John E. Hobbie

Use of Glucose and Acetate by Bacteria and Algae in Aquatic Ecosystems

New methods employing C14—Labeled organic compounds measure the uptake of organic solutes by planktonic microorganisms. By testing uptake over a wide range of substrate concentrations, two separate mechanisms can be differentiated and their kinetics measured. Using filtration and evidence from experiments with laboratory cultures of planktonic bacteria and algae, specific transport systems effective at very low substrate concentrations were traced to the bacteria, and a diffusion mechanism, effective only at higher substrate concentrations, to the algae. Studies have shown that V, the maximum velocity of uptake by bacterial transport systems, gives information about the size and function of the bacterial populations. A diffusion constant, kd, gives information on the rate of uptake of solutes by the algal populations. Turnover times, derived from kinetic parameters, indicate that the algal uptake of glucose and acetate in Lake Erken, Sweden, is always less than 10% of the bacterial uptake, even though the algal biomass may be orders of magnitude greater than the bacterial. Two new types of bioassay employ the kinetics of bacterial uptake systems as the measuring reaction. Acetate and glucose were found in 1—10 mg/liter concentrations in the several natural waters tested. At these very low concentrations, algal uptake of glucose and acetate is so low that effective heterotrophy is impossible. In contrast, the bacteria effectively remove substrate from solution at these low levels and probably keep the substrate at these low concentrations. By doing this, the bacteria may prevent heterotrophic growth of algae in nature.

Ecosystem Alteation of Boreal Forest Streams by Beaver (Castor Canadensis)

Beaver (Castor canadensis) alter the structure and dynamics of aquatic ecosystems with a minimum of direct energy or nutrient transfer. Through dam building and feeding activities, beaver act as a keystone species to alter hydrology, channel geomorphology, biogeochemical pathways, and community productivity. Here we consider the effects of beaver activity on several major ecosystem components and processes in boreal forest drainage networks in Quebec, Canada. The density of dams on the small streams (≤4th order) we studied average 10.6 dams/km; the streams retain up to 6500 m3 of sediment per dam, and the wetted surface area of the channel is increased up to several hundred—fold. Beaver are also active in large order streams (≥5th order), but their effects are most noticeable along riverbanks and in floodplains. Comparative carbon budgets per unit area for a riffle on 2nd order Beaver Creek and a beaver pond downstream show the pond receives only 42% of the carbon acquired by the riffle annually. However, because the pond has a surface area seven times greater than the riffle, it receives nearly twice as much carbon as the riffle per unit of channel length. Carbon in the pond has an estimated turnover time of °161 yr compared to ° 24yr for the riffle. Beaver ponds are important sites for organic matter processing; the stream metabolism index (SMI), a measure of ecosystem efficiency for the utilization or storage of organic inputs, is 1.63 for the pond compared to 0.30 for the riffle; the turnover length (S) for particulate carbon is 1.2 km for the pond compared to 8.0 km for the riffle. Beaver—induced alterations to the structure and function of streams suggest removal of beaver prior to 1900 AD had substantial effects on the dynamics of lotic ecosystems. Our results suggest that current concepts of the organization and diversity of unaltered stream ecosystems in North America should recognize the keystone role of beaver, as drainage networks with beaver are substantially different in their biogeochemical economies than those without beaver.

Aggregating Fine‐Scale Ecological Knowledge to Model Coarser‐Scale Attributes of Ecosystems

As regional and global scales become more important to ecologists, methods must be developed for the application of existing fine-scale knowledge to predict coarser-scale ecosystem properties. This generally involves some form of model in which fine-scale components are aggregated. This aggregation is necessary to avoid the cumulative error associated with the estimation of a large number of parameters. However, aggregation can itself produce errors that arise because of the variation among the aggregated components. The statistical expectation operator can be used as a rigorous method for translating fine-scale relationships to coarser scales without aggregation errors. Unfortunately this method is too cumbersome to be applied in most cases, and alternative methods must be used. These alternative methods are typically partial corrections for the variation in only a few of the fine-scale attributes. Therefore, for these methods to be effective, the attributes that are the most severe sources of error must be identified a priori. We present a procedure for making these identifications based on a Monte Carlo sampling of the fine-scale attributes. We then present four methods of translating fine-scale knowledge so it can be applied at coarser scales: (1) partial transformations using the expectation operator, (2) moment expansions, (3) partitioning, and (4) calibration. These methods should make it possible to apply the vast store of fine-scale ecological knowledge to model coarser-scale attributes of ecosystems.

