e Marine bacteria form one of the largest living surfaces on Earth, and their metabolic activity is of fundamental importance for global nutrient cycling. Here, we explored the largely unknown intracellular pathways in 25 microbes representing different classes of marine bacteria that use glucose: Alphaproteobacteria, Gammaproteobacteria, and Flavobacteriia of the Bacteriodetes phylum. We used 13 C isotope experiments to infer metabolic fluxes through their carbon core pathways. Notably, 90% of all strains studied use the Entner-Doudoroff (ED) pathway for glucose catabolism, whereas only 10% rely on the Embden-Meyerhof-Parnas (EMP) pathway. This result differed dramatically from the terrestrial model strains studied, which preferentially used the EMP pathway yielding high levels of ATP. Strains using the ED pathway exhibited a more robust resistance against the oxidative stress typically found in this environment. An important feature contributing to the preferential use of the ED pathway in the oceans could therefore be enhanced supply of NADPH through this pathway. The marine bacteria studied did not specifically rely on a distinct anaplerotic route, but the carboxylation of phosphoenolpyruvate (PEP) or pyruvate for fueling of the tricarboxylic acid (TCA) cycle was evenly distributed. The marine isolates studied belong to clades that dominate the uptake of glucose, a major carbon source for bacteria in seawater. Therefore, the ED pathway may play a significant role in the cycling of mono-and polysaccharides by bacterial communities in marine ecosystems.
Marine bacteria influence global environmental dynamics in fundamental ways by controlling the biogeochemistry and productivity of the oceans (1). Due to their importance, marine microorganisms have been studied intensively (2). In particular, their mechanisms for metabolizing carbon and other nutrients have attracted attention, because they directly or indirectly affect the biogeochemical status of seawater (3). A prominent nutrient in seawater is glucose, the most abundant free neutral aldose (4). Current estimates of glucose concentrations in seawater indicate an almost ubiquitous distribution in the oceans in a nanomolar range (5). Particularly, large amounts of glucose are available in coastal habitats, e.g., during bloom situations (6). In fact, a large fraction (Ͼ30%) of bacterial growth can be supported by this monosaccharide in some oceans (7,8). Furthermore, glucose is the dominant component of dissolved polysaccharides, which constitute up to 15% of marine dissolved organic matter (9). The turnover of the (monomeric and polymeric) glucose pool in different oceanic regions ranges from days to months, and glucose assimilation in marine surface waters may represent up to 40% of bacterial carbon production (5). Taken together, bacteria that use glucose are common in the sea (10), and glucose is a representative model nutrient to monitor carbon uptake by heterotrophic marine bacteria (11). At this point, questions that arise from current knowledge concern...
