Christopher G. Peterson

Mechanisms of lotic microalgal colonization following space-clearing disturbances acting at different spatial scales

Diel drift/colonization studies were conducted in Sycamore Creek, Arizona, U.S.A. in late June, 87 d post-spate and, again, 4 d after recession of a large mid-August spate to examine differences in benthic algal colonization of space cleared by small-scale, localized disturbance versus large-scale, system-wide disturbance. Cell densities and species composition in the algal drift pool were quantified with hourly samples, composited every 3 h for 36 h. Diel changes in colonization activity were assessed by sampling ceramic tiles incubated during each of the twelve 3-h periods (instantaneous colonization). Cumulative changes in algal communities were assessed by collecting tiles, introduced at the start of each study, at 3-h intervals for 36 h, then again at 48 and 96 h. Consecutive 3-h measures from instantaneous-colonization samples were summed to compute expected cell densities of taxa within different algal divisions, based on short-term colonization alone, after 6 to 36 h, and compared to actual cell densities on cumulative tiles. Tiles introduced during the June interflood period were colonized much more slowly than in August, and supported assemblages dominated by bluegreen algae. Comparisons of cell densities and changes in taxonomic structure in instantaneous- and cumulative-colonization assemblages indicated that, in June, algal reproductive activity was low and rates of emigration and death were high. In August, diatom and green algal densities in drift and colonization assemblages were significantly higher than in June, and accrual of these taxa on cumulative-colonization tiles exceeded that expected, indicating rapid reproduction. August drift and colonization assemblages exhibited clear diel changes in both cell densities (with mid-day maxima) and taxonomic structure, suggesting that rates of immigration, emigration, and reproduction varied among taxa. In June, mid-day peaks in algal drift activity were caused by passive entrainment of cells into the water column, likely by oxygen bubbles produced by photosynthesis. In August, live diatom cells exhibited higher drift peaks than dead cells, indicating reproductive activity and, likely, changes in cell buoyancy was influential in generating drift maxima. Many diatom taxa displayed clear mid-day minima in colonization efficiency, corresponding to periods of peak drift, suggesting diel changes in cell bouyancy. No such patterns were detected in bluegreen or green-algal taxa. Rapid recovery of diatom assemblages in Sycamore Creek after spates appears to be driven by rapid reproduction, emigration, and reimmigration of early successional diatom taxa. The rate and pattern of benthic algal colonization of open space in Sycamore Creek, and the mechanisms controlling this process, varied considerably depending on the circumstances under which substrata became available.

Influence of Algal Community Structure on Denitrification Rates in Periphyton Cultivated on Artificial Substrata

We conducted a field survey of periphyton cultivated on benthic mesh installations in freshwater aquatic systems, including two constructed wetlands and a pond, and also studied periphyton grown on a benthic mesh in laboratory mesocosms. The objectives of this study were to (1) determine if periphyton cultivated on benthic mesh denitrifies at higher rates than the underlying sediments and (2) determine if denitrification rates within periphyton vary with characteristics such as algal and bacterial community structure and biomass. We measured denitrification potential rates of field and laboratory periphyton by the acetylene inhibition method. We characterized algal community composition by algal identification and bacterial community composition by terminal restriction fragment length polymorphisms. Periphyton collected on benthic mesh from our field sites denitrified at significantly higher rates than the underlying sediments, regardless of sampling site or season. Results from both our field survey and laboratory studies indicated a significant, positive correlation between diatom presence and denitrification rate. In our laboratory studies, we found that periphyton with the highest diatom abundance showed the highest denitrification rates as well as a distinct bacterial community composition. These results suggest a synergistic relationship between diatoms and denitrifying bacteria that warrants further study.

Effects of anthropogenic inputs on the organic quality of urbanized streams

Due to arid conditions, population growth, and anthropogenic impacts from agricultural and urban development, wastewater effluent makes up an increasingly large percentage of surface water supplies promoting concerns about the potential ecological and human health effects associated with the organic quality of surface waters receiving treated wastewater discharge. Anthropogenic inputs alter the quality and quantity of organic carbon and also affect the ability of aquatic ecosystems to retain or transform carbon and other nutrients. In this paper, we use pyrolysis-GC/MS (Py-GC/MS) as a tool to examine whether the dissolved organic carbon (DOC) in suburban streams influenced by anthropogenic inputs displays an organic signature that is structurally different from natural organic material (NOM). Py-GC/MS was not only able to differentiate among stream sites that received discharge from upstream wastewater treatment plants and those that did not, but also distinguished stream sites influenced significantly by storm water. Distinct organic signatures were evident in stream waters with upstream wastewater treatment plant discharges regardless of the distance from effluent discharge, indicative of the persistent nature of effluent-derived organic material (EfOM). The pyrolysis fragments of 3-methyl-pyridine, 2-methyl-pyridine, pyrrole, and acetamide were identified as indicators of EfOM, supporting previous research that has suggested that protein and aminosugar derivitives are possible wastewater markers. Furthermore, pyrolysis fragments associated with soil polycarboxylic acids correlated highly with stream sites having the least anthropogenic influences.

