Steven T. Rier

Differential heterotrophic utilization of organic compounds by diatoms and bacteria under light and dark conditions

The heterotrophic utilization of organic substrates by diatoms is likely an important survival strategy when light levels are too low for photosynthesis. The objectives of this study were: (1) to determine if heterotrophic utilization of a large array of organic compounds by eight common freshwater benthic diatom taxa was light-dependent, and (2) to determine if organic substrate utilization patterns differed between darkgrown diatoms and bacteria as a possible means of reducing competition by niche separation. Eight lightand dark-grown diatom taxa and five bacterial species were incubated in 96-well Biolog® Microtiter plates with each well containing 1 of 95 different organic substrates. Oxidation rates of each organic substrate were measured through time. There was a substantial increase in the number of organic substrates oxidized by diatoms grown in the dark compared to their light-grown counterparts, indicating that the transport systems for these molecules may be light activated. Therefore, diatoms likely only utilize these metabolically expensive uptake mechanisms when they are necessary for survival, or when substrates are plentiful. A principal components analysis indicated discernible differences in the types of organic-C substrates utilized by dark-grown diatoms and bacteria. Although bacteria were able to oxidize a more diverse array of organic substrates including carboxylic acids and large polymers, diatoms appeared to more readily utilize the complex carbohydrates. By oxidizing different organic substrates than bacteria, heterotrophically metabolizing diatoms may be reducing direct competition and enhancing coexistence with bacteria.

Algal regulation of extracellular enzyme activity in stream microbial communities associated with inert substrata and detritus

Abstract We tested the hypothesis that algae influence the activities of extracellular enzymes involved in mineralization processes within microbial assemblages in streams. We tested the prediction that the factors that influence algal biomass and photosynthesis (i.e., diel fluctuations in photosynthetically active radiation [PAR], long-term variations in light regime, and community development stage) would have a corresponding effect on extracellular enzyme activities. We also tested the prediction that algae would influence enzyme activities on inorganic substrata and in detrital communities where they ultimately would influence plant litter decomposition rates. We allowed microbial communities to develop on inert substrata (glass-fiber filters) or on leaf litter in artificial streamside channels. For each community type, we examined the effects of long-term light manipulations, community development stage, and diel periodicity on the activities of β-glucosidase, alkaline phosphatase, leucine-aminopeptidase, and phenol oxidase. In addition, we measured the decomposition rates of the leaf litter substrata in the low- and high-light treatments. Our results support the prediction that factors that influence algal photosynthesis and biomass in the short (diel fluctuations in PAR) and long (shading, community development stage) term ultimately influence enzyme activities in microbial communities associated with both inorganic substrata and detritus. Furthermore, decomposition rates of organic detritus probably are enhanced by algal colonization and activity. Algal photosynthesis might enhance redox and pH conditions within microbial communities, and in turn, might increase the activities of oxidative and hydrolytic enzymes. As a consequence, photoautotrophic activities might stimulate heterotrophic pathways in stream ecosystems by creating conditions favorable for decomposition of both dissolved and particulate organic detritus.

Elevated Atmospheric CO2 Alters Soil Microbial Communities Associated with Trembling Aspen (Populus tremuloides) Roots

Global atmospheric CO2 levels are expected to double within the next 50 years. To assess the effects of increased atmospheric CO2 on soil ecosystems, cloned trembling aspen (Populustremuloides) seedlings were grown individually in 1 m3 open bottom root boxes under either elevated (720 ppm, ELEV) or ambient CO2 (360 ppm, AMB). After 5 years, soil cores (40 cm depth) were collected from the root boxes and divided into 0–20 cm and 20–40 cm fractions. ELEV treatment resulted in significant decreases in both soil nitrate and total soil nitrogen in both the 0–20 cm and 20–40 cm soil fractions, with a 47% decrease in soil nitrate and a 50% decrease in total soil nitrogen occurring in the 0–20 cm fraction. ELEV treatment did not result in a significant change in the amount of soil microbial biomass. However, analysis of indicator phospholipid fatty acids (PLFA) indicated that ELEV treatment did result in significant increases in PLFA indicators for fungi and Gram-negative bacteria in the 0–20 cm fraction. Terminal restriction fragment length polymorphism (T-RFLP) analysis was used to analyze the composition of the soil bacterial communities (using primers targeting the 16SrRNA gene) and the soil fungal communities (using primers targeting the intergenic transcribed spacer region). T-RFLP analysis revealed shifts in both bacterial and fungal community structure, as well as increases in both bacterial and fungal species richness with ELEV treatment. These results indicated that increased atmospheric CO2 had significant effects on both soil nutrient availability and the community composition of soil microbes associated with aspen roots.

