1.Aggregation and subsequent vertical transport of organic carbon derived from various activities of planktonic organisms in the upper ocean are important ocean processes controlling concentrations of atmospheric carbon dioxide and called as biological pump. To understand the actual mechanisms governing biological and physico-chemical processes of particle aggregation, we conducted in situ observation of aggregation processes in the picnocline by using the in situ laser particle analyzer and underwater camera. The observation showed importance of picnocline as the active site of aggregation of organic detritus in the upper ocean.

2.The aggregation process of marine colloids promotes material cycling in marine environment by connecting dissolved materials with larger suspended particles, and by supplying food particles to micro- and meso- zooplankton. The formation rate of aggregates and their fate depend on the physical properties of materials on their surface such as electric charge, hydrophobicity and etc... We examined the potential roles of microbial organisms in the aggregation process of colloidal particles because they have potential abilities to change surface properties of particles by releasing organic compounds into dissolved pool and by transforming dissolved materials by their ectoenzymes.

Consumption by bacteria of dissolved and colloidal organic matter represents a major flux of carbon in oceanic environments. In order to better understand the dynamics and regulation of microbes-organic matter interactions in marine bio-systems, we examine 1) biochemical mechanisms underlying the degradation of a complex biopolymer (peptidoglycan) in seawater, 2) growth and mortality of different bacterial groups in coastal environments, and 3) distribution and production of microbial communities in meso- and bathypelagic environments. The data obtained by these studies may contribute to better modeling carbon cycling and bacterial community dynamics in marine ecosystems.

Dissolved organic matter (DOM) in the ocean is utilized solely bybacteria as carbon and energy sources. However, the interaction between the two is complicated due to the involvement of other processes. Recent findings of the presence of bacterial cell components in DOM fraction lead us to investigate on the process of bacterial death, release of their components and selective preservation of particular compounds. As the first step, we clarified that Pseudomonas aeruginsa, a bacterium well known as an opportunistic pathogen to human, is commonly present as the source of porin protein in marine environments. Further investigations related to DOM-microbe interactions are being conducted.

Population dynamics of marine plankton at lower trophic levels are investigated with emphases on ecological succession induced by changes in nutrient concentrations, and a symbiotic association of microalgae with a heterotrophic host. The former includes distribution study of both nutrients at nanomolar levels and phytoplankton composition in the subtropical and tropical ocean, and growth responses of natural phytoplankton and protozoan assemblages upon nutrient pulse at nanomolar levels. The latter is examined using Noctiluca scintillans with the endosymbiont Pedinomonas noctilucae.

The coastal ocean is one of the most productive aquatic ecosytems but is often stressed by anthropogenic impact. The coastal pelagic ecosystem research deals primarily with the dynamics of the food chain from phytoplankton via zooplankton to fish/jellyfish. However, the reality of the food chain is so complex and its structure and function are shifting along with the environmental change. Research is undertaken to understand the mechanisms to control the productivity and to manipulate the food chain properly in order to maintain the coastal ocean ecosystem sustainable.

The pelagic, deep-sea ecosystem is fueled by organic fluxes from the upper ocean, through mechanisms collectively referred to as the "biological pump", including fluxes by fecal pellets, deadcarcasses, marine snow, and interzonal vertical migrants. The deep-sea food web, in turn, is maintained by organisms adapted for efficient use of these resources. Through our research into the deep-sea food web focusing on the above aspects, knowledge is accumulating on the patterns of species diversity, modes of resource partitioning and feeding tactics, and the role of metazoan plankters in the mesopelagic zone.