Coexistence of mixotrophs, autotrophs, and heterotrophs in planktonic microbial communities

We examine what circumstances allow the coexistence of microorganisms following different nutritional strategies, using a mathematical model. This model incorporates four nutritional types commonly found in planktonic ecosystems: (1) heterotrophic bacteria that consume dissolved organic matter and are prey to some of the other organisms; (2) heterotrophic zooflagellates that depend entirely on bacterial prey; (3) phototrophic algae that depend only on light and inorganic nutrients, and (4) mixotrophs that photosynthesize, take up inorganic nutrients, and consume bacterial prey. Mixotrophs are characterized by a parameter representing proportional mixing of phototrophic and heterotrophic nutritional strategies. Varying this parameter, a range of mixotrophic strategies was examined in hypothetical habitats differing in supplies of light, dissolved organic carbon, and dissolved inorganic phosphorous. Mixotrophs expressing a wide range of mixotrophic strategies persisted in model habitats with low phosphorus supply, but only those with a strategy that is mostly autotrophic persisted with high nutrient supply, and then only when light supply was also high. Organisms representing all four nutritional strategies were predicted to coexist in habitats with high phosphorus and light supplies. Coexistence involves predation by zooflagellates and mixotrophs balancing the high competitive ability of bacteria for phosphorus, the partitioning of partially overlapping resources between all populations, and possibly nonequlibrium dynamics. In most habitats, the strategy predicted to maximize the abundance of mixotrophs is to be mostly photosynthetic and supplement nutritional needs by consuming bacteria.     

Fluxes of dissolved organic carbon,other nutrients and microbial communities in a water-filled treehole ecosystem

Water-filled treeholes are temporally and spatially variable habitats that consist of communities of a limited number of insect orders, namely dipterans and beetles. Since these systems are largely heterotrophic, treeholes are dependent on the surrounding terrestrial ecosystem for their basal energy input. In this study, we observed a cyclic succession of three system states in a water-filled Fagus grandifolia treehole: ‘connected’ during rainfall; ‘isolated’ during periods without rain; and ‘dry’ when no freestanding water was present. During the isolated phase, a rapid, microbially mediated turn-over of nitrogen and sulphate took place, coincident with an accumulation of orthophosphate. Ammonium was the dominant form of nitrogen in the treehole water, and a net decrease in its concentration was observed when the water volume decreased. Normally, nitrate concentration showed only minor fluctuations (0.3–1.3 mg l⁻¹) and concentrations of nitrite were very low (3–18 μg l⁻¹). Concentration of sulphate showed a net decrease, coincident with an increase in sulphide. During the connected phase, the effect of stemflow and throughfall on the nutrient concentrations in the treehole water proved to be variable. Over time, both dilution of, and increase in, treehole water nutrient concentrations were observed. Dissolved organic carbon (DOC) concentration showed a net decrease during the isolated phase. Spring coarse particulate organic matter input into the treehole was variable (0.20–1.74 mg cm⁻² week⁻¹). The observed variability in precipitation inputs as well as fall-in of organic matter underlines the pulsed character of basal energy entering the treehole food-web. DOC concentrations varied mainly with depth (15–57.4 mg l⁻¹) as concentrations were almost three times greater within the detritus than within the water column. Bacterial abundances ranged from 7.3 × 10⁵ to 9.3 × 10⁶ cells ml⁻¹ and did not appear to vary within the water column or in the detrital sediment. Rates of bacterial production increased 24 h after rain events, suggesting that the combined nutrient fluxes due to increased stemflow stimulate the microbial community. Handling editor: J. Cole     

Adaptive response of microbial communities to soluble microbial products

We carried out two experiments to study the influence of soluble microbial products (SMP) on biomass concentration [defined as mixed liquor suspended solids (MLSS)] and removal of soluble biological and chemical oxygen demands (sBOD₅ and sCOD): (1) SMP were allowed to accumulate, and (2) SMP content was artificially reduced by washing the biomass. The daily initial sCOD in both experiments was kept constant at 859±6 mg/l for 16 days. In experiment 1, the highest sCOD removal (80%) occurred during the first day. Thereafter, it decreased successively to 40% [sludge retention time (SRT), 12 days], after which it increased steadily to 50±4%. Variations in residual sCOD were accompanied by variations in sBOD₅, showing that the biodegradability of the accumulated SMP components was changing. MLSS fluctuated within the range 1,200±25–1,993±58 mg/l. We attributed the irregular accumulation of the biomass to variations in the biodegradability of SMP components. The initial sBOD₅/MLSS ratio varied according to variations in initial sBOD₅ and MLSS, whereas the residual ratio was constant at 0.025±0.008. This indicated a direct relationship between the concentrations of biomass and SMP produced. In experiment 2, MLSS increased from 1,200±25 to a constant value (2,810±16 mg/l; SRT, 12 days). After this time, no decrease or increase in MLSS was observed. Correspondingly, sCOD and sBOD₅ removal increased from 80–97 to 84–99%. A stable microbial community that could consume organic matter efficiently was developed under these conditions.     

