Limitation of Bacterial Growth by Dissolved Organic Matter and Iron in the Southern Ocean

The importance of resource limitation in controlling bacterial growth in the high-nutrient, low-chlorophyll (HNLC) region of the Southern Ocean was experimentally determined during February and March 1998. Organic- and inorganic-nutrient enrichment experiments were performed between 42°S and 55°S along 141°E. Bacterial abundance, mean cell volume, and [³H]thymidine and [³H]leucine incorporation were measured during 4- to 5-day incubations. Bacterial biomass, production, and rates of growth all responded to organic enrichments in three of the four experiments. These results indicate that bacterial growth was constrained primarily by the availability of dissolved organic matter. Bacterial growth in the subtropical front, subantarctic zone, and subantarctic front responded most favorably to additions of dissolved free amino acids or glucose plus ammonium. Bacterial growth in these regions may be limited by input of both organic matter and reduced nitrogen. Unlike similar experimental results in other HNLC regions (subarctic and equatorial Pacific), growth stimulation of bacteria in the Southern Ocean resulted in significant biomass accumulation, apparently by stimulating bacterial growth in excess of removal processes. Bacterial growth was relatively unchanged by additions of iron alone; however, additions of glucose plus iron resulted in substantial increases in rates of bacterial growth and biomass accumulation. These results imply that bacterial growth efficiency and nitrogen utilization may be partly constrained by iron availability in the HNLC Southern Ocean.     

Limitation of bacterial growth by dissolved organic matter and iron in the Southern ocean

The importance of resource limitation in controlling bacterial growth in the high-nutrient, low-chlorophyll (HNLC) region of the Southern Ocean was experimentally determined during February and March 1998. Organic- and inorganic-nutrient enrichment experiments were performed between 42 degrees S and 55 degrees S along 141 degrees E. Bacterial abundance, mean cell volume, and [(3)H]thymidine and [(3)H]leucine incorporation were measured during 4- to 5-day incubations. Bacterial biomass, production, and rates of growth all responded to organic enrichments in three of the four experiments. These results indicate that bacterial growth was constrained primarily by the availability of dissolved organic matter. Bacterial growth in the subtropical front, subantarctic zone, and subantarctic front responded most favorably to additions of dissolved free amino acids or glucose plus ammonium. Bacterial growth in these regions may be limited by input of both organic matter and reduced nitrogen. Unlike similar experimental results in other HNLC regions (subarctic and equatorial Pacific), growth stimulation of bacteria in the Southern Ocean resulted in significant biomass accumulation, apparently by stimulating bacterial growth in excess of removal processes. Bacterial growth was relatively unchanged by additions of iron alone; however, additions of glucose plus iron resulted in substantial increases in rates of bacterial growth and biomass accumulation. These results imply that bacterial growth efficiency and nitrogen utilization may be partly constrained by iron availability in the HNLC Southern Ocean.     

Viral and bacterial abundance and production in the Western Pacific Ocean and the relation to other oceanic realms

We completed a transect through the Western Pacific Warm Pool to examine how environmental variables may influence viral and bacterial abundance and production rates in this globally important oceanic region. Of the variables analyzed, viral abundance and production had the most significant relationship to bacterial cell abundance: viral parameters were not significantly correlated to the measured environmental variables, including temperature. Bacterial production rates were significantly correlated to temperature in open ocean waters, but not in waters close to land masses. Analyses of 16S rRNA gene by pyrosequencing indicated only minor changes in eubacterial community structure across the transect, with α-proteobacteria dominating all sampled populations. Diversity within the prokaryotic community did not correlate directly with viral abundance or activity. Comparisons to two other ocean-scale transects (> 8000 km of open ocean in total) in the Atlantic Ocean indicated that correlations between viral and bacterial abundance and production relative to environmental variables are regime dependent. In particular, correlations to temperature showed remarkable differences across the three transects. Collectively, our observations suggest that seemingly similar oceanic regions may have very different microbial community responses to environmental variables. Our observations and analyses demonstrate that ocean-scale generalizations may not apply in the case of viral ecology.     