Relation of environmental factors to density of epilithic lotic bacteria in 2 ecoregions

Although heterotrophic bacteria are important components of stream ecosystems, little is known about the factors that contribute to their abundance. We used data from 69 streams to examine both the direct and indirect effects of algal biomass (chlorophyll a), dissolved organic C (DOC), NO3−, soluble reactive P (SRP), and NH4+ on bacterial density. All potential relationships were simultaneously evaluated using path analysis. Streams in 2 ecoregions (unglaciated and glaciated) were used to determine if important factors varied between ecoregions. The results indicated that algal biomass was the best predictor of bacterial density in both ecoregions. Furthermore, inorganic nutrients appeared to mainly affect bacteria indirectly through their influence on algal biomass. However, a quantitative relationship between algae and bacteria was not observed when algal biomass was <5 μg chlorophyll a/cm2. The lack of relationship between algae and bacteria at low algal biomass may help to explain why some studies have observed strong quantitative relationships between bacteria and algae in streams and others have not. Although the current study design was not capable of assessing the bioavailability of DOC to bacteria, the proposed conceptual framework used in the path analysis coupled with better assessments of the relationship between DOC and bacteria should prove useful for evaluating the importance of DOC, inorganic nutrients, and algae on epilithic bacteria in a number of different stream ecosystems.

Elevated-CO2-induced changes in the chemistry of quaking aspen (Populus tremuloides Michaux) leaf litter: subsequent mass loss and microbial response in a stream ecosystem

Changes in chemistry of quaking aspen (Populus tremuloides Michaux) leaf litter were examined under ambient (AMB = 360 ppm) and elevated (ELE = 720 ppm) levels of atmospheric CO2. Senesced ELE leaves were significantly higher in phenolic compounds, lignin, and C:N than AMB leaves. A 30-d in situ experiment in a northern Michigan stream analyzed changes in leaf mass, the concentration of phenolic compounds as a result of chemical leaching, and the growth responses of fungi and bacteria. ELE leaves lost less mass than AMB leaves after a 30-d incubation. Although ELE leaves were initially higher in total phenolic compounds and condensed tannins, differences between the treatments were no longer observed after 48 h of chemical leaching. Bacterial biomass and community respiration were higher on the AMB leaves for the first 12 d of incubation, whereas fungal biomass and community respiration were higher in the AMB treatment by the end of the 30-d experiment. Fungal biomass was negatively correlated with C:N and positively correlated with bacterial biomass on the ELE leaves, but not on the AMB leaves. These results indicate that a doubling in concentration of atmospheric CO2 could lead to leaf litter that is more recalcitrant toward microbial breakdown, which may decrease the availability of C and N for higher trophic levels.

Abandoned coal mine drainage and its remediation: impacts on stream ecosystem structure and function

The effects of abandoned mine drainage (AMD) on streams and responses to remediation efforts were studied using three streams (AMD-impacted, remediated, reference) in both the anthracite and the bituminous coal mining regions of Pennsylvania (USA). Response variables included ecosystem function as well as water chemistry and macroinvertebrate community composition. The bituminous AMD stream was extremely acidic with high dissolved metals concentrations, a prolific mid-summer growth of the filamentous alga, Mougeotia, and > 10-fold more chlorophyll than the reference stream. The anthracite AMD stream had a higher pH, substrata coated with iron hydroxide(s), and negligible chlorophyll. Macroinvertebrate communities in the AMD streams were different from the reference streams, the remediated streams, and each other. Relative to the reference stream, the AMD stream(s) had (1) greater gross primary productivity (GPP) in the bituminous region and undetectable GPP in the anthracite region, (2) greater ecosystem respiration in both regions, (3) greatly reduced ammonium uptake and nitrification in both regions, (4) lower nitrate uptake in the bituminous (but not the anthracite) region, (5) more rapid phosphorus removal from the water column in both regions, (6) activities of phosphorus-acquiring, nitrogen-acquiring, and hydrolytic-carbon-acquiring enzymes that indicated extreme phosphorus limitation in both regions, and (7) slower oak and maple leaf decomposition in the bituminous region and slower oak decomposition in the anthracite region. Remediation brought chlorophyll concentrations and GPP nearer to values for respective reference streams, depressed ecosystem respiration, restored ammonium uptake, and partially restored nitrification in the bituminous (but not the anthracite) region, reduced nitrate uptake to an undetectable level, restored phosphorus uptake to near normal rates, and brought enzyme activities more in line with the reference stream in the bituminous (but not the anthracite) region. Denitrification was not detected in any stream. Water chemistry and macroinvertebrate community structure analyses capture the impact of AMD at the local reach scale, but functional measures revealed that AMD has ramifications that can cascade to downstream reaches and perhaps to receiving estuaries.

