Nutrient Constraints on Metabolism Affect the Temperature Regulation of Aquatic Bacterial Growth Efficiency

Inorganic nutrient availability and temperature are recognized as major regulators of organic carbon processing by aquatic bacteria, but little is known about how these two factors interact to control bacterial metabolic processes. We manipulated the temperature of boreal humic stream water samples within 0–25°C and measured bacterial production (BP) and respiration (BR) with and without inorganic nitrogen?+?phosphorus addition. Both BP and BR increased exponentially with temperature in all experiments, with Q ₁₀ values varying between 1.2 and 2.4. The bacterial growth efficiency (BGE) showed strong negative relationships with temperature in nutrient-enriched samples and in natural stream water where community-level BP and BR were not limited by nutrients. However, there were no relationships between BGE and temperature in samples where BP and BR were significantly constrained by the inorganic nutrient availability. The results suggest that metabolic responses of aquatic bacterial communities to temperature variations can be strongly dependent on whether the bacterial metabolism is limited by inorganic nutrients or not. Such responses can have consequences for both the carbon flux through aquatic food webs and for the flux of CO₂ from aquatic systems to the atmosphere.     

Soil carbon storage,respiration potential,and organic matter quality across an age and climate gradient in southwestern Greenland

Geological factors influence biological cycling of organic carbon in soils but are not well represented in our understanding of Arctic carbon dynamics. Landscape age, for instance, directly affects quantity and quality of soil carbon, which are two strong controls of the temperature sensitivity of soil organic matter. We investigated soil carbon storage, respiration potential, and organic matter quality for microbial decomposition across a climate and landscape age gradient in southwest Greenland that deglaciated during the Holocene. We measured soil respiration during a 370-day laboratory incubations of active layer soils collected from four study areas across this gradient (ages 1.8 × 10², 6.8 × 10³, and 1.0 × 10⁴, coinciding with a climate gradient from drier inland to wetter coastal terrain) and used a soil respiration model comparison approach to assess the substrate quality of stored organic matter for microbial decomposers. Soils store more than three times greater organic carbon at the 10,000-year-old, maritime climate study areas than the 180-year-old, continental climate study areas. Respiration rates were highest in the surface soils of the coastal areas. Model comparisons reveal important heterogeneity in the quality of organic matter for microbial decomposition between areas: coastal soils were best modeled by both one- and two-pooled models, and inland soils were best represented by one-pooled respiration models. Together, the measures of carbon quality (C:N, CO₂ production, and model parameters estimating initial CO₂ production rates from different organic matter pools) show that shallow soils at the southern coastal area, Kobbefjord, had the highest respiration rates from the recalcitrant carbon pool. This study reveals differences in carbon storage and turnover associated with landscape age and climate factors in western Greenland. When applied to thermodynamic theory, which predicts that temperature sensitivity increases with carbon recalcitrance, our findings suggest that carbon stored in coastal soils may be more sensitive to climate warming than inland soils.     

Bacterial metabolism in small temperate streams under contemporary and future climates

1. We examined the detailed temperature dependence (0–40 °C) of bacterial metabolism associated with fine sediment particles from three Danish lowland streams to test if temperature dependence varied between sites, seasons and quality of organic matter and to evaluate possible consequences of global warming. 2. A modified Arrhenius model with reversible denaturation at high temperatures could account for the temperature dependence of bacterial metabolism and the beginning of saturation above 35 °C and it was superior to the unmodified Arrhenius model. Both models overestimated respiration rates at very low temperatures (<5 °C), whereas Ratkowsky's model – the square root of respiration – provided an excellent linear fit between 0 and 30 °C. 3. There were no indications of differences in temperature dependence among samples dominated by slowly or easily degradable organic substrates. Optimum temperature, apparent minimum temperature, Q₁₀‐values for 0–40 °C and activation energies of bacterial respiration were independent of season, stream site and degradability of organic matter. 4. Q₁₀‐values of bacterial respiration declined significantly with temperature (e.g. 3.31 for 5–15 °C and 1.43 for 25–35 °C) and were independent of site and season. Q₁₀‐values of bacterial production behaved similarly, but were significantly lower than Q₁₀‐values of respiration implying that bacterial growth efficiency declined with temperature. 5. A regional warming scenario for 2071–2100 (IPCC A2) predicted that mean annual temperatures will increase by 3.5 °C in the air and 2.2–4.3 °C in the streams compared with the control scenario for 1961–1990. Temperature is expected to rise more in cool groundwater‐fed forest springs than in open, summer‐warm streams. Mean annual bacterial respiration is estimated to increase by 26–63% and production by 18–41% among streams assuming that established metabolism–temperature relationships and organic substrate availability remain the same. To improve predictions of future ecosystem behaviour, we further require coupled models of temperature, hydrology, organic production and decomposition.     