Cascading influence of inorganic nitrogen sources on DOM production,composition, lability and microbial community structure in the open ocean

Nitrogen frequently limits oceanic photosynthesis and the availability of inorganic nitrogen sources in the surface oceans is shifting with global change. We evaluated the potential for abrupt increases in inorganic N sources to induce cascading effects on dissolved organic matter (DOM) and microbial communities in the surface ocean. We collected water from 5 m depth in the central North Pacific and amended duplicate 20 liter polycarbonate carboys with nitrate or ammonium, tracking planktonic carbon fixation, DOM production, DOM composition and microbial community structure responses over 1 week relative to controls. Both nitrogen sources stimulated bulk phytoplankton, bacterial and DOM production and enriched Synechococcus and Flavobacteriaceae; ammonium enriched for oligotrophic Actinobacteria OM1 and Gammaproteobacteria KI89A clades while nitrate enriched Gammaproteobacteria SAR86, SAR92 and OM60 clades. DOM resulting from both N enrichments was more labile and stimulated growth of copiotrophic Gammaproteobacteria (Alteromonadaceae and Oceanospirillaceae) and Alphaproteobacteria (Rhodobacteraceae and Hyphomonadaceae) in weeklong dark incubations relative to controls. Our study illustrates how nitrogen pulses may have direct and cascading effects on DOM composition and microbial community dynamics in the open ocean.     

Bacterial Consumption of Humic and Non-Humic Low and High Molecular Weight DOM and the Effect of Solar Irradiation on the Turnover of Labile DOM in the Southern Ocean

The decomposition of dissolved organic matter (DOM) in pelagic ecosystems is mediated primarily by heterotrophic bacteria, but transformation by short-wave solar radiation may play an important role in surface waters, in particular when humic substances constitute a substantial fraction of the DOM pool. Most of the studies examining bacterial decomposition and photochemical transformation of DOM stem from limnetic and coastal marine systems and much less information is available from oceanic environments. To examine the bacterial decomposition of humic and non-humic DOM in the Southern Ocean we carried out microcosm experiments in which we measured bacterial growth on isolated fractions of humic and non-humic DOM of the size classes <3 kDa and >3 kDa. Experiments carried out at the Polar Front showed a preferential bacterial growth on non-humic DOM and in particular on the size fraction <3 kDa. Bacterial growth, measured as bacterial biomass production, on non-humic DOM accounted for 74% to 88% of the total growth on all four DOM fractions. In experiments in the Antarctic circumpolar current and the coastal current under pack ice, bacterial growth was 6× lower than at the Polar Front, and humic and non-humic DOM was consumed to equal amounts. The size fraction <3 kDa was always preferred. Experiments examining the effect of solar radiation on the release of dissolved amino acids (DAA) and carbohydrates (DCHO) and their subsequent bacterial utilization showed a stimulating effect on glucose uptake and the release of DAA at the Polar Front but an inhibition in the eastern Weddell Sea. Ultraviolet-B was the most effective component of the solar radiation spectrum tested. Effects of UV-B on glucose uptake and release of DAA were positively correlated with concentrations of humic-bound DAA. The data imply that at low concentrations, e.g., <100 nM (amino acid equivalent), UV-irradiation reduces, whereas at concentrations >100 nM UV-irradiation stimulates glucose uptake and release of DAA as compared to dark conditions.     

Effects of Photochemical Transformations of Dissolved Organic Matter on Bacterial Metabolism and Diversity in Three Contrasting Coastal Sites in the Northwestern Mediterranean Sea during Summer