Summer fluctuations of microbial planktonic communities in a eutrophic lake--Cierva Point, Antarctica

A eutrophic lake at Cierva Point, Antarctic Peninsula was surveyedduring the summers of 1997 and 1998. Phytoplankton size fractions(micro-, nano- and picoplankton) were analysed, as well as theabundance of bacterioplankton and planktonic ciliates. No permanentvertical stratification was found owing to the shallowness ofthe lake. Both nutrient concentrations and chlorophyll a valuesindicated highly eutrophic conditions, which are a consequenceof a natural enrichment by seabirds. Significant differencesin temperature between the 1997 and 1998 seasons strongly influencedmost of the biological features. The phytoplankton communityshowed a high algal species-richness, with important contributionsof epilithic, cryobiontic and soil algae. The dominant algalgroup was Chlorophyta, mainly represented by Chlamydomonas aff.celerrima, followed by Chl. aff. braunii. Some replacement ofphytoplanktonic species took place in summer and was more evidentin 1998. Picophytoplankton reached high densities, similar tothose reported from other Antarctic lakes. Bacterioplanktonabundances were typical of eutrophic and hypereutrophic lakes.There was a positive correlation between bacterial and totalphytoplankton abundance. Ciliates reached some very high peaks,with higher figures than those reported for other Antarcticsystems with similar trophic status.     

Response of bacterial extracellular enzymes to inundation of floodplain sediments

1. Bacterial extracellular enzymes provide a measure of microbial response to organic matter supply, pivotal to the recovery of riverine food webs after disturbances such as floods.
2. We examined the effects of flood duration on extracellular enzyme response from riverbank and floodplain wetland sediments from the Murrumbidgee River, south-east Australia.
3. There were strong temporal peaks in enzyme activity from both riverbank and billabong sites, peaking between 1 and 5 days following flooding and with a general decline by 21 days. A dominance of non-glucosidase and xylosidase enzymes resulted in no significant differences between billabong and riverbank sediments. This supports the hypothesis that regulated Australian river systems are driven by autochthonous carbon sources.
4. The short response time of the glucosidase after flooding suggests that even short pulses (24 h) in high flows may stimulate bacterial activity, as dissolved organic carbon (DOC) loads also peak at this time. However, a longer wetting time may be needed to drive hydrolysis of the proteins, fatty acids and longer chain polysaccharides, whether in the littoral zone of the river or connection with the floodplain.     

The phylogenetic composition and structure of soil microbial communities shifts in response to elevated carbon dioxide

One of the major factors associated with global change is the ever-increasing concentration of atmospheric CO₂. Although the stimulating effects of elevated CO₂ (eCO₂) on plant growth and primary productivity have been established, its impacts on the diversity and function of soil microbial communities are poorly understood. In this study, phylogenetic microarrays (PhyloChip) were used to comprehensively survey the richness, composition and structure of soil microbial communities in a grassland experiment subjected to two CO₂ conditions (ambient, 368 p.p.m., versus elevated, 560 p.p.m.) for 10 years. The richness based on the detected number of operational taxonomic units (OTUs) significantly decreased under eCO₂. PhyloChip detected 2269 OTUs derived from 45 phyla (including two from Archaea), 55 classes, 99 orders, 164 families and 190 subfamilies. Also, the signal intensity of five phyla (Crenarchaeota, Chloroflexi, OP10, OP9/JS1, Verrucomicrobia) significantly decreased at eCO₂, and such significant effects of eCO₂ on microbial composition were also observed at the class or lower taxonomic levels for most abundant phyla, such as Proteobacteria, Firmicutes, Actinobacteria, Bacteroidetes and Acidobacteria, suggesting a shift in microbial community composition at eCO₂. Additionally, statistical analyses showed that the overall taxonomic structure of soil microbial communities was altered at eCO₂. Mantel tests indicated that such changes in species richness, composition and structure of soil microbial communities were closely correlated with soil and plant properties. This study provides insights into our understanding of shifts in the richness, composition and structure of soil microbial communities under eCO₂ and environmental factors shaping the microbial community structure.     