Persistent nitrogen limitation of stream biofilm communities along climate gradients in the Arctic

Climate change is rapidly reshaping Arctic landscapes through shifts in vegetation cover and productivity, soil resource mobilization, and hydrological regimes. The implications of these changes for stream ecosystems and food webs is unclear and will depend largely on microbial biofilm responses to concurrent shifts in temperature, light, and resource supply from land. To study those responses, we used nutrient diffusing substrates to manipulate resource supply to biofilm communities along regional gradients in stream temperature, riparian shading, and dissolved organic carbon (DOC) loading in Arctic Sweden. We found strong nitrogen (N) limitation across this gradient for gross primary production, community respiration and chlorophyll‐a accumulation. For unamended biofilms, activity and biomass accrual were not closely related to any single physical or chemical driver across this region. However, the magnitude of biofilm response to N addition was: in tundra streams, biofilm response was constrained by thermal regimes, whereas variation in light availability regulated this response in birch and coniferous forest streams. Furthermore, heterotrophic responses to experimental N addition increased across the region with greater stream water concentrations of DOC relative to inorganic N. Thus, future shifts in resource supply to these ecosystems are likely to interact with other concurrent environmental changes to regulate stream productivity. Indeed, our results suggest that in the absence of increased nutrient inputs, Arctic streams will be less sensitive to future changes in other habitat variables such as temperature and DOC loading.     

Microbial Response of a Freshwater Benthic Community to a Simulated Diatom Sedimentation Event: Interactive Effects of Benthic Fauna

Abstract The response of a sediment microbial assemblage to a pulse of diatoms was studied over 36 days by measuring bacterial activity and biomass, ATP concentration, and overall community respiration in laboratory microcosms. Also, the contribution of macrofaunal chironomids to the decomposition of settling diatoms in benthic communities, and the relative importance of benthic meiofauna in community metabolism, were determined. The addition of diatoms resulted in an immediate response by sediment bacteria, with higher bacterial production recorded after only 2 h, and a more than tenfold increase within one day. The rapid response by sediment bacteria was accompanied by relatively high initial concentrations of dissolved organic carbon. In treatments receiving diatoms, higher bacterial production was sustained throughout the experiment. Surprisingly, neither these elevated production estimates, nor the starvation of controls affected bacterial abundance. Mean bacterial cell volume, however, was markedly affected by the addition of diatoms. Combining community respiration measurements and bacterial production estimates showed that growth efficiencies for sediment bacteria ranged from 14.6 to 34.5%. The contribution of ambient meiozoobenthos to carbon metabolism was less than 1%. Carbon budgets showed that 1.3 mg C was cooxidized along with 4.3 mg added diatom C. Sediment reworking by Chironomus larvae initially enhanced bacterial production, but the presence of Chironomus resulted in lower bacterial production estimates after 16 and 36 days. This was interpreted as a result of faster decomposition of diatoms in treatments with chironomids, which was validated by a faster decline of ATP and chlorophyll a in the sediment. Our results indicate that Chironomus larvae compete with sediment bacteria for available organic substrates. Received: 11 June 1996; Accepted: 13 August 1996     