The effects of phototransformation of dissolved organic matter (DOM) on bacterial growth, production, respiration, growth efficiency, and diversity were investigated during summer in two lagoons and one oligotrophic coastal water samples from the Northwestern Mediterranean Sea, differing widely in DOM and chromophoric DOM concentrations. Exposure of 0.2-μm filtered waters to full sun radiation for 1 d resulted in small changes in optical properties and concentrations of DOM, and no changes in nitrate, nitrite, and phosphate concentrations. After exposure to sunlight or dark (control) treatments, the water samples were inoculated with the original bacterial community. Phototransformation of DOM had contrasting effects on bacterial production and respiration, depending on the water’s origin, resulting in an increase of bacterial growth efficiency for the oligotrophic coastal water sample (120%) and a decrease for the lagoon waters (20 to 40%) relative to that observed in dark treatments. We also observed that bacterial growth on DOM irradiated by full sun resulted in changes in community structure of total and metabolically active bacterial cells for the three locations studied when compared to the bacteria growing on un-irradiated DOM, and that changes were mainly caused by phototransformation of DOM by UV radiation for the eutrophic lagoon and the oligotrophic coastal water and by photosynthetically active radiation (PAR) for the mesoeutrophic lagoon. These initial results indicate that phototransformation of DOM significantly alters both bacterial metabolism and community structure in surface water for a variety of coastal ecosystems in the Mediterranean Sea. Further studies will be necessary to elucidate a more detailed appreciation of potential temporal and spatial variations of the effects measured.     

The Response of Vibrio- and Rhodobacter-Related Populations of the NW Mediterranean Sea to Additions of Dissolved Organic Matter, Phages, or Dilution

We investigated the growth response of the heterotrophic prokaryotic community focusing on Vibrio- and Rhodobacter-related populations (SRF3) to variation in the availability of dissolved organic matter (DOM), population density-dependent effects, and prokaryotic virus (phage) infection in coastal and offshore waters of the NW Mediterranean Sea. We tested the response of the prokaryotic community to three different DOM fractions prepared by ultrafiltration. One of the DOM fractions contained phages (<0.2 m), a second was virus-free (<100 kDa), and a third contained only low molecular weight (<1 kDa). The proportion of Vibrio and SRF3 populations as determined by fluorescent in situ hybridization in the community ranged from <1 to 6.2% and from 3.2 to 6.3%, respectively. Based on changes in cell numbers, growth rates ranged from 2.1 to 3.1 day⁻¹ for Vibrio and from 0.8 to 1.2 day⁻¹ for SRF3. Growth rates of Vibrio were similar or higher than those of the total prokaryotic community, whereas the ability of Vibrio to use high molecular weight (HMW) DOM and the responses to additions of phage-rich material were lower. Growth rates of SRF3 were lower than that of the community. Susceptibility to infection of SRF3 was sometimes lower than in the community, whereas the growth stimulation of HMW DOM was similar or lower. Reducing the cell concentrations of the prokaryotic community by dilution stimulated the overall growth of the community, including that of its constituent Vibrio and SRF3 populations, but the effect was smaller on the SRF3 and greater on Vibrio populations than for the total community. Comparisons with the community also revealed that life strategy traits of bacterial populations differed between coastal and offshore waters. Overall, our data suggest that Vibrio is an r-strategist or opportunistic population in the NW Mediterranean Sea, whereas SRF3 is a K-strategist or equilibrium population.     

Comparing temperature sensitivity of bacterial growth in Antarctic marine water and soil

The western Antarctic Peninsula is an extreme low temperature environment that is warming rapidly due to global change. Little is known, however, on the temperature sensitivity of growth of microbial communities in Antarctic soils and in the surrounding oceanic waters. This is the first study that directly compares temperature adaptation of adjacent marine and terrestrial bacteria in a polar environment. The bacterial communities in the ocean were adapted to lower temperatures than those from nearby soil, with cardinal temperatures for growth in the ocean being the lowest so far reported for microbial communities. This was reflected in lower minimum (T_min) and optimum temperatures (T_opt) for growth in water (?17 and +20°C, respectively) than in soil (?11 and +27°C), with lower sensitivity to changes in temperature (Q₁₀; 0–10°C interval) in Antarctic water (2.7) than in soil (3.9). This is likely due to the more stable low temperature conditions of Antarctic waters than soils, and the fact that maximum in situ temperatures in water are lower than in soils, at least in summer. Importantly, the thermally stable environment of Antarctic marine water makes it feasible to create a single temperature response curve for bacterial communities. This would thus allow for calculations of temperature‐corrected growth rates, and thereby quantifying the influence of factors other than temperature on observed growth rates, as well as predicting the effects of future temperature increases on Antarctic marine bacteria.     