Response of three arid zone floodplain plant species to inundation

For species to persist on floodplains and in temporary wetlands in arid climates, where large and unpredictable water level fluctuations are common, at least one life history stage must be able to survive inundation. We investigated the survival and performance (RGR, total biomass and above-to-belowground biomass (A:B)) of three common and often coexisting arid zone floodplain species: Xanthium strumarium, Cyperus gymnocaulos and Ludwigia peploides. Observations suggested the species had different responses to inundation, which was tested in a controlled pond experiment. Plants were held at three elevations (+ 10 cm, ? 20 and ? 70 cm) and subjected to three hydrological regimes (static 90 cm, 1 and 5 cm day^?1 inundation) for 16 weeks. Xanthium strumarium died when completely inundated for longer than 4 weeks but when partially flooded survived, showed lower growth rates, increased A:B and produced adventitious roots. C. gymnocaulos showed reduced growth rates when partially flooded and senesced to rhizomes when completely inundated for longer than 4 weeks, which re-sprouted after inundation pressure was removed. L. peploides responded positively to flooding with increased A:B and the production of adventitious roots. The species exhibited three contrasting responses to inundation, which do not necessarily fit neatly within existing water regime functional classification frameworks.     

Responses of billabong rotifer communities to inundation

Daily plankton collections were taken from a billabong of the River Murray for two weeks prior to inundation in March 1990, and continued for ten days after flooding. Quantitative responses of the plankton community and the component species were analysed against measured environmental variables and between species. Rotifers and copepod nauplii were the predominant net plankton (> 53 µm). Significant negative or positive responses to inundation were detected for most common taxa of 63 rotifer species recorded. A four-fold dilution from intrusion of river water masked rapid population increases. Opportunistic responses to inundation appear to be a survival strategy in the highly unpredictable billabong environment.     

Influence of inorganic and organic nutrients on aerobic biodegradation and on the adaptation response of subsurface microbial communities

The influence of inorganic and organic amendments on the mineralization of ethylene dibromide, p-nitrophenol, phenol, and toluene was examined in subsurface soil samples from a pristine aquifer near Lula, Okla. The responses indicate that the metabolic abilities and nutrient requirements of groundwater microorganisms vary substantially within an aquifer. In some samples, additions of inorganic nutrients resulted in a more rapid adaptation to the test substrate and a higher rate of metabolism, indicating that metabolism may have been limited by these nutrients. In other samples from the same aquifer layer, inorganic amendments had little or no influence on mineralization. In general, the addition of multiple inorganic nutrients resulted in a greater enhancement of degradation than did the addition of single substances. Additions of alternate carbon sources, such as glucose or amino acids, inhibited the mineralization of the xenobiotic substrates. This inhibition appears to be the result of the preferential utilization of the more easily degradable carbon amendments.     

Influence of inorganic and organic nutrients on aerobic biodegradation and on the adaptation response of subsurface microbial communities.

The influence of inorganic and organic amendments on the mineralization of ethylene dibromide, p-nitrophenol, phenol, and toluene was examined in subsurface soil samples from a pristine aquifer near Lula, Okla. The responses indicate that the metabolic abilities and nutrient requirements of groundwater microorganisms vary substantially within an aquifer. In some samples, additions of inorganic nutrients resulted in a more rapid adaptation to the test substrate and a higher rate of metabolism, indicating that metabolism may have been limited by these nutrients. In other samples from the same aquifer layer, inorganic amendments had little or no influence on mineralization. In general, the addition of multiple inorganic nutrients resulted in a greater enhancement of degradation than did the addition of single substances. Additions of alternate carbon sources, such as glucose or amino acids, inhibited the mineralization of the xenobiotic substrates. This inhibition appears to be the result of the preferential utilization of the more easily degradable carbon amendments.     

