Search for: All records

Abstract Heterotrophic bacteria process a large fraction of the polysaccharides produced by phytoplankton in the surface ocean. Some of these polysaccharides are taken up by “selfish” bacteria, which bind, partially hydrolyze, and transport large fragments of polysaccharides into the cell with little loss of hydrolysis products. Recently, selfish bacteria have also been identified in deep ocean water. Whether these bacteria effectively capture polysaccharides under hydrostatic pressures typical of the deep ocean, however, remained an open question. Here, for the first time, we measured the extent of selfish uptake of laminarin—a common marine polysaccharide—in surface and deep waters, under pressures ranging from atmospheric up to ~50 MPa (equivalent to 5000 m depth). We incubated seawater from Japan, Denmark, and the western North Atlantic Ocean in pressure vessels and found that the extent of selfish uptake of laminarin varied somewhat by site but was insensitive to hydrostatic pressure. Deployment of an in situ incubator in the North Atlantic Ocean also showed evidence of selfish uptake in situ. These results suggest that selfish uptake can play a key role in polysaccharide degradation, especially in the deep ocean where other modes of organic matter degradation may be more inhibited by high hydrostatic pressure. 

Abstract Heterotrophic microorganisms are responsible for transforming and respiring a substantial fraction of the organic matter produced by phytoplankton in the surface ocean. Much of this organic matter is composed of polysaccharides, high‐molecular weight (HMW) sugars. To initiate degradation of polysaccharides, microorganisms must produce extracellular enzymes of the right structural specificity to hydrolyze these complex structures. To date, most measurements of enzyme activities are made at in situ temperatures, but at atmospheric pressure. However, previous studies have shown that hydrostatic pressure can impact the functionality of enzymes. Since deep sea communities may be seeded by microbes from shallow waters, we aimed to determine if pressure affects the performance of enzymes from coastal waters. To determine the extent to which enzymatic activities of coastal microbial communities are affected by pressure, we quantified the degradation of seven polysaccharides under pressures ranging from 0.1 MPa (atmospheric) to 40 MPa (equivalent to 4,000 m). Enzyme activities of pelagic communities were inhibited with increased pressure, while enzyme activities of benthic microbial communities were more resistant to increased pressure. Addition of HMW organic matter resulted in communities with enzyme activities that were more resistant to increased pressure. However, the freely‐dissolved enzymes (<0.2 μm) produced by these communities were strongly inhibited by increased hydrostatic pressure, suggesting that the pressure‐resistant enzymes were cell‐surface attached. Because pressure inhibition of enzyme activities varied strongly by polysaccharide, we surmise that the structural complexity of a polysaccharide—and therefore the number of distinct enzymes required for hydrolysis—is likely closely associated with pressure inhibition. 

Abstract. Heterotrophic bacteria process nearly half of the organic matter produced by phytoplankton in the surface ocean. Much of this organic matter consists of high-molecular-weight (HMW) biopolymers such as polysaccharides and proteins, which must initially be hydrolyzed to smaller sizes by structurally specific extracellular enzymes. Few previous studies, however, have investigated the structural complexity of polysaccharides among regions and depths. To simultaneously investigate substrate structure and microbial community composition and function, we concurrently determined carbohydrate abundance and structural complexity, bacterial community composition, and peptidase and polysaccharide hydrolase activities across depth gradients from surface to bottom water at four distinct stations in the western North Atlantic Ocean. Although the monosaccharide constituents of particulate organic matter (POM) were similar among stations, the structural complexity of POM-derived polysaccharides varied by depth and station, as demonstrated by polysaccharide-specific antibody probing. Bacterial community composition and polysaccharide hydrolase activities also varied substantially by depth, suggesting that the structure and function of bacterial communities may be related to substrate structural complexity. Thus, the extent to which bacteria can transform organic matter in the ocean is dependent on both the structural complexity of the organic matter and their enzymatic capabilities in different depths and regions of the ocean. 

Abstract Heterotrophic bacteria hydrolyze high molecular weight (HMW) organic matter extracellularly prior to uptake, resulting in diffusive loss of hydrolysis products. An alternative ‘selfish’ uptake mechanism that minimises this loss has recently been found to be common in the ocean. We investigated how HMW organic matter addition affects these two processing mechanisms in surface and bottom waters at three stations in the North Atlantic Ocean. A pulse of HMW organic matter increased cell numbers, as well as the rate and spectrum of extracellular enzymatic activities at both depths. The effects on selfish uptake were more differentiated: in Gulf Stream surface waters and productive surface waters south of Newfoundland, selfish uptake of structurally simple polysaccharides increased upon HMW organic matter addition. The number of selfish bacteria taking up structurally complex polysaccharides, however, was largely unchanged. In contrast, in the oligotrophic North Atlantic gyre, despite high external hydrolysis rates, the number of selfish bacteria was unchanged, irrespective of polysaccharide structure. In deep bottom waters (> 4000 m), structurally complex substrates were processed only by selfish bacteria. Mechanisms of substrate processing—and the extent to which hydrolysis products are released to the external environment—depend on substrate structural complexity and the resident bacterial community. 

