Abundance, cellular contents of carbon and nitrogen, and C/N ratio of planktonic thraustochytrids in the Seto Inland Sea and the coastal Hyuga-nada, Japan, were determined. Thraustochytrid cells were stained with acrflavine and counted dlrectly by epifluorescence rnicroscopy.Thraustochytrids were found in the water column at a density of 2.5 X 103 to 4.5 X 104 cells I-', with an overall average of 1.6 X 104 cells 1-l. In order to estimate the impact of thraustochytrid biomass on the coastal plankton community, cellular carbon and nitrogen contents of a cultured thraustochytrid were determned. The carbon and nitrogen for 10 pm cell contents were estimated to be 1.65 X 10-4 pg C cell-' and 1.58 X 1W5 pg N cell-', respectively. The cellular C/N ratio of 10.4 was higher than that known for pelagic bacteria. The estimated population biomass of thraustochytrids varied from 0.41 to 7.44 pg C I-', with an overall average of 2.60 pg C 1-'; and 0.039 to 0.71 pg N I-', with an overall average of 0.23 pg N I-'. The thraustochytrid carbon biomass corresponded to about 3.4 to 29% of total planktonic decomposers [bacteria and thraustochytrids). It is suggested that the contribution of thraustochytrids to the microbial food chain and carbon cycling through organic degradation and production may not be negligible.

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We have developed and tested a functionally integrated in situ analyzer, the IISA-ATP system, for microbial activity assays based on a quantitative determination of the total (particulate and dissolved) ATP in ocean environments. The IISA-ATP utilizes a PDMS-glass hybrid microfluidic device as its core functional element, which can perform cell lysis and total ATP quantification by a luciferin-luciferase bioluminescence assay in situ. Transparent heaters and a temperature sensor fabricated on a glass substrate provide temperature control. As a result of the evaluation using the microfluidic device with ATP standard solutions, the bioluminescence intensity was linearly correlated with 2 × 10(-12) to 2 × 10(-8) M of ATP. A detection limit of 1.1 × 10(-11) M was determined using the completed IISA-ATP system, which includes a miniature pumping module and a control module. As a result of the evaluation using the environmental seawater sample collected from Tokyo Bay, Japan, 2.7 × 10(-10) M of total ATP was successfully determined in the laboratory by the IISA-ATP. The system was operated at a shallow submarine hot spring area in Okinawa, Japan for an in situ trial. The result shows the system was successfully operated in situ and the total ATP was determined to be 3.4 × 10(-10) M.