Production of N2 through Anaerobic Ammonium Oxidation Coupled to Nitrate Reduction in Marine Sediments

In the global nitrogen cycle, bacterial denitrification is recognized as the only quantitatively important process that converts fixed nitrogen to atmospheric nitrogen gas, N₂, thereby influencing many aspects of ecosystem function and global biogeochemistry. However, we have found that a process novel to the marine nitrogen cycle, anaerobic oxidation of ammonium coupled to nitrate reduction, contributes substantially to N₂ production in marine sediments. Incubations with ¹⁵N-labeled nitrate or ammonium demonstrated that during this process, N₂ is formed through one-to-one pairing of nitrogen from nitrate and ammonium, which clearly separates the process from denitrification. Nitrite, which accumulated transiently, was likely the oxidant for ammonium, and the process is thus similar to the anammox process known from wastewater bioreactors. Anaerobic ammonium oxidation accounted for 24 and 67% of the total N₂ production at two typical continental shelf sites, whereas it was detectable but insignificant relative to denitrification in a eutrophic coastal bay. However, rates of anaerobic ammonium oxidation were higher in the coastal sediment than at the deepest site and the variability in the relative contribution to N₂ production between sites was related to large differences in rates of denitrification. Thus, the relative importance of anaerobic ammonium oxidation and denitrification in N₂ production appears to be regulated by the availability of their reduced substrates. By shunting nitrogen directly from ammonium to N₂, anaerobic ammonium oxidation promotes the removal of fixed nitrogen in the oceans. The process can explain ammonium deficiencies in anoxic waters and sediments, and it may contribute significantly to oceanic nitrogen budgets.     

Balance between primary and bacterial production in North Patagonian shallow lakes

The shallow Andean North Patagonian lakes are suitable environments for the evaluation of autotrophic and heterotrophic production under a scenario of high radiation in high dissolved organic matter (DOM) systems. We aimed to study the balance between primary and bacterial production in three shallow Andean lakes, in a summer sampling (high irradiance condition). Our hypothesis is that two factors would interact: high light and high DOM, affecting bacteria and algae. We carried out experiments of bacterial production (BP) by measuring [¹⁴C]-l-leucine incorporation and PP by ¹⁴C uptake in two fractions: picophytoplankton and phytoplankton >2 μm. Cell abundance, chlorophyll a, nutrients, DOM, light, and temperature were also measured. The contribution of picophytoplankton to total primary production (PP) was, in general, very high exceeding 50%. Picophytoplankton was photosynthetically more efficient than the larger autotrophs in all lakes. We observed a decrease in PP at surface levels due the high solar irradiances, while BP was not affected. Changes in the PP:BP ratios were observed in relation to DOM content and light effect. Our data indicate that the amount of available DOM drives the balance between PP and BP. However, solar radiation should be included as an important factor since PP:BP ratio may decrease because of PP photoinhibition.     

Experimental evidence that terrestrial carbon subsidies increase CO<Subscript>2</Subscript> flux from lake ecosystems

Subsidies are donor-controlled inputs of nutrients and energy that can affect ecosystem-level processes in a recipient environment. Lake ecosystems receive large inputs of terrestrial carbon (C) in the form of dissolved organic matter (DOM). DOM inputs may energetically subsidize heterotrophic bacteria and determine whether lakes function as sources or sinks of atmospheric CO₂. I experimentally tested this hypothesis using a series of mesocosm experiments in New England lakes. In the first experiment, I observed that CO₂ flux increased by 160% 4 days following a 1,000 m C addition in the form of DOM. However, this response was relatively short lived, as there was no effect of DOM enrichment on CO₂ flux beyond 8 days. In a second experiment, I demonstrated that peak CO₂ flux from mesocosms in two lakes increased linearly over a broad DOM gradient (slope for both lakes=0.02±0.001 mm CO₂·m^–2 day^–1 per m DOC, mean±SE). Concomitant changes in bacterial productivity and dissolved oxygen strengthen the inference that increasing CO₂ flux resulted from the metabolism of DOM. I conducted two additional studies to test whether DOM-correlated attributes were responsible for the observed change in plankton metabolism along the subsidy gradient. First, terrestrial DOM reduced light transmittance, but experimental shading revealed that this was not responsible for the observed patterns of CO₂ flux. Second, organically bound nitrogen (N) and phosphorus (P) accompanied DOM inputs, but experimental nutrient additions (without organic C) caused mesocosms to be satuated with CO₂. Together, these results suggest that C content of terrestrial DOM may be an important subsidy for freshwater bacteria that can influence whether recipient aquatic ecosystems are sources or sinks of atmospheric CO₂.     

