Pelagic Microbial Communities Research Articles

Adding a cost of resistance description extends the ability of virus–host model to explain observed patterns in structure and function of pelagic microbial communities

By adding a generic description of cost of resistance (COR) to the existing 'killing-the-winner' model, we show how this expands the model's explanatory power to include rank-abundance relationships in the host population. The model can predict a counter-intuitive relationship previously suggested in the literature, where abundant viruses are associated with rare hosts and vice versa. The model explains the observed dominance of slow-growing prokaryotes as the result of successful defence strategies, rather than as dormancy of hosts lacking essential substrates. In addition to these important conceptual aspects, the model is able to reproduce realistic values for virus : host ratios and partitioning of bacterial production between predatory loss and viral lysis. A high COR is also shown to increase the community's richness and Shannon diversity index. This model thus not only couples life strategies at the cellular level with system properties, but it also links the two system level properties of biogeochemical flows and diversity to each other. The model operates with host groups, and consequences for biodiversity when interpreting these groups in terms of species and strains are discussed.

Patterns of extracellular enzyme activities and microbial metabolism in an Arctic fjord of Svalbard and in the northern Gulf of Mexico: contrasts in carbon processing by pelagic microbial communities

Patterns of extracellular enzyme activities and microbial metabolism in an Arctic fjord of Svalbard and in the northern Gulf of Mexico: contrasts in carbon processing by pelagic microbial communities

Patterns of extracellular enzyme activities and microbial metabolism in an Arctic fjord of Svalbard and in the northern Gulf of Mexico: contrasts in carbon processing by pelagic microbial communities

The microbial community composition of polar and temperate ocean waters differs substantially, but the potential functional consequences of these differences are largely unexplored. We measured bacterial production, glucose metabolism, and the abilities of microbial communities to hydrolyze a range of polysaccharides in an Arctic fjord of Svalbard (Smeerenburg Fjord), and thus to initiate remineralization of high-molecular weight organic matter. We compared these data with similar measurements previously carried out in the northern Gulf of Mexico in order to investigate whether differences in the spectrum of enzyme activities measurable in Arctic and temperate environments are reflected in “downstream” aspects of microbial metabolism (metabolism of monomers and biomass production). Only four of six polysaccharide substrates were hydrolyzed in Smeerenburg Fjord; all were hydrolyzed in the upper water column of the Gulf. These patterns are consistent on an interannual basis. Bacterial protein production was comparable at both locations, but the pathways of glucose utilization differed. Glucose incorporation rate constants were comparatively higher in Svalbard, but glucose respiration rate constants were higher in surface waters of the Gulf. As a result, at the time of sampling ca. 75% of the glucose was incorporated into biomass in Svalbard, but in the northern Gulf of Mexico most of the glucose was respired to CO2. A limited range of enzyme activities is therefore not a sign of a dormant community or one unable to further process substrates resulting from extracellular enzymatic hydrolysis. The ultimate fate of carbohydrates in marine waters, however, is strongly dependent upon the specific capabilities of heterotrophic microbial communities in these disparate environments.

Functional variation among polysaccharide-hydrolyzing microbial communities in the Gulf of Mexico

Marine polysaccharides are structurally diverse, as are the microbial communities capable of remineralizing them. Variations in the diversity, richness, and metagenome content of pelagic microbial communities are well documented, but variation in the spectrum of substrates accessible to those communities is less well understood. Here we investigate variability in the abilities of microbial communities to access specific polysaccharides along lateral and depth gradients in the Gulf of Mexico. Patterns of polysaccharide degradation during long-term incubations varied in ways that could not be predicted from bulk community measurements of bacterial production and glucose metabolism. There was greater diversity of function among epipelagic communities than among mesopelagic communities, and the communities in the two deepest water samples (700m and 905m) were more specialized in their abilities to access specific polysaccharide structures than were shallower-water communities. Timecourses of polysaccharide hydrolysis suggest that the capacity of communities to access specific polysaccharides may be influenced in part by variability in the composition or activity of the rare biosphere.