Microbial Biogeography along an Estuarine Salinity Gradient: Combined Influences of Bacterial Growth and Residence Time

ABSTRACT Shifts in bacterioplankton community composition along the salinity gradient of the Parker River estuary and Plum Island Sound, in northeastern Massachusetts, were related to residence time and bacterial community doubling time in spring, summer, and fall seasons. Bacterial community composition was characterized with denaturing gradient gel electrophoresis (DGGE) of PCR-amplified 16S ribosomal DNA. Average community doubling time was calculated from bacterial production ([14C]leucine incorporation) and bacterial abundance (direct counts). Freshwater and marine populations advected into the estuary represented a large fraction of the bacterioplankton community in all seasons. However, a unique estuarine community formed at intermediate salinities in summer and fall, when average doubling time was much shorter than water residence time, but not in spring, when doubling time was similar to residence time. Sequencing of DNA in DGGE bands demonstrated that most bands represented single phylotypes and that matching bands from different samples represented identical phylotypes. Most river and coastal ocean bacterioplankton were members of common freshwater and marine phylogenetic clusters within the phyla Proteobacteria, Bacteroidetes, and Actinobacteria. Estuarine bacterioplankton also belonged to these phyla but were related to clones and isolates from several different environments, including marine water columns, freshwater sediments, and soil.

Bacterioplankton Community Shifts in an Arctic Lake Correlate with Seasonal Changes in Organic Matter Source

ABSTRACT Seasonal shifts in bacterioplankton community composition in Toolik Lake, a tundra lake on the North Slope of Alaska, were related to shifts in the source (terrestrial versus phytoplankton) and lability of dissolved organic matter (DOM). A shift in community composition, measured by denaturing gradient gel electrophoresis (DGGE) of 16S rRNA genes, occurred at 4°C in near-surface waters beneath seasonal ice and snow cover in spring. This shift was associated with an annual peak in bacterial productivity ([14C]leucine incorporation) driven by the large influx of labile terrestrial DOM associated with snow meltwater. A second shift occurred after the flux of terrestrial DOM had ended in early summer as ice left the lake and as the phytoplankton community developed. Bacterioplankton communities were composed of persistent populations present throughout the year and transient populations that appeared and disappeared. Most of the transient populations could be divided into those that were advected into the lake with terrestrial DOM in spring and those that grew up from low concentrations during the development of the phytoplankton community in early summer. Sequencing of DNA in DGGE bands demonstrated that most bands represented single ribotypes and that matching bands from different samples represented identical ribotypes. Bacteria were identified as members of globally distributed freshwater phylogenetic clusters within the α- and β-Proteobacteria, the Cytophaga-Flavobacteria-Bacteroides group, and the Actinobacteria.

THE STABLE NITROGEN ISOTOPE RATIO AS A MARKER OF FOOD‐WEB INTERACTIONS AND FISH MIGRATION

We used stable nitrogen isotopes to describe the pelagic food-web structure of three coastal Baltic Sea areas, each of which was sampled twice. Two of the areas were influenced by 15N-rich nutrient discharges from a sewage treatment plant. Analyses were made of particulate organic matter (<35 μm, mainly phytoplankton), zooplankton, mysids (Mysis mixta and M. relicta), sprat (Sprattus sprattus), smelt (Osmerus eperlanus), four size classes of herring (Clupea harengus), and pikeperch (Stizostedion lucioperca). Discharges from the sewage treatment plant significantly increased δ15N values in the whole food web, from phytoplankton to piscivorous fish. Based on nitrogen isotopic compositions, consistent trophic food-web structures were observed on both occasions and in all three areas. The results indicate that zooplankton and mysids may have more complex diets than assumed before. Apparent trophic fractionation, i.e., differences in δ15N between a consumer and its assumed food, averaged 2.4‰ with a standard e...

15N IN SYMBIOTIC FUNGI AND PLANTS ESTIMATES NITROGEN AND CARBON FLUX RATES IN ARCTIC TUNDRA

When soil nitrogen is in short supply, most terrestrial plants form symbioses with fungi (mycorrhizae): hyphae take up soil nitrogen, transport it into plant roots, and receive plant sugars in return. In ecosystems, the transfers within the pathway fractionate nitrogen isotopes so that the natural abundance of 15N in fungi differs from that in their host plants by as much as 12% per hundred. Here we present a new method to quantify carbon and nitrogen fluxes in the symbiosis based on the fractionation against 15N during transfer of nitrogen from fungi to plant roots. We tested this method, which is based on the mass balance of 15N, with data from arctic Alaska where the nitrogen cycle is well studied. Mycorrhizal fungi provided 61-86% of the nitrogen in plants; plants provided 8-17% of their photosynthetic carbon to the fungi for growth and respiration. This method of analysis avoids the disturbance of the soil-microbe-root relationship caused by collecting samples, mixing the soil, or changing substrate concentrations. This analytical technique also can be applied to other nitrogen-limited ecosystems, such as many temperate and boreal forests, to quantify the importance for terrestrial carbon and nitrogen cycling of nutrient transfers mediated by mycorrhizae at the plant-soil interface.

Global deforestation: contribution to atmospheric carbon dioxide.