Effect of soil microorganisms and labile C availability on soil respiration in response to litter inputs in forest ecosystems: A meta‐analysis

Litter inputs can influence soil respiration directly through labile C availability and, indirectly, through the activity of soil microorganisms and modifications in soil microclimate; however, their relative contributions and the magnitude of any effect remain poorly understood. We synthesized 66 recently published papers on forest ecosystems using a meta‐analysis approach to investigate the effect of litter inputs on soil respiration and the underlying mechanisms involved. Our results showed that litter inputs had a strong positive impact on soil respiration, labile C availability, and the abundance of soil microorganisms, with less of an impact related to soil moisture and temperature. Overall, soil respiration was increased by 36% and 55%, respectively, in response to natural and doubled litter inputs. The increase in soil respiration induced by litter inputs showed a tendency for coniferous forests (50.7%)> broad‐leaved forests (41.3%)> mixed forests (31.9%). This stimulation effect also depended on stand age with 30‐ to 100‐year‐old forests (53.3%) and ≥100‐year‐old forests (50.2%) both 1.5 times larger than ≤30‐year‐old forests (34.5%). Soil microbial biomass carbon and soil dissolved organic carbon increased by 21.0%‐33.6% and 60.3%‐87.7%, respectively, in response to natural and doubled litter inputs, while soil respiration increased linearly with corresponding increases in soil microbial biomass carbon and soil dissolved organic carbon. Natural and doubled litter inputs increased the total phospholipid fatty acid (PLFA) content by 6.6% and 19.7%, respectively, but decreased the fungal/bacterial PLFA ratio by 26.9% and 18.7%, respectively. Soil respiration also increased linearly with increases in total PLFA and decreased linearly with decreases in the fungal/bacterial PLFA ratio. The contribution of litter inputs to an increase in soil respiration showed a trend of total PLFA > fungal/bacterial PLFA ratio > soil dissolved organic carbon > soil microbial biomass carbon. Therefore, in addition to forest type and stand age, labile C availability and soil microorganisms are also important factors that influence soil respiration in response to litter inputs, with soil microorganisms being more important than labile C availability.     

Temperature and substrate controls on intra-annual variation in ecosystem respiration in two subarctic vegetation types

Arctic ecosystems are important in the context of climate change because they are expected to undergo the most rapid temperature increases, and could provide a globally significant release of CO₂ to the atmosphere from their extensive bulk soil organic carbon reserves. Understanding the relative contributions of bulk soil organic matter and plant‐associated carbon pools to ecosystem respiration is critical to predicting the response of arctic ecosystem net carbon balance to climate change. In this study, we determined the variation in ecosystem respiration rates from birch forest understory and heath tundra vegetation types in northern Sweden through a full annual cycle. We used a plant biomass removal treatment to differentiate bulk soil organic matter respiration from total ecosystem respiration in each vegetation type. Plant‐associated and bulk soil organic matter carbon pools each contributed significantly to ecosystem respiration during most phases of winter and summer in the two vegetation types. Ecosystem respiration rates through the year did not differ significantly between vegetation types despite substantial differences in biomass pools, soil depth and temperature regime. Most (76–92%) of the intra‐annual variation in ecosystem respiration rates from these two common mesic subarctic ecosystems was explained using a first‐order exponential equation relating respiration to substrate chemical quality and soil temperature. Removal of plants and their current year's litter significantly reduced the sensitivity of ecosystem respiration to intra‐annual variations in soil temperature for both vegetation types, indicating that respiration derived from recent plant carbon fixation was more temperature sensitive than respiration from bulk soil organic matter carbon stores. Accurate assessment of the potential for positive feedbacks from high‐latitude ecosystems to CO₂‐induced climate change will require the development of ecosystem‐level physiological models of net carbon exchange that differentiate the responses of major C pools, that account for effects of vegetation type, and that integrate over summer and winter seasons.     