Heat output by marine microbial and viral communities

The Marine Microbial Food Web (MMFW) includes heterotrophicmicrobes and their protist and viral predators. These microbes consume dissolved organic matter thereby making the MMFW a major component of global biogeochemical and energy cycles. However, quantification of the MMFW contribution to these cycles is dependent on a handful of techniques, all of which require laboratory-derived conversion factors. Here we describe a differential calorimeter capable of measuring the small amounts of heat produced by marine microbes and viruses at natural populations. Using this ultra-sensitive calorimeter, we show that heat production in the presence of viruses is significantly larger than in their absence. This increased heat output occurs despite a net decrease in the number of microbes. This provides direct evidence for top-down control of microbial populations by viruses and shows that there is increased re-mineralization. A comparative statics model was developed to interpret the calorimeter measurements. The spirit of the model is thermodynamic – it restricts its view to net changes in the populations and net heat produced. The model predicts that approximately 25% of the total heat production during the growth phase of a pelagic microbial community is due directly to viral activities. This result has implications for the energy budget of our planet and for climate prediction.

Diversity of Archaea and detection of crenarchaeotal amoA genes in the rivers Rhine and Têt

Pelagic archaeal phylogenetic diversity and the potential for crenarchaeotal nitrification of Group 1.1a were determined in the rivers Rhine and Tet by 16S rRNA sequencing, catalyzed reported deposition-fluorescence in situ hybridization (CARD–FISH) and quantification of 16S rRNA and functional genes. Euryarchaeota were, for the first time, detected in temperate river water even though a net predominance of crenarchaeotal phylotypes was found. Differences in phylogenic distribution were observed between rivers and seasons. Our data suggest that a few archaeal phylotypes (Euryarchaeota Groups RC-V and LDS, Crenarchaeota Group 1.1a) are widely distributed in pelagic riverine environments whilst others (Euryarchaeota Cluster Sagma-1) may only occur seasonally in river water. Crenarchaeota Group 1.1a has recently been identified as a major nitrifier in the marine environment and phylotypes of this group were also present in both rivers, where they represented 0.3% of the total pelagic microbial community. Interestingly, a generally higher abundance of Crenarchaeota Group 1.1a was found in the Rhine than in the Tet, and crenarchaeotal ammonia monooxygenase gene (amoA) was also detected in the Rhine, with higher amoA copy numbers measured in February than in September. This suggests that some of the Crenarchaeota present in river waters have the ability to oxidize ammonia and that riverine crenarchaeotal nitrification of Group 1.1a may vary seasonally.

Sources of Methylmercury to a Wetland-Dominated Lake in Northern Wisconsin

Several lines of evidence suggest that wetlands may be a major source of methylmercury (MeHg) to receiving waters, perhaps explaining the strong correlation between concentrations of waterborne MeHg and dissolved organic carbon (DOC) in regions such as northern Wisconsin. We evaluated the relative importance of wetland export in the MeHg budget of a wetland-dominated lake in northern Wisconsin using mass balance. Channelized runoff from a large headwater wetland was the major source of water and total mercury (HgT) to the lake during the study period. The wetland also exported MeHg in high concentrations (0.2-0.8 ng L(-1)), resulting in an export rate similar to those reported for other northern wetlands (ca. 0.3 microg MeHg m(-2) y(-1)). Yet, based on intensive sampling during 2002, the mass of MeHg that accumulated in the lake during summer was an order of magnitude greater than the export of MeHg from the wetland to the lake. Hence, a large in-lake source of MeHg is inferred from the mass balance. Most of the accumulated MeHg built-up in anoxic hypolimnetic waters; and the build-up was roughly balanced by losses of inorganic Hg (Hg(II)) implying a chemical transformation within the anoxic water column. An abundance of sulfate-reducing bacteria (SRB) in hypolimnetic waters, established by DNA analysis of the pelagic microbial community, along with a previous report documenting high methylation rates in the hypolimnion of this lake (ca. 10% d(-1)), suggest that this transformation was microbially mediated. These findings indicate that the direct effect of wetland runoff may be outweighed by indirect effects on the lacustrine MeHg cycle, enhancing the load of Hg(II), the activity of SRB, and the retention of MeHg, especially in northern lakes with flushing times longer than six months.