Increases in soil respiration following labile carbon additions linked to rapid shifts in soil microbial community composition

Organic matter decomposition and soil CO₂ efflux are both mediated by soil microorganisms, but the potential effects of temporal variations in microbial community composition are not considered in most analytical models of these two important processes. However, inconsistent relationships between rates of heterotrophic soil respiration and abiotic factors, including temperature and moisture, suggest that microbial community composition may be an important regulator of soil organic matter (SOM) decomposition and CO₂ efflux. We performed a short-term (12-h) laboratory incubation experiment using tropical rain forest soil amended with either water (as a control) or dissolved organic matter (DOM) leached from native plant litter, and analyzed the effects of the treatments on soil respiration and microbial community composition. The latter was determined by constructing clone libraries of small-subunit ribosomal RNA genes (SSU rRNA) extracted from the soil at the end of the incubation experiment. In contrast to the subtle effects of adding water alone, additions of DOM caused a rapid and large increase in soil CO₂ flux. DOM-stimulated CO₂ fluxes also coincided with profound shifts in the abundance of certain members of the soil microbial community. Our results suggest that natural DOM inputs may drive high rates of soil respiration by stimulating an opportunistic subset of the soil bacterial community, particularly members of the Gammaproteobacteria and Firmicutes groups. Our experiment indicates that variations in microbial community composition may influence SOM decomposition and soil respiration rates, and emphasizes the need for in situ studies of how natural variations in microbial community composition regulate soil biogeochemical processes.     

Depleted dissolved organic carbon and distinct bacterial communities in the water column of a rapid-flushing coral reef ecosystem

Coral reefs are highly productive ecosystems bathed in unproductive, low-nutrient oceanic waters, where microbially dominated food webs are supported largely by bacterioplankton recycling of dissolved compounds. Despite evidence that benthic reef organisms efficiently scavenge particulate organic matter and inorganic nutrients from advected oceanic waters, our understanding of the role of bacterioplankton and dissolved organic matter (DOM) in the interaction between reefs and the surrounding ocean remains limited. In this study, we present the results of a 4-year study conducted in a well-characterized coral reef ecosystem (Paopao Bay, Moorea, French Polynesia) where changes in bacterioplankton abundance and dissolved organic carbon (DOC) concentrations were quantified and bacterial community structure variation was examined along spatial gradients of the reef:ocean interface. Our results illustrate that the reef is consistently depleted in concentrations of both DOC and bacterioplankton relative to offshore waters (averaging 79 μmol l⁻¹ DOC and 5.5 × 10⁸ cells l⁻¹ offshore and 68 μmol l⁻¹ DOC and 3.1 × 10⁸ cells l⁻¹ over the reef, respectively) across a 4-year time period. In addition, using a suite of culture-independent measures of bacterial community structure, we found consistent differentiation of reef bacterioplankton communities from those offshore or in a nearby embayment across all taxonomic levels. Reef habitats were enriched in Gamma-, Delta-, and Betaproteobacteria, Bacteriodetes, Actinobacteria and Firmicutes. Specific bacterial phylotypes, including members of the SAR11, SAR116, Flavobacteria, and Synechococcus clades, exhibited clear gradients in relative abundance among nearshore habitats. Our observations indicate that this reef system removes oceanic DOC and exerts selective pressures on bacterioplankton community structure on timescales approximating reef water residence times, observations which are notable both because fringing reefs do not exhibit long residence times (unlike those characteristic of atoll lagoons) and because oceanic DOC is generally recalcitrant to degradation by ambient microbial assemblages. Our findings thus have interesting implications for the role of oceanic DOM and bacterioplankton in the ecology and metabolism of reef ecosystems.     