Catabolic diversity of periphyton and detritus microbial communities in a subtropical wetland

The catabolic diversity of wetland microbial communities may be a sensitive indicator of nutrient loading or changes in environmental conditions. The objectives of this study were to assess the response of periphyton and microbial communities in water conservation area-2a (WCA-2a) of the Everglades to additions of C-substrates and inorganic nutrients. Carbon dioxide and CH₄ production rates were measured using 14 days incubation for periphyton, which typifies oligotrophic areas, and detritus, which is prevalent at P-impacted areas of WCA-2a. The wetland was characterized by decreasing P levels from peripheral to interior, oligotrophic areas. Microbial biomass and N mineralization rates were higher for oligotrophic periphyton than detritus. Methane production rates were also higher for unamended periphyton (80 mg CH₄-C kg⁻¹ d⁻¹) than detritus (22 mg CH₄-C kg⁻¹ d⁻¹), even though the organic matter content was higher for detritus (80%) than periphyton (69%). Carbon dioxide production for unamended periphyton (222 mg CO₂-C kg⁻¹ d⁻¹) was significantly greater than unamended detritus (84 mg CO₂-C kg⁻¹ d⁻¹). The response of the heterotrophic microbial community to added C-substrates was related to the nutrient status of the wetland, as substrate-induced respiration (SIR) was higher for detritus than periphyton. Amides and polysaccharides stimulated SIR more than other C-substrates, and methanogenesis was greater contributor to SIR for periphyton than detritus. Inorganic P addition stimulated CO₂ and CH₄ production for periphyton but not detritus, indicating a P limitation in the interior areas of WCA-2a. Continued nutrient loading into oligotrophic areas of WCA-2a or enhanced internal nutrient cycling may stimulate organic matter decomposition and further contribute to undesirable changes to the Everglades ecosystem caused by nutrient enrichment.     

Characterization of microbial communities and composition in constructed dairy wetland wastewater effluent

Constructed wetlands have been recognized as a removal treatment option for high concentrations of contaminants in agricultural waste before land application. The goal of this study was to characterize microbial composition in two constructed wetlands designed to remove contaminants from dairy washwater. Water samples were collected weekly for 11 months from two wetlands to determine the efficiency of the treatment system in removal of chemical contaminants and total and fecal coliforms. The reduction by the treatment was greatest for biological oxygen demand, suspended solids, chemical oxygen demand, nitrate, and coliforms. There was only moderate removal of total nitrogen and phosphorus. Changes in the total bacterial community and ammonia-oxidizing bacterial composition were examined by using denaturing gradient gel electrophoresis (DGGE) and sequencing of PCR-amplified fragments of the gene carrying the alpha subunit of the ammonia monooxygenase gene (amoA) recovered from soil samples and DGGE bands. DGGE analysis of wetlands and manure samples revealed that the total bacterial community composition was dominated by bacteria from phylogenetic clusters related to Bacillus, Clostridium, Mycoplasma, Eubacterium, and Proteobacteria originally retrieved from the gastrointestinal tracts of mammals. The population of ammonia-oxidizing bacteria showed a higher percentage of Nitrosospira-like sequences from the wetland samples, while a higher percentage of Nitrosomonas-like sequences from manure, feces, raw washwater, and facultative pond was found. These results show that the wetland system is a natural process dependent upon the development of healthy microbial communities for optimal wastewater treatment.     

Soil microbial eco-physiological response to nutrient enrichment in a sub-tropical wetland

Eutrophication in subtropical wetland ecosystems can lead to extensive displacements of vegetative communities and as a result changes in overall environmental conditions (loss of indigenous habitat, substrate quality, etc.). This has generated a demand for a set of sensitive indicator(s) that prelude these structural changes. The functional response of bacterial communities may indicate the effect and extent of the impact on the overall system. The effects of nutrient enrichment on the microbial community and its ecophysiology were measured in a subtropical marsh (Water Conservation Area 2a) in the northern Everglades, USA. We investigated the microbially mediated organic matter decomposition processes and nutrient cycling in three areas of the marsh, a nutrient enriched site, an intermediate site and a unimpacted (oligotrophic) site. We chose measures associated to the hydrolytic enzyme activities of alkaline phosphatase, β-glucosidase and aminopeptidase. We also monitored microbial biomass carbon (C), nitrogen (N) and phosphorus (P) and the associated elemental turnover rates (C, N and P). We found a significant (α = 0.05) spike in microbial biomass C, N, and P in the intermediate site. The elemental turnover rates (C, N and P) where significantly higher in the impacted and intermediate site when compared to the unimpacted site. The enzymatic profiles at the unimpacted site illustrate a system regulated for optimal use of P. In the intermediate zone between the overall P-limited and P-impacted areas, the nutrient inputs alleviates the stress imposed by the P-limitation. Microbial biomass increased dramatically without a decrease in the overall microbial metabolic efficiency. The metabolic coefficients (particularly q-Potentially Mineralizable P – qPMP and qCO₂) indicated that after the disturbance, the impacted areas in the Everglades are characterized by relatively open, inefficient nutrient cycles. The nonlinear shifts (threshold behavior) in microbial parameters indicate that microbial indicators function effectively as early warning signals.     