Illustrating the importance of particulate organic matter to pelagic microbial abundance and community structure—an Arctic case study

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 40:217-227 (2005) - doi:10.3354/ame040217 Illustrating the importance of particulate organic matter to pelagic microbial abundance and community structure—an Arctic case study Lisa R. Hodges1,2, Nasreen Bano1, James T. Hollibaugh1, Patricia L. Yager1,* 1Department of Marine Sciences, University of Georgia, 220 Marine Sciences Building, Athens, Georgia 30602-3636, USA2Present address: Centers for Disease Control & Prevention, 1600 Clifton Road, Mail-Stop C16, Atlanta, Georgia 30333, USA *Corresponding author. Email: pyager@uga.edu ABSTRACT: The pelagic bacterial community composition of the summertime Chukchi Sea (coastal Alaskan Arctic) was investigated under varying particle concentrations. Free-living and wholecommunity assemblages were compared using PCR-amplified 16S rDNA with denaturing gradient gel electrophoresis (PCR/DGGE) alongside traditional geochemical and microbial inventories. Algal blooms were characterized by increased microbial abundance, decreased species richness, and decreased similarity between free-living and whole-community bacterial assemblages. Bacterial and viral abundance correlated positively with a bloom index identified by principle components analysis that included particulate organic carbon and nitrogen, total organic carbon, and chlorophyll a, but not dissolved organic carbon or inorganic nutrients. The species richness of the free-living community correlated negatively with microbial abundance; differences between free-living and whole-community assemblages correlated positively with viral abundance. Algal blooms may therefore increase microbial abundance while stimulating a succession of the bacterial assemblage to fewer free-living species and more specialized particle-associated bacteria. Such community-level shifts are very likely to impact the fate of carbon in high production and export regions like the coastal Arctic. We speculate about the importance of viral lysis to such a succession. KEY WORDS: Bacterioplankton · Virioplankton · Bacterial community composition · Particleassociated bacteria · Free-living bacteria · Psychrophiles · Arctic · Carbon fluxes Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 40, No. 3. Online publication date: October 18, 2005 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2005 Inter-Research.

Patterns of extracellular enzyme activities among pelagic marine microbial communities: implications for cycling of dissolved organic carbon

The potential for pelagic microbial communities to hydrolyze high molecular weight substrates and therefore to initiate remineralization of high-molecular-weight dissolved organic carbon (DOC) was assessed at 8 different stations spanning a range of locations. Six structurally diverse polysaccharides were used to investigate activities and structural specificities of a related class of microbial extracellular enzymes, the polysaccharide hydrolases. Three high-latitude (57 to 79° N) stations showed similar rates and patterns of enzyme activity, with only 3 of 6 polysaccharides hydrolyzed. Hydrolysis at the other stations (39° S to 29° N) showed a variety of patterns, in which 2 to 5 of the polysaccharides were hydrolyzed. One of the polysaccharides, fucoidan, was not hydrolyzed at any station, while only laminarin was hydrolyzed at every station. A limited ability of microbial communities in some locations to hydrolyze high-molecular-weight substrates could help explain the persistence of a fraction of DOC in the ocean. Potential hydrolysis rates showed no corre- lation with factors such as environmental temperature or total cell numbers. Denaturing gradient gel electrophoresis analysis of community 16S rDNA indicated that the microbial communities at these locations were diverse, consistent with the diversity in patterns of enzyme activities.