Beneficial and detrimental interactive effects of dissolved organic matter and ultraviolet radiation on Zooplankton in a transparent lake

While changes in dissolved organic matter (DOM) concentrations are expected to affect zooplankton species through attenuation of potentially damaging ultraviolet (UV) radiation, generation of potentially beneficial or harmful photoproducts, pH alteration, and microbial food web stimulation, the combined effects of such changes on zooplankton community structure have not been studied previously. Our purpose was to determine how an increase in allochthonous DOM and associated changes in pH in an initially transparent lake may affect zooplankton community structure, and how exposure to solar UV may alter these DOM and pH effects. We ran microcosm experiments manipulating UV, DOM, and pH near the surface of Lake Giles in northeastern Pennsylvania. We found that when DOM was added in the presence of ambient UV, Daphnia and copepod UV-mortality was reduced by approximately three and two times compared to UV exposure without extra DOM. When DOM was added in the absence of UV, adult Daphnia and copepods were reduced compared to no DOM addition in the absence of UV. Daphnia and cyclopoid egg production and rotifer abundance were generally higher in the presence of DOM, regardless of UV treatment. The lower abundance yet high egg production in the presence of DOM and absence of UV may be explained by higher abundance of egg-bearing adults compared to non-egg-bearers. We conclude that allochthonous DOM benefits some zooplankton in a high-UV environment, but may be detrimental under low-UV conditions. Overall, Daphnia abundance and egg production were higher than that of calanoid copepods in the DOM additions, indicating that in some lakes an increase in allochthonous DOM may lead to a zooplankton community shift favoring Daphnia over calanoid copepods.     

Transport-limited fermentation and growth of Saccharomyces cerevisiae and its competitive inhibition

Summary The anaerobic glucose uptake (at 20°, pH 3.5) by resting cells of Saccharomyces cerevisiae followed unidirectional Michaelis-Menten kinetics and was competitively inhibited by l-sorbose; K _m and K _i were respectively 5.6×10^-4 m and 1.8×10^-1 m; V _max was 6.5×10^-8 moles mg^-1 min^-1. The aerobic uptake of glucose by resting yeast was also inhibited by l-sorbose but did not follow unidirectional Michaelis-Menten kinetics. Glucose-limited growth in the chemostat of a respiration-deficient mutant of S. cerevisiae was competitively inhibited by l-sorbose. As predicted by theory for transport-limited growth in the chemostat (van Uden, 1967) the steady state glucose concentrations were linear functions of the l-sorbose concentrations with different slopes at different dilution rates; K _m and K _i were respectively 7.2×10^-4 m and 1.8×10^-1 m. It is concluded that glucose transport was the rate-limiting step of anaerobic fermentation of S. cerevisiae and of growth of the mutant and that l-sorbose is a competitive inhibitor of active glucose transport in this yeast. The latter conclusion is accommodated in the transport model of van Steveninck and Rothstein (1965).     

Uptake of Dissolved Organic Carbon by Gammaproteobacterial Subgroups in Coastal Waters of the West Antarctic Peninsula

Heterotrophic bacteria are well known to be key players in the turnover of dissolved organic material (DOM) in the oceans, but the relationship between DOM uptake and bacterial clades is still not well understood. Here we explore the turnover and single-cell use of glucose, an amino acid mixture, N-acetylglucosamine (NAG), and protein by gammaproteobacterial clades in coastal waters of the West Antarctic Peninsula in summer and fall. More than 60% of the cells within two closely related gammaproteobacterial clades, Ant4D3 and Arctic96B-16, were active in using the amino acid mixture, protein, and NAG. In contrast, an average of only 7% of all SAR86 cells used amino acids and protein even in summer when DOM use was high. In addition to DOM uptake within a group, we explored the contribution of the three gammaproteobacterial groups to total community uptake of a compound. SAR86 contributed 5- to 10-fold less than the other gammaproteobacterial subgroups to the uptake of all compounds. We found that the overall contribution of the Ant4D3 clade to DOM uptake was highest, whereas the SAR86 clade contributed the least to DOM turnover in West Antarctic Peninsula waters. Our results suggest that the low growth activity of a bacterial clade leads to low abundance, fewer active cells and a low contribution to the turnover of DOM components.     