Spatial distribution of viruses associated with planktonic and attached microbial communities in hydrothermal environments

Viruses play important roles in marine surface ecosystems, but little is known about viral ecology and virus-mediated processes in deep-sea hydrothermal microbial communities. In this study, we examined virus-like particle (VLP) abundances in planktonic and attached microbial communities, which occur in physical and chemical gradients in both deep and shallow submarine hydrothermal environments (mixing waters between hydrothermal fluids and ambient seawater and dense microbial communities attached to chimney surface areas or macrofaunal bodies and colonies). We found that viruses were widely distributed in a variety of hydrothermal microbial habitats, with the exception of the interior parts of hydrothermal chimney structures. The VLP abundance and VLP-to-prokaryote ratio (VPR) in the planktonic habitats increased as the ratio of hydrothermal fluid to mixing water increased. On the other hand, the VLP abundance in attached microbial communities was significantly and positively correlated with the whole prokaryotic abundance; however, the VPRs were always much lower than those for the surrounding hydrothermal waters. This is the first report to show VLP abundance in the attached microbial communities of submarine hydrothermal environments, which presented VPR values significantly lower than those in planktonic microbial communities reported before. These results suggested that viral lifestyles (e.g., lysogenic prevalence) and virus interactions with prokaryotes are significantly different among the planktonic and attached microbial communities that are developing in the submarine hydrothermal environments.     

Breeding waterbird wetland habitat availability and response to water-level management in Saint John River floodplain wetlands, New Brunswick

Vegetated areas of rivers and estuaries are capable of affecting the concentration of dissolved and particulate matter in water masses traversing those plant beds. We examined whether different sizes of water chestnut (Trapa natans) beds in the Hudson River, USA, alter dissolved oxygen, nutrients and turbidity of water masses. Ebb–tide water was sampled from four water chestnut beds in the tidal freshwater portion of the Hudson River estuary and each site was sampled multiple times during the growing season and once following plant senescence. Water quality variables included dissolved oxygen, turbidity, dissolved organic carbon, and inorganic nutrients. Samples from the small beds (575 m² and 624 m²) were compared with large beds (16 600 m² and 24 820 m²). Dissolved oxygen of water flooding vegetated beds in the hour before high tide was 7.18±1.03 mg/l (mean±standard deviation) with a range of 5.5–9.8 mg/l throughout the growing season. Water samples collected as water ebbed from the plant beds showed that only the large beds had an effect on dissolved oxygen with the largest declines in oxygen exhibited by the largest bed. Decline of dissolved oxygen in the water ebbing from the largest bed averaged 1.5±0.4 mg/l/h with a minimum of 4.5 mg/l, equivalent to 54% of saturation, a level at which sensitive fauna are negatively affected. There were no significant relationships between bed size or plant presence and inorganic nutrients, turbidity or DOC. Ebb–tide nitrate was never lower than 87% of flood tide means. Effective management of invasive plants must consider both the variability in effects among plant beds and the areal coverage of plant bed sizes.     