Urban infrastructure influences dissolved organic matter quality and bacterial metabolism in an urban stream network

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Soil respiration under prolonged soil warming: are rate reductions caused by acclimation or substrate loss?

The world's soils contain a large amount of carbon so that even a fractionally small loss or gain could have a quantitatively important feedback effect on net CO₂ emissions to the atmosphere. It is therefore important to fully understand the temperature dependence of soil‐carbon decomposition. Evidence from various observations can be used to quantify the temperature dependence of carbon efflux, but it is important to ensure that confounding factors, such as changing water relations or availability of readily decomposable substrate, are fully considered in inferring an underlying temperature response from observed response patterns. A number of recent findings from soil‐warming experiments have led to the suggestion that stimulation of soil‐carbon efflux by increasing temperature is only transitory before acclimation takes place and carbon efflux rates return to similar rates as before the increase in temperature. It is shown here that this response pattern can be explained through a simple two‐pool soil‐carbon model with no acclimation response needing to be invoked. The temporal pattern is, instead, due to depletion of readily decomposable substrate. It shows that findings of reduced respiration rate in soil‐warming experiments are consistent with unchanged high temperature sensitivity of organic carbon decomposition and affirms that there is, indeed, a danger of positive feedback between global warming and the release of soil organic carbon that can lead to further warming.     

Does declining carbon‐use efficiency explain thermal acclimation of soil respiration with warming?

Enhanced soil respiration in response to global warming may substantially increase atmospheric CO₂ concentrations above the anthropogenic contribution, depending on the mechanisms underlying the temperature sensitivity of soil respiration. Here, we compared short‐term and seasonal responses of soil respiration to a shifting thermal environment and variable substrate availability via laboratory incubations. To analyze the data from incubations, we implemented a novel process‐based model of soil respiration in a hierarchical Bayesian framework. Our process model combined a Michaelis–Menten‐type equation of substrate availability and microbial biomass with an Arrhenius‐type nonlinear temperature response function. We tested the competing hypotheses that apparent thermal acclimation of soil respiration can be explained by depletion of labile substrates in warmed soils, or that physiological acclimation reduces respiration rates. We demonstrated that short‐term apparent acclimation can be induced by substrate depletion, but that decreasing microbial biomass carbon (MBC) is also important, and lower MBC at warmer temperatures is likely due to decreased carbon‐use efficiency (CUE). Observed seasonal acclimation of soil respiration was associated with higher CUE and lower basal respiration for summer‐ vs. winter‐collected soils. Whether the observed short‐term decrease in CUE or the seasonal acclimation of CUE with increased temperatures dominates the response to long‐term warming will have important consequences for soil organic carbon storage.     

Seasonal changes in temperature and nutrient control of photosynthesis, respiration and growth of natural phytoplankton communities

1. To investigate the influence of elevated temperatures and nutrients on photosynthesis, respiration and growth of natural phytoplankton assemblages, water was collected from a eutrophic lake in spring, summer, autumn, winter and the following spring and exposed to ambient temperature and ambient +2, +4 and +6 °C for 2 weeks with and without addition of extra inorganic nutrients. 2. Rates of photosynthesis, respiration and growth generally increased with temperature, but this effect was strongly enhanced by high nutrient availability, and therefore was most evident for nutrient amended cultures in seasons of low ambient nutrient availability. 3. Temperature stimulation of growth and metabolism was higher at low than high ambient temperature showing that long‐term temperature acclimation of the phytoplankton community before the experiments was of great importance for the measured rates. 4. Although we found distinct responses to relatively small temperature increases, the interaction between nutrient availability, time of the year and, thus, ambient temperature was responsible for most of the observed variability in phytoplankton growth, photosynthesis and respiration. 5. Although an increase in global temperature will influence production and degradation of organic material in lakes, the documented importance of ambient temperatures and nutrient conditions suggests that effects will be most pronounced during winter and early spring, while the remaining part of the growth season will be practically unaffected by increasing temperatures.     

