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Changes in the size and neutral aldose (NAld) composition of phytoplankton photosynthetic products during the early diagenetic process were experimentally examined using 13 C as a tracer. Most (94.7%) of the photosynthetically produced (P-) organic carbon (OC) was found to be particulate organic carbon (POC) after a 12-h light incubation. An increase in the concentration of P-dissolved organic carbon (DOC) was found after the shift to the dark incubation, probably due to a leakage of cellular constituents. An increase in the concentration of P-DOC was mainly due to an increase in the high molecular weight (HMW: Ͼ10 kDa) fraction that reached its maximum concentration on day 3 and then declined at a relatively high rate. The change in the concentration of the low molecular weight (LMW: Ͻ10 kDa) DOC fraction was less marked than that in the HMW fraction, and the rate of decrease was much slower, indicating a more biorefractory nature of P-LMW DOC. P-dissolved carbohydrates (DCHO) accounted for 38% and 50% in P-HMW DOC and P-LMW DOC, respectively, after a 12-h light incubation. The concentration of P-LMW DCHO showed a rapid decrease in the early stage of the dark incubation, whereas the concomitant increase in the concentration of P-LMW dissolved noncarbohydrate (DnonCHO) was noticed. The decrease in the contribution of CHO was noticed both in P-POC and P-HMW DOC fractions, but the decline rates were slower than that in P-LMW DOC fraction. On day 60, the remaining P-OC accounted for 4.6% of photosynthetic material originally produced. The distribution of the size fractions of the remaining P-OC on day 60 (POC, 14%; HMW DOC, 22%; LMW DOC, 64%) indicates that phytoplankton photosynthetic products were rapidly degraded to the less bioreactive LMW DOM during early diagenesis. The present results indicate that the annual global ocean production rate of semilabile DOC with lifetimes exceeding 2 months is 1.91 PgC yr Ϫ1 .

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Combined 13C and gas chromatography ‐mass spectrometry (GC‐MS) were applied to determine the rate of amino acid production of marine phytoplankton populations. The accuracy of the estimate of the 13C ratio in each amino acid using the relative intensity of isotope peaks was investigated with 13C‐enriched amino acid standards. Production rates of 12 amino acids of a natural phytoplankton population were determined. The gross amino acid composition of photosynthetic products is generally comparable with previous reports. Production of amino acid at night was confirmed by the increase in 13C atom%; glucose appeared to supply carbon for amino acid production at night. The ratio of carbon : amino acid nitrogen in photosynthetic products showed a marked diurnal change, due mainly to the loss of glucose carbon at night and the continuing uptake of nitrogen.

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