Speed bumps and barricades in the carbon cycle: substrate structural effects on carbon cycling

The observation that a fraction of organic matter produced in marine systems evades the concerted efforts of microbial communities and is buried in sediments suggests that there are ‘speed bumps’ in carbon degradation pathways that impede microbially driven remineralization processes. The initial step in degradation of macromolecules, extracellular enzymatic hydrolysis, is often stated to be ‘the’ rate-limiting step in carbon remineralization. Experimental investigations described here, however, demonstrate that at least in certain cases, microbes produce extracellular enzymes on time scales of hours to tens of hours in response to substrate addition, and hydrolysis is extremely rapid. If enzymatic hydrolysis can be rapid, what factors slow or stop organic matter degradation? A lack of the correct inducer to initiate enzyme production, and/or a lack of the correct organism to produce the required enzyme, may result in a complete lack of hydrolysis in certain environments—a barricade, rather than a speed bump. Preliminary evidence supporting this hypothesis includes a comparison of polysaccharide hydrolysis in seawater and sediments, which demonstrates that the spectrum of enzymes active in seawater and sediments are fundamentally different. Furthermore, a survey of enzyme activities in surface waters from a range of locations suggests that pelagic microbial communities also differ widely in their abilities to express specific extracellular enzymes. Trans-membrane transport through porins is yet another potential location of structure-related selectivity. Our efforts to identify speed bumps and barricades are hampered by our inability to structurally characterize in sufficient detail the macromolecular structures present in marine systems. Furthermore, assessments of organic matter ‘quality’ from a chemical perspective do not necessarily accurately reflect the availability of organic carbon to microbial communities. For these communities, in fact, ‘quality’ may be a variable, which depends on the enzymatic and uptake capabilities of community members. To begin to assess substrate structure and quality from a microbial perspective, we will have to combine specific knowledge of macromolecular structures with detailed investigations of the enzymatic and transport capabilities of heterotrophic marine microbes.

Members of a readily enriched beta-proteobacterial clade are common in surface waters of a humic lake.

Humic lakes are systems often characterized by irregular high input of dissolved organic carbon (DOC) from the catchment. We hypothesized that specific bacterial groups which rapidly respond to changes in DOC availability might form large populations in such habitats. Seasonal changes of microbial community composition were studied in two compartments of an artificially divided bog lake with contrasting DOC inputs. These changes were compared to community shifts induced during short-term enrichment experiments. Inocula from the two compartments were diluted 1:10 into water from the more DOC-rich compartment, and inorganic nutrients were added to avoid microbial N and P limitation. The dilutions were incubated for a period of 2 weeks. The microbial assemblages were analyzed by cloning and sequencing of 16S rRNA genes and by fluorescence in situ hybridization with specific oligonucleotide probes. beta-Proteobacteria from a cosmopolitan freshwater lineage related to Polynucleobacter necessarius (beta II) were rapidly enriched in all treatments. In contrast, members of the class Actinobacteria did not respond to the enhanced availability of DOC by an immediate increase in growth rate, and their relative abundances declined during the incubations. In lake water members of the beta II clade seasonally constituted up to 50% of all microbes in the water column. Bacteria from this lineage annually formed a significantly higher fraction of the microbial community in the lake compartment with a higher allochthonous influx than in the other compartment. Actinobacteria represented a second numerically important bacterioplankton group, but without clear differences between the compartments. We suggest that the pelagic microbial community of the studied system harbors two major components with fundamentally different growth strategies.

