were affiliated with Methanosarcinales, four clones (7.4%) were related to Methanomicrobiales, three clones (5.6%) were related to Halobacterium (with 93% similarity), and the remaining clones (81.5%) were related to those uncultured Euryarchaeota in the aquatic sediment ecosystem. None of the crenarchaeal clones were ssociated with any known cultured lineages. The selective dispersal of the archaeal population indicates that their ecological niches are associated with environmental characteristics. Novel phylotypes of Archaea would expand our understanding of the genetic diversity of Archaea in aquatic sediment systems and would be significant in the phylogenetic study of Archaea.

Prochlorococcus, a tiny oxygenic photosynthetic picoplankton with unique pigment composition, has been found to be ubiquitous and abundant in the world oceans, and has been recognized to be closely related to living resources and environmental issues. It has attracted the interest of marine biologists since its discovery, and field data on it over global oceans have accumulated rapidly in the past 10 years. In China, we have studied Prochlorococcus for 8 years, achieving a basic cological understanding. The presence of Prochlorococcus in China seas, marginal seas of the west Pacific, was confirmed, and its distribution patterns were also brought to light. Prochlorococcus is very abundant in the South China Sea and the offshore regions of the East China Sea. It is seasonally present in the southeast part of the Yellow Sea and absent in the Bohai Sea. Temporal and spatial variations of the abundance of Prochlorococcus and their affecting factors, physiological and ecological characteristics of Prochlorococcus and their relationships to the other groups of picoplankton, and the importance of Prochlorococcus in total biomass and possible roles in living resources and environmental problems are discussed. In the future, isolation of different Prochlorococcus strains from the China seas and their physiological characteristics, genetic diversity, phylogenies and gene exploiture, etc. are important issues to be addressed.