Species identity, diversity and microbial carbon flow in reassembling macrobenthic communities

Intense disturbance may locally destroy patches of habitat and shape the landscape into a mosaic of reassembling communities. The development of ecosystem properties during such community reassembly is poorly understood. In intertidal bare sediments, trophic relations between microphytobenthos or heterotrophic bacteria and macrofauna invertebrates may guarantee fundamental ecosystem properties such as carbon flow through the food web. We studied the dynamic relation between reassembling macrofauna communities and such microbial carbon flow during recovery after severe disturbance. We deliberately induced prolonged hypoxia in winter and early summer and allowed recolonisation for periods of two and five months. Carbon flow was quantified from basal resources (microphytobenthos and bacteria) to intermediate consumers using ¹³C as a tracer. Within the period of study (5 months), microbial carbon flow fully recovered, although macrofauna diversity was still very low compared to the natural communities (ranging from 6 to 17 species). More than 90% of microbial carbon flow to macrofauna was due to the consumers that recolonised within two months. Two of these species were dominant contributors to microphytobenthos carbon transfer to fauna. Furthermore, at an early stage of reassembly, this ecosystem property was remarkably similar when disturbance took place at different times of the year (winter or early summer), although there were differences in assemblage composition and functional diversity. We conclude that species assemblages and ecosystem function developed relatively independently in this benthic system. We discuss which ecological factors may have caused such non-parallel development of macrofaunal communities and carbon flow.     

Impact of natural metazooplankton assemblage on planktonic microbial communities in a newly flooded reservoir

The grazing impact of a natural assemblage of metazoan zooplankton on pigmented flagellates (PF), heterotrophic nanoflagellates (HNF), ciliates, and non-flagellate algae and microcyanobacteria (NFAM) was measured in situ during the period of thermal stratification in a newly flooded reservoir (Reservoir de la Sep, France). Experiments were conducted with diffusion chambers in the meta- and epilimnion over a period of 7 h. The mean mortalities of PF in the epi- and metalimnion (0.08 0.02 and 0.06 ± 0.03 h^-1, respectively), of HNF (0.04 ± 0.02 and 0.05 ± 0.02 h^-1) and ciliates (0.09 ± 0.04 and 0.11 ± 0.04 h^-1) demonstrate the impact of the metazoan zooplankton, and particularly of the rotifer Asplanchna priodonta, on the components of the microbial loop. The mortality of NFAM, accounting for 14 and 18% of total mortality, remained low throughout the study. The taxa with the highest mortality were pigmented flagellates of 4-19 m, HNF, and small-sized ciliates such as Halteria sp. (0.10 ± 0.02 h^-1 at 1 m and 0.30 ± 0.38 h^-1 at 7 m) and Urotricha furcata (0.11 ± 0.05 h^-1 at 1 m and 0.12 ± 0.06 h^-1 at 7 m). Large-sized ciliates (Paradileptus elephantinus) and sessile ciliates (Suctorida, Vorticella sp.) had a very low mortality (>0.04 h^-1). After reservoir flooding, the organisms in the microbial trophic loop, favoured by the high quantities of allochthonous organic matter, are subject to a higher mortality than the phytoplankton.      

The use of multiple-vessel,open flow systems to investigate carbon flow in anaerobic microbial communities

Five vessels, connected in series, were used for a continuous flow system to model carbon flow in anaerobic microbial communities. Two such 5-vessel systems were constructed, the inflows containing 10 mM sulfate and either 10 mM glucose or benzoate. Dilution was slow (D=0.0018 h^?1 for the whole system). Analyses of dissolved organic and inorganic carbon, and of CO₂ and CH₄, showed that the systems attained steady states in which biomass was constant, although there was net biosynthesis in the early vessels and net mineralization in succeeding vessels. Examination of the distributions of sulfate reduction, methanogenesis, and of H₂+CO₂-utilizing fatty acid-forming bacteria revealed spatial separation of these functional groups of bacteria in different vessels of the array, resembling the vertical spatial separation found in many natural sediments. Such model systems should, therefore, prove valuable in investigating the many microbial activities that contribute to the flow of carbon in anaerobic microbial communities.     

Inorganic nutrients,bacteria, and the microbial loop

The realization that natural assemblages of planktonic bacteria may acquire a significant fraction of their nitrogen and phosphorus via the uptake of dissolved inorganic nutrients has modified our traditional view of these microorganisms as nutrient remineralizers in plankton communities. Bacterial uptake of inorganic nitrogen and phosphorus may place bacteria and phytoplankton in competition for growth-limiting nutrients, rather than in their traditional roles as the respective source and sink for these nutrients in the plankton. Bacterial nutrient uptake also implies that bacterivorous protozoa may play a pivotal role in the remineralization of these elements in the microbial loop. The overall contribution of bacterial utilization of inorganic nutrients to total nutrient uptake in the ocean is still poorly understood, but some generalizations are emerging with respect to the geographical areas and community physiological conditions that might elicit this behavior.