Increased Light Availability Reduces the Importance of Bacterial Carbon in Headwater Stream Food Webs

Many ecosystems rely on subsidies of carbon and nutrients from surrounding environments. In headwater streams that are heavily shaded by riparian forests, allochthonous inputs from terrestrial systems often comprise a major part of the organic matter budget. Bacteria play a key role in organic matter cycling in streams, but there is limited evidence about how much bacterial carbon is actually assimilated by invertebrate and fish consumers, and how bacterial carbon assimilation varies among streams. We conducted stable isotope tracer additions of ¹³C-acetate, that is assimilated only by bacteria, and ¹⁵N-ammonium, that is assimilated by both bacteria and algae, in two small, shaded streams in the Adirondack region of New York State, USA. Our goal was to determine whether there is an important trophic link between bacteria and macroconsumers, and whether the link changes when the light environment is experimentally altered. In 2009, we evaluated bacterial carbon use in both streams with natural canopy cover using 10-day dual-isotope tracer releases. The canopy was then thinned in one stream to increase light availability and primary production and tracer experiments were repeated in 2010. As part of the tracer experiments, we developed a respiration assay to measure the δ ¹³C content of live bacteria, which provided critical information for determining how much of the carbon assimilated by invertebrate consumers is from bacterial sources. Some invertebrate taxa, including scraper mayflies (Heptagenia spp.) that feed largely on biofilms assimilated over 70% of their carbon from bacterial sources, whereas shredder caddisflies (Pycnopsyche spp.) that feed on decomposing leaves assimilated less than 1% of their carbon from bacteria. Increased light availability led to strong declines in the magnitude of bacterial carbon fluxes to different consumers (varying from ?17 to ?91% decrease across invertebrate taxa), suggesting that bacterial energy assimilation differs not only among consumer taxa but also within the same consumer taxa in streams with different ecological contexts. Our results demonstrate that fluxes of bacterial carbon to higher trophic levels in streams can be substantial, that is over 70% for some taxa, but that invertebrate taxa vary considerably in their reliance on bacterial carbon, and that local variation in carbon sources controls how much bacterial carbon invertebrates use.     

Links between Terrestrial Primary Production and Bacterial Production and Respiration in Lakes in a Climate Gradient in Subarctic Sweden

We compared terrestrial net primary production (NPP) and terrestrial export of dissolved organic carbon (DOC) with lake water heterotrophic bacterial activity in 12 headwater lake catchments along an altitude gradient in subarctic Sweden. Modelled NPP declined strongly with altitude and annual air temperature decreases along the altitude gradient (6°C between the warmest and the coldest catchment). Estimated terrestrial DOC export to the lakes was closely correlated to NPP. Heterotrophic bacterial production (BP) and respiration (BR) were mainly based on terrestrial organic carbon and strongly correlated with the terrestrial DOC export. Excess respiration over PP of the pelagic system was similar to net emission of CO₂ in the lakes. BR and CO₂ emission made up considerably higher shares of the terrestrial DOC input in warm lakes than in cold lakes, implying that respiration and the degree of net heterotrophy in the lakes were dependant not only on terrestrial export of DOC, but also on characteristics in the lakes which changed along the gradient and affected the bacterial metabolization of allochthonous DOC. The study showed close links between terrestrial primary production, terrestrial DOC export and bacterial activity in lakes and how these relationships were dependant on air temperature. Increases in air temperature in high latitude unproductive systems might have considerable consequences for lake water productivity and release of CO₂ to the atmosphere, which are ultimately determined by terrestrial primary production.     