Abundance, biomass and growth rates of pelagic microorganisms in a tropical coastal ecosystem

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 18:175-185 (1999) - doi:10.3354/ame018175 Abundance, biomass and growth rates of pelagic microorganisms in a tropical coastal ecosystem Petra Wallberg1,*, Per R. Jonsson2, Ron Johnstone1 1Department of Zoology, Stockholm University, S-106 91 Stockholm, Sweden 2Tjärnö Marine Biological Laboratory, S-452 96 Strömstad, Sweden *E-mail: petra.wallberg@itm.su.se ABSTRACT: In situ growth rates of ciliates and heterotrophic nanoflagellates were determined after size fractionation during rainy and dry seasons at 2 coastal stations off Zanzibar Island, Tanzania. In addition, bacterial and primary production and the biomass components in the pelagic microbial community were measured. The bacterial and primary production, and growth rates of heterotrophic nanoflagellates were significantly higher during the rainy season. For the ciliate community the growth rate between the seasons did not differ, but the number of positive growth estimates increased by a factor of 5 in the rainy seasons compared with the dry seasons. Further, the diversity of ciliates increased with the amount of rain, and the size distribution indicated a shift towards a community with larger species. Data obtained from these experiments are summarised in a carbon budget for each season. Carbon budgets indicate that the carbon flow through the heterotrophic microbial food web was approximately 3 times higher during the rainy season compared with the dry season. However, due to the relatively lower production of larger diatoms, heterotrophic microplankton may be relatively more important as a carbon source for higher trophic levels, such as copepods, during the dry season. KEY WORDS: Ciliates · Microbial food web · Growth rates · Carbon budget · East Africa Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 18, No. 2. Publication date: August 09, 1999 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 1999 Inter-Research.

Ectoaminopeptidase specificity and regulation in Antarctic marine pelagic microbial communities

Ectoaminopeptidase activities of pelagic marine microbial communities were investigated on several research cruises in the western Antarctic Peninsula region from 1992 to 1994, using the fluorigenic substrate analogue L-leucyl-P-naphthylamine. K,,, values at in situ temperature were comparable to those observed by other investigators for a variety of aquatic environments. Competitive inhibition by dipeptides of a variety of amino acids showed that the aminopeptidases present were broadly specific; no strong tendency toward greater affinity for hydrophobic or hydrophilic amino acids was observed. Seawater cultures (1.0 pm filtrate) were inoculated with a variety of monomeric organic compounds and incubated for 24 to 72 h prior to activity determination; histidine and phenylalanine were found to consistently inhibit aminopeptidase expression. It is hypothesized that auxotrophy for hstidine and phenylalanine may be widespread in these assemblages, giving rise to high levels of constitutive, nonspecific aminopeptidase activity which results in significant respiration of the more common amino acids. Whatever the exact mechanism, aminopeptidase activity is strongly affected by the specific compounds present and not simply by the carbon-to-nitrogen ratio.

Seasonal and spatial variations in pelagic community respiration on the southeastern U.S. continental shelf

Pelagic microbial community respiratory rates on the southeastern continental shelf show strong seasonal changes, with summer maxima and winter minima. In nearshore waters <20 m, community respiratory rates of 0.1–3 μM O 2 l −1 h −1 are similar to recent estimates of primary production. In outer shelf water >40 m, strongly influenced by the Gulf Stream, community respiration is 0.1–2.7 μM O 2 l −1 h −1, exceeding primary production in summer. The changes in microbial community respiration are partially explained by changes in bacterial numbers. A related potential cause is the inverse relationship between limiting substrate concentration and temperature, affecting rates of growth and respiration, that has been found to exist for aerobic heterotrophy bacterial isolates in the laboratory. These processes result in shifts between net autotrophy in winter and net heterotrophy in summer for the community as a whole, with little excess organic production remaining for export to the ocean's interior.