A study of effects of terrestrial biota on the amount of carbon dioxide in the atmosphere suggests that the global net release of carbon due to forest clearing between 1860 and 1980 was between 135 x 1015 and 228 x 1015 grams. Between 1.8 x 1015 and 4.7 x 1015 grams of carbon were released in 1980, of which nearly 80 percent was due to deforestation, principally in the tropics. The annual release of carbon from the biota and soils exceeded the release from fossil fuels until about 1960. Because the biotic release has been and remains much larger than is commonly assumed, the airborne fraction, usually considered to be about 50 percent of the release from fossil fuels, was probably between 22 and 43 percent of the total carbon released in 1980. The increase in carbon dioxide in the atmosphere is thought by some to be increasing the storage of carbon in the earths remaining forests sufficiently to offset the release from deforestation. The interpretation of the evidence presented here suggests no such effect; deforestation appears to be the dominant biotic effect on atmospheric carbon dioxide. If deforestation increases in proportion to population, the biotic release of carbon will reach 9 x 1015 grams per year before forests are exhausted early in the next century. The possibilities for limiting the accumulation of carbon dioxide in the atmosphere through reduction in use of fossil fuels and through management of forests may be greater than is commonly assumed.

Climate change effects on hydroecology of arctic freshwater ecosystems.

Abstract In general, the arctic freshwater-terrestrial system will warm more rapidly than the global average, particularly during the autumn and winter season. The decline or loss of many cryospheric components and a shift from a nival to an increasingly pluvial system will produce numerous physical effects on freshwater ecosystems. Of particular note will be reductions in the dominance of the spring freshet and changes in the intensity of river-ice breakup. Increased evaporation/evapotranspiration due to longer ice-free seasons, higher air/water temperatures and greater transpiring vegetation along with increase infiltration because of permafrost thaw will decrease surface water levels and coverage. Loss of ice and permafrost, increased water temperatures and vegetation shifts will alter water chemistry, the general result being an increase in lotic and lentic productivity. Changes in ice and water flow/levels will lead to regime-specific increases and decreases in habitat availability/quality across the circumpolar Arctic.

The Utilization of Dissolved Free Amino Acids by Estuarine Microorganisms

The importance of vacteria in the cycling of carbon in the Pamlico River estuary was studied by measuring the rates of uptake of organic compounds. Our methods allowed analysis with the Michaelis—Menten kinetics equations, and both the rates of uptake of dissolved free amino acids (DFAA) and glucose as well as the percentage of carbon subsequently respired as CO2 were determined. In addition, the concentrations of the amino acids in the water were determined using ion exchange chromatography. Other tests included measurements of primary productivity and of the effects of the other amino acids in the water upon the uptake of one amino acid. There was considerable variation in the heterotrophic activity over time and distance probably caused by patchiness in distribution of plankton and dissolved compounds in the water. Although there is some competition between amino acids being taken up, the effect upon kinetics measurements is probably negligible. Tests made every 3 hr showed a coefficient of variability (CV) of the measured maximum velocity of uptake (Vmax) of aspartic acid to be only 26%, and a similar CV was found for daily samples. In several instances the uptake of one amino acid was found to be competitively inhibited by the presence of another amino acid, but the concentrations necessary to inhibit were far above natural concentrations and such effects are probably unimportant in nature. Mutual inhibition was found between the similar amino acid pairs glutamic acid and aspartic acid, threonine and serine, glycine and alanine, and leucine and alanine. Highest Vmax values were found during the summer months and early fall and ranged from a high of 69.42 mg C/1°hr for alanine in August to less than 0.20 mg C/1°hr for most of the substrates tested in the colder months. The Vmax values for glucose uptake (0.06 to 9.64 mg C/1°hr) indicate that this estuarine system is one of the most microbially—active environments tested. The DFAA were presented in the water at concentrations of from 10 to 30 mg C/1; over half of this was ornithine, glycine, and serine. The DFAA were only about 0.2% of the total dissolved organic carbon in the water. Further, seasonal variations of DFAA concentrations, generally paralleling those of primary productivity, suggested that the amino acids originated from algal excretion and the decay of algal cells. The orders of abundance and concentrations of individual amino acids were similar to those reported for other bodies of water. When the natural concentration of a substrate is known the actual velocity of uptake (Vn) or flux for that substrate may be found. Flux rates were only 1%—10% of the Vmax values in the coldest months; the highest values were found in the warmest months. At each experimental concentration of amino acid, a certain amount was taken up, and a percentage of this amount was oxidized to caron dioxide. This percentage was constant for a particular amino acid in spite of varying experimental times, substrate concentrations, and temperatures. Leucine had the lowest percent respired (13%) while aspartic and glutanic acids had the highest (50%). Failure to correct uptake data for this respiratory loss introduces significant underestimation. The production of particulate material was calculated by correcting total uptake figures for each amino acid by its characteristic respiration percentage. Over 60% of the particulate production from amino acids was by uptake of alanine, leucine, valine, serine, glycine, aspartic acid, and glutamic acid. Such particulate production averaged 0.79 mg C/1.hr for the year and ranged from 0..6 to 2.37 mg C/1.hr; this is about 10% of the rate of production by algae during the summer months. This amount of particulate organic material is a significant contribution to this estuarine food chain.