Responses of physiological groups of tropical heterotrophic bacteria to temperature and dissolved organic matter additions: food matters more than warming

Compared to higher latitudes, tropical heterotrophic bacteria may be less responsive to warming because of strong bottom-up control. In order to separate both drivers, we determined the growth responses of bacterial physiological groups to temperature after adding dissolved organic matter (DOM) from mangroves, seagrasses and glucose to natural seawater from the Great Barrier Reef. Low (LNA) and high (HNA) nucleic acid content, membrane-intact (Live) and membrane-damaged (Dead) plus actively respiring (CTC+) cells were monitored for 4 days. Specific growth rates of the whole community were significantly higher (1.9 day^-1) in the mangrove treatment relative to the rest (0.2–0.4 day^-1) at in situ temperature and their temperature dependence, estimated as activation energy, was also consistently higher. Strong bottom-up control was suggested in the other treatments. Cell size depended more on DOM than temperature. Mangrove DOM resulted in significantly higher contributions of Live, HNA and CTC+ cells to total abundance, while the seagrass leachate reduced Live cells below 50%. Warming significantly decreased Live and CTC+ cells contributions in most treatments. Our results suggest that only in the presence of highly labile compounds, such as mangroves DOM, can we anticipate increases in heterotrophic bacteria biomass in response to warming in tropical regions.     

Bacterial Growth on Photochemically Transformed Leachates from Aquatic and Terrestrial Primary Producers

Abstract We measured bacterial growth on phototransformed dissolved organic matter (DOM) leached from eight different primary producers. Leachates (10 mg C liter⁻¹) were exposed to artificial UVA + UVB radiation, or kept in darkness, for 20 h. DOM solutions were subsequently inoculated with lake water bacteria. Photoproduction of dissolved inorganic carbon (DIC), ranging from 3 to 16 μg C liter⁻¹ h⁻¹, and changes in the absorptive characteristics of the DOM were observed for all leachates upon UV irradiation. The effects of irradiation exposure on DOM bioavailability varied greatly, depending on leachate and type of bacterial growth criterion. Bacterial carbon utilization (biomass production plus respiration) over the entire incubation period (120 h) was enhanced by UV radiation of leachate from the terrestrial leaves, relative to carbon utilization in non-irradiated leachates. Conversely, carbon utilization was reduced by radiation of the leachates from aquatic macrophytes. In a separate experiment, the stable C and N isotope composition of bacteria grown on irradiated and non-irradiated DOM was estimated. Bacterial growth on UV-irradiated DOM was enriched in ¹³C relative to the bacteria in the non-irradiated treatments; this result may be explained by selective assimilation of photochemically produced, isotopically enriched labile compounds. Received: 17 February 2000; Accepted: 1 May 2000; Online Publication: 28 August 2000     

Nutrient limitation of bacterial production in clear water Amazonian ecosystems

The aim of this research was to determine the main limiting nutrient (carbon, nitrogen or phosphorus) to bacterial production in different clear water Amazonian ecosystems during the high water period, when there is influence of the flooded land, mainly as sources of organic matter. Five stations were sampled in three clear water ecosystems: Trombetas River, Lake Batata and Caranã Stream. We estimated in each station the nutrient concentration, bacterial production and bacterial abundance. The experiment was set up with GF/F filtered water from all stations together with additions of glucose (400 M C), KNO₃ (15 M N) and KH₂PO₄ (5 M P) in accordance with each treatment (C, N, P ,CN, CP, NP, CNP and no amends). Bacterial production was estimated after 24 h of incubation. We observed that the values of bacterial production after additions of phosphate alone (P treatment) were 2- to 6-fold greater than the values measured in control flasks. Additions of nitrate (N treatment) and glucose alone (C treatment) had no effect on the bacterial production in four out of five ecosystems studied. However, additions of glucose with phosphate (CP treatment) strongly stimulated bacterial production in all ecosystems studied, including treatments with phosphate addition only. We conclude that phosphorus is the main limiting nutrient to bacterioplankton production in these clear water Amazonian ecosystems during the high water period. In addition, we conclude that, together with phosphorus, additions of glucose stimulated the bacterial production mainly due to the low quality of the carbon pool present in these ecosystems.