Bacteria-organic matter coupling and its significance for oceanic carbon cycling

This paper synthesizes current ideas on the role of the microbial loop in carbon fluxes in the ocean and proposes some directions for future research. Organic matter flux into bacteria is highly variable, which can significantly influence the pathways of carbon flow in the ocean. A goal for future research is to elucidate the mechanistic bases of bacteria-organic matter coupling. This research should take into consideration the micrometer-scale distribution of bacteria and the composition, structure, and dynamics of the organic matter field in the bacterium's microhabitat. The ideas on the interactions of bacteria with the particulate organic phase need to be revised in view of recent findings of highly abundant, previously unknown particles ranging in size from nanometers to hundreds of micrometers. The "hot-spots" in the distribution of organic matter and remineralized nutrients can influence the rates as well as the direction of biogeochemical fluxes. Slow-to-degrade dissolved organic matter (DOM) may be produced because of loose bacteria-organic matter coupling resulting in DOM storage. Its use at a later time and place has profound implications for carbon fluxes and food web dynamics. A fundamental research need for the future is to understand the ecological interactions among the members of the microbial loop in an appropriate microhabitat context. While this goal was previously intractable, new molecular and optical techniques should make it possible to understand the biogeochemical activities of the microbial loop in terms of the ecology and evolution of pelagic microbial communities.

The annual cycle of planktonic ciliates in nearshore waters at Signy Island, Antarctica

The abundance and biomass of marine planktonic ciliates in Borge Bay, Signy Island, were determined at monthly intervals between April 1990 and June 1991. At least 24 different ciliate taxa were recorded from samples preserved in Lugol's iodine, including the tintinnids Codonellopsis balechi, Cymalocylis convallaria, Laackmaniella naviculaefera and Salpingella sp., and the aloricate taxa Didinium sp. and Mesodinium rubrum. Ciliate abundance and biomass exhibited a clear seasonal cycle with high values during the austral summer and low values in the austral winter. Abundance ranged from 0.3 × 103l−1 in September to 2.3 × 103l−1 in January, while biomass ranged from 0.5 μg C l−1 in October to 12.6 μg C l−1 in December. Small ciliates dominated abundance throughout the year, and biomass during winter. Larger ciliates contributed most to biomass during summer. Aloricate ciliates were common throughout the year, while tintinnids contributed substantially to abundance and biomass only during summer. Salpingella sp. was the commonest tintinnid, but C.convallaria contributed most to tintinnid biomass. The seasonal pattern of ciliate abundance and biomass matched that of chlorophyll a concentration and bacterial biomass, suggesting tight trophic coupling between ciliates and other components of the pelagic microbial community.

The Seasonal Dynamics of The Chesapeake Bay Ecosystem

The full suite of carbon exchanges among the 36 most important components of the Chesapeake Bay mesohaline ecosystem is estimated to examine the seasonal trends in energy flow and the trophic dynamics of the ecosystem. The networks provide information on the rates of energy transfer between the trophic components in a system wherein autochthonous production is dominated by phytoplankton production. A key seasonal feature of the system is that the summer grazing of primary producers by zooplankton is greatly reduced due to top—down control of zooplankton by ctenophores and sea nettles. Some of the ungrazed phytoplankton is left to fuel the activities of the pelagic microbial community, and the remainder falls to the bottom where it augments the deposit—feeding assemblage of polychaetes, amphipods, and blue crabs. There is a dominant seasonal cycle in the activities of all subcommunities, which is greatest in the summer and least in the cold season. However, the overall topology of the ecosystem does not appear to change substantially from season to season. Matrix operations can be employed to assess the various direct and indirect pathways by which each trophic group obtains energy. Often, indirect linkages reveal interesting differences. For example, although the bluefish and striped bass are both piscivorous predators, 63% of bluefish intake depends indirectly on benthic organisms, whereas striped bass depends mainly on planktonic organisms. Nearly all higher trophic species exhibit significant indirect dependencies upon the upper components of the microbial loop, especially during summer. The complicated trophic network can be mapped into an eight—level trophic chain in the sense of Lindeman. Such analysis reveals that detritivory is about 10 times greater than herbivorous grazing in the Chesapeake system and that 70% of detritus results from internal recycle. Annual efficiencies of trophic levels decrease as one ascends the chain. Major seasonal shifts in trophic efficiencies at higher levels appear to be modulated by how effectively microscopic zooplankton (mostly ciliates) are cropped by their predators. Average trophic efficiency is 9.6%. Despite the existence of eight trophic levels, the average level at which each species feeds always remains below 5. One "pest" species (the coelenterate Chrysaora quinquecirrha) feeds rather high on the trophic pyramid and may exert a heretofore unappreciated level of control on the planktonic food chain. The number of cycles present in the network is surprisingly few, despite the fact that a relatively large and seemingly constant amount (23.2%) of total system activity is devoted to recycling. This combination of factors possibly indicates a stressed ecosystem. A study of the rate—limiting links in the seasonal networks of recycling of material within the plankton reconfirms the shift of predator control from crustaceous zooplankton in springtime to the sea nettle (Chrysaora quinquecirrha) during summer months. The collection of cycles present in the system is disjoint; there is no overlap between the cycles among the planktonic community and the circulations among the deposit feeders and nekton. The filter—feeding benthos and fish do not participate in any cycling, but serve rather as bridges to shift carbon and energy from the planktonic community into the benthic—nektonic subsystems. Neither do most of the members of the microbial loop engage in any recycle of carbon, functioning instead as a dissipative shunt of energy out of the system.

