Decrease in molecular weight of photosynthetic products of marine phytoplankton during early diagenesis

Abstract

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 .

Release of dissolved organic matter by coastal diatoms

“…This study highlights the necessity for future work to determine whether field measurements of POM and DOM are biased by species-specific differences. The bound or aggregated polysaccharides may have a much different fate than ''true'' DOC, both in terms of the potentials for export through sinking (reviewed by Passow 2002) and in differences in the potential rates of microbial turnover of the polysaccharides versus smaller monomeric compounds (e.g., Hama et al 2004). Hence, the relative amount of these different types of organic matter that are produced during blooms may significantly affect the C cycle dynamics.…”

Concentration and UV reactivity of total organic carbon in the upper layer in the oligotrophic subtropical western North Pacific

“…Hama et al [2004] observed that carbohydrates accounted for the major fraction (up to 58% of total DOC) of the freshly produced DOC during the early diagenesis of phytoplankton organic matter. They also observed that the concentration of heteropolysaccharides released from phytoplankton gradually decreased during the decomposition experiment in the time scale of a few weeks [Hama et al, 2004]. Carbohydrates are also chemically identified as the most abundant biochemical components of DOC in natural seawaters [Benner, 2002].…”

“…Although the "S-DOC" fraction has been proposed to explain the depth and seasonal variation in DOC concentration, its biogeochemical properties have not been confirmed experimentally. In contrast, the existence and time scales of the other two DOC fractions have been experimentally confirmed, i.e., the short time scales from hours to days of labile DOC (L-DOC) such as free amino acids and monosaccharides [Benner et al, 1992;Skoog et al, 1999;Hama et al, 2004] and the prolonged time scales up to several thousand years determined by the 14 C age of refractory DOC (R-DOC) [Williams and Druffel, 1987;Bauer et al, 1992;Druffel et al, 1992]. Thus, "S-DOC" can be considered as only an conceptual definition because the "semilability" has not been directly estimated.…”

“…Since then, a considerable number of studies on the marine organic carbon pool have revealed its variability in space and time [Carlson et al, , 2000Hansell and Carlson, 2001;Körtzinger et al, 2001], vertical transport [Carlson et al, 1994;Ogawa et al, 1999;Taki and Suzuki, 2001], and microbial availability Carlson and Ducklow, 1995;Hama et al, 2004]. The seasonal accumulation of DOC in the upper layer has been widely observed in mid-to high latitudinal oceans during the period of surface stratification [Williams, 1995;Carlson et al, 1998].…”

Diatom Polysaccharides: Extracellular Production, Isolation and Molecular Characterization

“…Although the excretion of photosynthetic compounds is recognised as the major source of carbohydrates in seawater [10], so far there are not many papers reporting on the molecular composition of carbohydrates in the exudates from diatom cultures [see for examples [6][7]10,[22][23][24][25][26][27][28]. The molecular-level characterization of dissolved polysaccharides may provide basic information on the origin, the bioreactivity and the fate of these biopolymers.…”