Effects of inorganic sedimentation and riparian clearing on benthic community metabolism in an agriculturally-disturbed stream

An in situ chamber technique was used to obtain seasonal estimates of benthic community metabolism at three stations in an agriculturally disturbed stream. Two stations with open canopies were examined. Sand was the dominant substrate at one site, cobble at the other. The third station was shaded by riparian vegetation and had a sand substrate.Seasonal estimates of net community productivity (NCP) and community respiration (CR) at the cobble section were significantly higher than those calculated for the sand sections (p>0.05). Ratios of gross community productivity (GCP) to 24 h respiration indicated autotrophic conditions in the cobble and extreme heterotrophy in the sand. NCP was significantly higher (p<0.05) in the open canopy sand than in the riparian shaded sand only when turbidity and discharge were low. Measurements of periphyton ash-free dry mass (AFDM) and chlorophyll a support metabolism estimates. Measurements of loose detrital AFDM were very low and variable compared to others reported in the literature. Therefore, allochthonously derived detritus may not be an important energy source for the benthic community.

Photo-acclimation response of benthic stream algae across experimentally manipulated light gradients : A comparison of growth rates and net primary productivity

The objective of this study was to measure the photo‐acclimation response of stream algae inhabiting thin biofilms across a range of light treatments comparing both growth rates and net primary productivity (NPP). Algae were grown on clay tiles incubated in artificial stream channels where light levels were manipulated by layering a neutral‐density shade cloth over each channel. We measured NPP and algal growth rates in the early stages of community development and compared assemblages that were either acclimated or unacclimated to a given light treatment. Algal growth rates did acclimate to light treatment, with saturation occurring at light levels that were substantially lower for the acclimated communities. Growth efficiency calculated from algal biovolume increased by a factor of 3. However, algal NPP showed a weaker photo‐acclimation response, with only a 30% increase in photosynthetic efficiency. Our results indicate that diatom‐dominated periphyton in thin biofilms are probably not light limited in many shaded streams with respect to growth rates, but are light limited with respect to NPP.

Alteration of microbial communities colonizing leaf litter in a temperate woodland stream by growth of trees under conditions of elevated atmospheric CO2.

ABSTRACT Elevated atmospheric CO2 can cause increased carbon fixation and altered foliar chemical composition in a variety of plants, which has the potential to impact forested headwater streams because they are detritus-based ecosystems that rely on leaf litter as their primary source of organic carbon. Fungi and bacteria play key roles in the entry of terrestrial carbon into aquatic food webs, as they decompose leaf litter and serve as a source of nutrition for invertebrate consumers. This study tested the hypothesis that changes in leaf chemistry caused by elevated atmospheric CO2 would result in changes in the size and composition of microbial communities colonizing leaves in a woodland stream. Three tree species, Populus tremuloides, Salix alba, and Acer saccharum, were grown under ambient (360 ppm) or elevated (720 ppm) CO2, and their leaves were incubated in a woodland stream. Elevated-CO2 treatment resulted in significant increases in the phenolic and tannin contents and C/N ratios of leaves. Microbial effects, which occurred only for P. tremuloides leaves, included decreased fungal biomass and decreased bacterial counts. Analysis of fungal and bacterial communities on P. tremuloides leaves via terminal restriction fragment length polymorphism (T-RFLP) and clone library sequencing revealed that fungal community composition was mostly unchanged by the elevated-CO2 treatment, whereas bacterial communities showed a significant shift in composition and a significant increase in diversity. Specific changes in bacterial communities included increased numbers of alphaproteobacterial and cytophaga-flavobacter-bacteroides (CFB) group sequences and decreased numbers of betaproteobacterial and firmicutes sequences, as well as a pronounced decrease in overall Gram-positive bacterial sequences.

Response of biofilm extracellular enzymes along a stream nutrient enrichment gradient in an agricultural region of north central Pennsylvania, USA

Anthropogenic nitrogen and phosphorus loading to streams has the potential to shift ecosystems from inorganic nutrient limitation toward increased carbon flux. We tested the hypothesis that relative patterns in the activities of carbon-, phosphorus- and nitrogen-acquiring extracellular enzymes associated with stream microbial communities could potentially indicate a transition from nutrient limitation to increased carbon flux. Biofilm extracellular enzyme activities (β-glucosidase, β-xylosidase, phosphatase, and leucine-aminopeptidase) were measured in 23 streams along an agricultural nutrient gradient in north central Pennsylvania. We also measured water column total nitrogen and total phosphorus along with biofilm chlorophyll a, ash free dry mass, and phosphorus concentrations. Although most absolute enzyme activities tended to be positively correlated with phosphorus concentrations, relative activities between carbon- and phosphorus-acquiring enzymes indicated a shift away from phosphorus limitation within a relatively narrow and low range of phosphorus concentrations. These results have potential implications for establishing nutrient guidelines for streams because they indicate that nutrients such as phosphorus might elicit a significant ecosystem-level response at relatively low concentrations of phosphorus before more apparent changes in community structure or biomass are observed.