Trophic interactions within pelagic microbial communities: Indications of feedback regulation of carbon flow

Future considerations of carbon-energy flows within pelagic food webs should include internal, biotic feedback controls, in addition to abiotic forcing functions, in the regulation of these flows. Over the past two decades, research on microbial communities of pelagic ecosystems has yielded data suggestive of cybernetic-like regulation operating within these communities. As presently conceived, phagotrophic protozoa have a pivotal role in such regulation as a consequence of their rapid growth, grazing, and nutrient regenerative capabilities. Feedback controls within microbial food webs may have significant effects on distal portions of pelagic ecosystems, including the fate of organic detritus and metazoan production.

Photolithotrophy, photoheterotrophy, and chemoheterotrophy: Patterns of resource utilization on an annual and a diurnal basis within a pelagic microbial community

An annual investigation of rates of photolithotrophy, photoheterotrophy, and chemoheterotrophy utilizing glucose and bicarbonate was made within the pelagic zone of a small, hardwater, southwestern Michigan lake. Sampling proceeded on a monthly, diurnal, and depth-wise basis. Annual mean photoheterotrophic uptake was estimated at 2.6μg C m(-3)h(-1). Two periods of relatively high activity were observed: one during spring overturn and the second during the late summer period. In general, greatest contributions to overall carbon cycling occurred during morning to midday incubation periods and at intermediate depths within the water column. Rates of chemoheterotrophy averaged 6.9μg C m(-3)h(-1) and were relatively uniform throughout the annual period. Greatest overall chemoheterotrophic activity was associated with periods of overturn. In general, this activity increased throughout the day and with increasing depth within the water column. The annual mean for photolithotrophic fixation was 1.33 mg C m(-3)h(-1). Greatest contributions to rates of photosynthesis were associated with epilimnetic waters during early morning and midday incubations. Relatively minor contributions to inorganic fixation were made by waters below the 6-meter contour. Spring overturn and late summer represented periods of particularly great photolithotrophic activity. Quantitative comparisons among carbon pathways indicate that rates of pelagic heterotrophy, both photo- and chemoheterotrophy combined, contribute small quantities of carbon to overall carbon metabolism in this oligotrophic system. Qualitative comparisons among pathways indicate strong spatial and temporal separation. The late summer period showed greatest seasonal separation of the three pathways. Spring values represented a period of relatively high activity for all three pathways. On a depth-wise basis, photolithotrophic activity was greatest near the surface and chemolithotrophic activity greatest near the bottom. Photoheterotrophy took an intermediate position between the two. Diurnally, photoheterotrophy and photolithotrophy showed greatest activity during midday and early morning periods, whereas chemoheterotrophy increased throughout the daylight period and reached maximal values in sunset incubations.