Metabolic diversity of heterotrophic bacterioplankton over winter and spring in the coastal Arctic Ocean

Metabolic diversity of heterotrophic bacterioplankton was tracked from early winter through spring with Biolog Ecoplates under the seasonally ice covered arctic shelf in the Canadian Arctic (Franklin Bay, Beaufort Sea). Samples were taken every 6 days from December 2003 to May 2004 at the surface, the halocline where a temperature inversion occurs, and at 200 m, close to the bottom. Despite the low nutrient levels and low chlorophyll a , suggesting oligotrophy in the winter surface waters, the number of substrates used (NSU) was greater than in spring, when chlorophyll a concentrations increased. Denaturing gradient gel electrophorisis analysis also indicated that the winter and spring bacterial communities were phylogenetically distinct, with several new bands appearing in spring. In spring, the bacterial community would have access to the freshly produced organic carbon from the early phytoplankton bloom and the growth of rapidly growing specialist phenotypes would be favoured. In contrast, in winter bacterioplankton consumed more complex organic matter originated during the previous year's phytoplankton production. At the other depths we tested the NSU was similar to that for the winter surface, with no seasonal pattern. Instead, bacterioplankton metabolism seemed to be influenced by resuspension, advection, and sedimentation events that contributed organic matter that enhanced bacterial metabolism.     

Land use controls stream ecosystem metabolism by shifting dissolved organic matter and nutrient regimes

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Evidence for a pan-Arctic sea-ice diatom diet in Strongylocentrotus spp

The Arctic marine food web is based on organic matter produced by both phytoplankton and sea-ice algae. With the decline of Arctic sea ice, the sustained availability of organic carbon of sea-ice origin is unclear. Recently, the detection of the sea-ice diatom biomarker IP₂₅ in a range of Arctic benthic macrofauna indicated that this is a highly suitable biomarker for the identification of organic carbon derived from sea-ice primary production in Arctic food webs. However, the data presented previously were restricted to a single geographical region in the Canadian Arctic. Here, we show that IP₂₅ is present in sea urchins of the genus Strongylocentrotus collected from ten locations with seasonal sea-ice cover from the Canadian Archipelago, Greenland and Spitsbergen. In contrast, IP₂₅ was not found in specimens of Echinus esculentus collected from the southwest UK, where sea ice is absent. Our findings provide evidence that the presence of IP₂₅ in macrobenthic organisms can be used across different Arctic regions as a versatile indicator of a diet containing carbon of sea-ice origin.     

Physical and chemical limnology of 204 lakes from the Canadian Arctic Archipelago

The physical and chemical limnology of 204 lakes from across the Canadian Arctic Archipelago was examined. Mean summer air temperature did not correlate well with lake chlorophyll levels due to the predominance of ultra-oligotrophic hard-water lakes located in a polar climate. Local geology influences ion budgets and is an important factor in determining pelagic phosphorus availability, carbon cycling and metal concentrations. Ratios of particulate carbon, particulate nitrogen and chlorophyll a indicate that planktonic microorganisms are not always the major producers of organic carbon in arctic lakes. Allochthonous particulate matter contributes significantly to the carbon and phosphorus budgets of small and mid-sized lakes across the Arctic, although the availability of these elements is controlled by many interacting geochemical and biological factors. Phosphorus is generally limiting, however, increases in available phosphorus, nitrogen and carbon are all required to make significant long-term differences in lake productivity. Particulate phosphorus levels can be high in lakes where phosphorus-rich shales or carbonatite bedrock are present. These phosphorus-enriched lakes are found in several areas across the mid-arctic islands, however, only small amounts of this nutrient are available as soluble reactive phosphorus. Although lakes throughout the Arctic are typically ultra-oligotrophic, they still represent an important sink for allochthonous nutrient deposition.     

