Bacterial growth and the chemical composition of dissolved organic matter (DOM) were followed during a 10-d decomposition experiment with fresh, algal-derived DOM from an Arctic ice floe. During the experiment ϳ30% of the dissolved organic carbon (DOC) was used by bacteria, indicating the highly reactive nature of this fresh DOM. Over half of the DOC consumption was accounted for as losses of combined neutral sugars and amino acids. The initial composition of the DOM was characterized by high neutral sugar (14% DOC) and amino acid (7.4% DOC) yields and the dominance of glucose (ϳ75 mol%) and glutamic acid (ϳ25 mol%). During microbial degradation the neutral sugar and amino acid yields decreased, and the molecular composition of the DOM became more uniform. The relatively constant abundance of D amino acids and the dramatic changes in the neutral sugar and amino acid compositions indicated that bacteria were important in shaping the chemical composition of marine DOM by selectively removing bioreactive components and by leaving behind biorefractory components. Based on principal component analysis and other parameters, neutral sugars and amino acids were found to be excellent indicators of the diagenetic state and bioavailability of marine DOM.The interactions between bacteria and marine dissolved organic matter (DOM), one of the largest active reservoirs of organic carbon, play a major role in the global carbon cycle. Our understanding of the origin, composition, and reactivity of DOM in the ocean is still very limited. The majority of marine DOM has not been characterized on a molecular level, is very resistant to degradation, and appears to be of low molecular weight (Benner et al. 1992;Ogawa and Ogura 1992;Amon and Benner 1994). In order to account for the low molecular weight, low bioavailability, and uncharacterized nature of DOM, Amon and Benner (1996) proposed the size-reactivity continuum model. This conceptual model links the physical size of organic matter to its diagenetic state, suggesting a decrease in size with increasing diagenesis and chemical alteration. The model provides a framework for interpreting diagenetic alterations but it does not explain the mechanism for the production of biorefractory low molecular weight DOM. The concept of a diage-1 Corresponding author (ramon@awi-bremerhaven.de). AcknowledgmentsWe thank the captain, scientists and crew of the RV Polarstern for their professional assistance during the cruise ARK XIII/3. The help by I. Bussmann, R. Engbrodt, and A. Terbrüggen during the recovery of the ice floe is highly acknowledged. Special thanks go to B. Rost for his help with the experiment on board and the measurement of bacterial leucine incorporation, K. Kaiser for his crucial help with the neutral sugar analysis, and E. Helmke and J. Jürgens for the bacteria cultures.

A broad variety of new acyclic vinyl ethers (see 6-41) have been synthesized viu the vinyl-interchange reaction of ethyl vinyl ether at room temperature using mercury(I1) trifluoroacetate as a highly efficient catalyst. The appropriate vinyl ethers were reacted under acidic conditions with 3',5'-0-silyl-protected uridine 42 to the corresponding 2-0-(1-alkoxyethyl) derivatives 43 --83 which gave, on desilylation of Fions, in high yields the uridinc-2-0-acetal derivatives 84-124. The relative stabilities of the newly synthesized compounds under acidic and basic conditions were determined using TLC and HPLC techniques. Protected protecting groups offer the best properties for oligoribonucleotide syntheses. Interestingly, the very acid-stable acetals of the /$substituted ethyltype 118-121 and 123 can be cleaved by a [Mimination process providing a series of base-labile acetals of potcntial synthetic value. HFLVETICA CHIMICA ACTA -VOI 81 (1998) Scheme 2 -, P H Me H Me 43 -83 84 -124 42 6 -4 1 C1 H SO