Preservation and degradation of organic matter

All phytoplankton can assimilate the more labile forms of dissolved inorganic Fe(ll) and Fe(lll), and this is the main assimilation mode for bacteria under iron-replete conditions (Granger Price 1999). Extracellular bioreduction is an important route to these labile forms, but the photoreductive dissociation of Fe(lll) chelates in oceanic surface waters may play a major role in sustaining phyto-planktonic productivity (Sunda Huntsman 1995), as has been proposed for the aquachelins (Barbeau et al. 2001). 

The differences between prokaryotic and eukaryotic phytoplanktonic uptake of iron must affect competition and hence the composition of primary producer communities (Hutchins et al. 1999). The rate of transport of iron into cells depends upon the number of receptors on the membrane surface, so low iron concentrations favour growth of the pi-coplankton, which have a large surface area to volume ratio. 

Much of the organic material produced by phytoplankton is consumed within the euphotic zone by herbivorous zooplankton. The most important members of the 

Photolysis in surface waters may transform a part of this recalcitrant DOM into low molecular weight compounds that are available for microbial uptake (Mopper et al. 1991 Hedges 1992). Manganese oxides have also been reported to oxidize humic substances spontaneously, forming some simple compounds such as acetaldehyde and pyruvate, which are readily assimilable by microorganisms (Sunda Kieber 1994). 

After death, cells self-destruct (the process of autolysis) under the influence of hydrolytic enzymes, which, in life, aided the recycling of cellular components. This process makes proteins and other components more readily available to the decomposers. Bacteria and fungi preferentially remove the more labile components from detritus and the residue becomes increasingly refract-ory. Much of the soluble product of the microbial breakdown of organic matter diffuses upward within pore waters to the sediment—water interface and is returned to the water column. Bacteria are important in all environments, but fungi are relatively 

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Burrows, and transport of solute in them, may contribute to dissolution by enhancing oxic degradation of organic matter near the burrow walls. However, the situation is complex, and depending on factors such as the type of burrow wall produced, cementation rather than dissolution of carbonates may be promoted. Aller s observation that the best carbonate preservation takes place in the most physically disturbed and biologically underdeveloped environments points to the need for studies of continental shelf and slope environments where carbonate dissolution could be even more intense than that observed at the sites studied in Long Island Sound. 

Studies such as those of Berger and Soutar (1970) and Sholkovitz (1973) also point to the importance of chemical parameters in controlling calcium carbonate preservation. These authors noted that carbonate preservation is substantially greater in the sulfidic Santa Barbara basin sediments than in adjacent slope sediments that are overlain by oxic waters. This observation probably results from the fact that oxic degradation of organic matter and oxidation of sulfides are not likely to occur in this anoxic basin. 

Use the simple model approach from Figure 12.12 and the general values given (5 = 85%, cp = 2,5 g cm= 500 yr) to calculate the degree of organic carbon preservation when accumulation rates are 1 and 0,1 (mg cm yr ) for NRP and, respectively, and a reactive mixed layer of 10 cm thickness. Please consider that the degradation of organic matter has an effect on the sedimentation rate (co). 

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