Abstract Heterotrophic bacteria in the ocean invest carbon, nitrogen, and energy in extracellular enzymes to hydrolyze large substrates to smaller sizes suitable for uptake. Since hydrolysis products produced outside of a cell may be lost to diffusion, the return on this investment is uncertain. Selfish bacteria change the odds in their favor by binding, partially hydrolyzing, and transporting polysaccharides into the periplasmic space without loss of hydrolysis products. We expected selfish bacteria to be most common in the upper ocean, where phytoplankton produce abundant fresh organic matter, including complex polysaccharides. We, therefore, sampled water in the western North Atlantic Ocean at four depths from three stations differing in physiochemical conditions; these stations and depths also differed considerably in microbial community composition. To our surprise, we found that selfish bacteria are common throughout the water column of the ocean, including at depths greater than 5500 m. Selfish uptake as a strategy thus appears to be geographically—and phylogenetically—widespread. Since processing and uptake of polysaccharides require enzymes that are highly sensitive to substrate structure, the activities of these bacteria might not be reflected by measurements relying on uptake only of low molecular weight substrates. Moreover, even at the bottom of the ocean, the supply of structurally-intact polysaccharides, and therefore the return on enzymatic investment, must be sufficient to maintain these organisms. 

Abstract High hydrostatic pressure is characteristic of the deep ocean and is presumed to influence microbial functions and viability. However, marine microbial processes are typically measured only at atmospheric pressure (0.1 MPa), limiting our understanding of pressure effects on the activities of microbes that sink as part of the biological carbon pump, as well as those that reside in the deep ocean. To test pressure effects on microbial functions, we measured extracellular enzymatic activities—the first step in organic matter remineralization—of a moderate piezophile (Photobacterium profundumSS9), as well as of microbial communities in waters and sediments from shallow to abyssal (5500 m) depths and their cell‐free enzymes (< 0.2 μm). Activities were measured at 0.1–100 MPa to assess the pressure effects across the nearly full range of oceanic depths.Photobacterium profundumSS9 enzymes show varying pressure effects, from considerable stimulation at optimal pressure (28 MPa) to near complete inhibition (100 MPa). Pressure effects were measured for diverse protein‐ and carbohydrate‐degrading and phosphorus‐acquiring enzymes among pelagic and benthic microbial communities. The most common pressure effect was partial activity inhibition, indicating a dampening of the initial step of carbon remineralization at increasing pressures. However, the retention of cell‐free enzymatic activities at higher pressures was occasionally observed even for enzymes from surface‐originating assemblages, indicating functionality down to hadal depths and their potential for piezotolerance. These variable pressure effects must be considered when quantifying degradation rates of sinking and deposited particulate matter at increasing pressures in the deep ocean. 

Heterotrophic bacteria initiate the degradation of high molecular weight organic matter by producing an array of extracellular enzymes to hydrolyze complex organic matter into sizes that can be taken up into the cell. These bacterial communities differ spatially and temporally in composition, and potentially also in their enzymatic complements. Previous research has shown that particle-associated bacteria can be considerably more active than bacteria in the surrounding bulk water, but most prior studies of particle-associated bacteria have been focused on the upper ocean - there are few measurements of enzymatic activities of particle-associated bacteria in the mesopelagic and bathypelagic ocean, although the bacterial communities in the deep are dependent upon degradation of particulate organic matter to fuel their metabolism. We used a broad suite of substrates to compare the glucosidase, peptidase, and polysaccharide hydrolase activities of particle-associated and unfiltered seawater microbial communities in epipelagic, mesopelagic, and bathypelagic waters across 11 stations in the western North Atlantic. We concurrently determined bacterial community composition of unfiltered seawater and of samples collected via gravity filtration (>3 μm). Overall, particle-associated bacterial communities showed a broader spectrum of enzyme activities compared with unfiltered seawater communities. These differences in enzymatic activities were greater at offshore than at coastal locations, and increased with increasing depth in the ocean. The greater differences in enzymatic function measured on particles with depth coincided with increasing differences in particle-associated community composition, suggesting that particles act as ‘specialty centers’ that are essential for degradation of organic matter even at bathypelagic depths.