Large Plankton Enhance Heterotrophy Under Experimental Warming in a Temperate Coastal Ecosystem

Microbes are key players in oceanic carbon fluxes. Temperate ecosystems are seasonally variable and thus suitable for testing the effect of warming on microbial carbon fluxes at contrasting oceanographic conditions. In four experiments conducted in February, April, August and October 2013 in coastal NE Atlantic waters, we monitored microbial plankton stocks and daily rates of primary production, bacterial heterotrophic production and respiration at in situ temperature and at 2 and 4°C over ambient values during 4-day incubations. Ambient total primary production (TPP) exceeded total community respiration (< 200 µm, TR) in winter and fall but not in spring and summer. The bacterial contribution to ecosystem carbon fluxes was low, with bacterial production representing on average 6.9 ± 3.2% of TPP and bacterial respiration (between 0.8 and 0.2 µm) contributing on average 35 ± 7% to TR. Warming did not result in a uniform increase in the variables considered, and most significant effects were found only for the 4°C increase. In the summer and fall experiments, under warm and nutrient-deficient conditions, the net TPP/TR ratio decreased by 39 and 34% in the 4°C treatment, mainly due to the increase in respiration of large organisms rather than bacteria. Our results indicate that the interaction of temperature and substrate availability in determining microbial carbon fluxes has a strong seasonal component in temperate planktonic ecosystems, with temperature having a more pronounced effect and generating a shift toward net heterotrophy under more oligotrophic conditions as found in summer and early fall.     

Pelagic microbial activity in the Northeast Water polynya, summer 1992

 Heterotrophic activity and related measures of pelagic microorganisms in the Northeast Water (NEW) polynya, the largest and northernmost summer polynya in the Arctic, were studied during the Polar Sea cruise of July/August 1992 at stations spanning a range of ice conditions. Utilization (incorporation and respiration) of ¹⁴C-labelled amino acids was measured at in situ temperature as a proxy for microheterotrophic activity in samples from the chlorophyll maxima, intermediate water depths, and the benthic boundary layer. Total activity and bacterial abundance (measured by epifluorescence microscopy) were highest in the sub-zero surface water layer that dominates the northeast Greenland shelf, and particularly in areas most influenced by ice-edge processes and lateral advection. In this cold-water layer activity correlated strongly with particulate organic carbon (POC), to a lesser degree with phytoplankton-derived pigments and bacterial abundance, but not with median bacterial cell size. No significant correlations were detected in the warmer, Atlantic-influenced deep waters of the regional trough system where POC concentrations and other parameters were low. In comparison with temperate environments, pelagic heterotrophic activity in the NEW polynya appears to be low, a finding that bears upon the fate of dissolved organic matter and the efficient supply of particulate food to the benthos in this unique polar setting. Received: 3 December 1995/Accepted:29 May 1996     

Bacterial consumption of total and dissolved organic carbon in the Great Barrier Reef

Heterotrophic bacteria typically take up directly dissolved organic matter due to the small molecular size, although both particulate and dissolved organic matter have labile (easily consumed) compounds. Tropical coastal waters are important ecosystems because of their high productivity. However, few studies have determined bacterial cycling (i.e. carbon uptake by bacteria and allocation for bacterial biomass and respiration) of dissolved organic carbon in coastal tropical waters, and none has determined bacterial cycling of total and dissolved organic carbon simultaneously. In this study we followed bacterial biomass and production, and organic carbon changes over short-term (12 days) dark incubations with (total organic carbon, TOC) and without particulate organic carbon additions (dissolved organic carbon, DOC). The study was performed at three sites along the middle stretch of the Great Barrier Reef (GBR) during the dry and wet seasons. Our results show that the bacterial growth efficiency is low (0.1–11.5%) compared to other coastal tropical systems, and there were no differences in the carbon cycling between organic matter sources, seasons or locations. Nonetheless, more carbon was consumed in the TOC compared to the DOC incubations, although the proportion allocated to biomass and respiration was similar. This suggests that having more bioavailable substrate in the particulate form did not benefit bacteria. Overall, our study indicates that when comparing the obtained respiration rates with previously measured primary production rates, the GBR is a heterotrophic system. More detailed studies are required to fully explore the mechanisms used by bacteria to cycle TOC and DOC in tropical coastal waters.