Algal Exudates and Stream Organic Matter Influence the Structure and Function of Denitrifying Bacterial Communities

Within aquatic ecosystems, periphytic biofilms can be hot spots of denitrification, and previous work has suggested that algal taxa within periphyton can influence the species composition and activity of resident denitrifying bacteria. This study tested the hypothesis that algal species composition within biofilms influences the structure and function of associated denitrifying bacterial communities through the composition of organic exudates. A mixed population of bacteria was incubated with organic carbon isolated from one of seven algal species or from one of two streams that differed in anthropogenic inputs. Pyrolysis-gas chromatography-mass spectrometry (Py-GC/MS) revealed differences in the organic composition of algal exudates and stream waters, which, in turn, selected for distinct bacterial communities. Organic carbon source had a significant effect on potential denitrification rates (DNP) of the communities, with organics isolated from a stream with high anthropogenic inputs resulting in a bacterial community with the highest DNP. There was no correlation between DNP and numbers of denitrifiers (based on nirS copy numbers), but there was a strong relationship between the species composition of denitrifier communities (as indicated by tag pyrosequencing of nosZ genes) and DNP. Specifically, the relative abundance of Pseudomonas stutzeri-like nosZ sequences across treatments correlated significantly with DNP, and bacterial communities incubated with organic carbon from the stream with high anthropogenic inputs had the highest relative abundance of P. stutzeri-like nosZ sequences. These results demonstrate a significant relationship between bacterial community composition and function and provide evidence of the potential impacts of anthropogenic inputs on the structure and function of stream microbial communities.

Development of associations between microalgae and denitrifying bacteria in streams of contrasting anthropogenic influence

We compared the development of microalgal and bacterial-denitrifier communities within biofilms over 28 days in a restored-prairie stream (RP) and a stream receiving treated wastewater effluent (DER). Inorganic nutrient concentrations were an order of magnitude greater in DER, and stream waters differed in the quality of dissolved organics (characterized via pyrolysis-GC/MS). Biofilm biomass and the densities of algae and bacteria increased over time in both systems; however, algal and denitrifier community composition and the patterns of development differed between systems. Specifically, algal and denitrifier taxonomic composition stabilized more quickly in DER than RP, whereas the rates of algal and denitrifier succession were more closely coupled in RP than DER. We hypothesize that, under unenriched conditions, successional changes in algal assemblages influence bacterial denitrifiers due to their dependence on algal exudates, while under enriched conditions, this relationship is decoupled. Between-system differences in organic signatures supported this, as RP biofilms contained more labile, aliphatic compounds than DER. In addition, potential denitrification rates (DNP) were negatively correlated with the percentage of aromatic compounds within the biofilm organic signatures, suggesting a significant relationship between algal exudate composition and denitrification. These results are significant because anthropogenic factors that affect biofilm community composition may alter their capacity to perform critical ecosystem services.

Diatom viability in insect fecal material: comparison between two species, Achnanthidium lanceolatum and Synedra ulna

We examined differences in digestibility and viability following gut passage through water penny larvae (Psephenus herricki) of Synedra ulna and Achnanthidium lanceoloatum, two common diatom taxa that differ in growth habit and autecological characteristics. Prior to the experiment, diatoms were cultured in Chu-10 media in petri plates to establish a monospecific biofilm to offer grazers. After collection, insects were left to clear their guts over night, allowed to graze for 3 hours on diatom biofilms, and then placed in vials over 1-mm mesh to defecate. Samples from source material and from insect feces were mounted in syrup media and the ratio of chloroplast-containing to empty diatom frustules was microscopically assessed. In addition, subsamples from source material and feces were sprayed onto agar plates prepared with Chu-10 and individual cells were mapped and tracked for 5 days to quantify reproduction. Cells of both S. ulna and A. lanceolatum taken from source material formed colonies on agar. Achnanthidium lanceolatum cells from insect feces also formed colonies, but with lower densities than those from source material. In contrast, none of the S. ulna cells tracked from fecal cultures formed colonies, and the percentage of S. ulna cells that were dead was significantly greater in feces relative to source material. Dead cell percentages of A. lanceolatum were also higher in feces relative to source material, but to a lesser degree than observed for S. ulna. These findings have potential implications for linking patterns of energy transfer in stream ecosystems and the structure and dynamics of benthic microalgal communities.

Chronic addition of a common engineered nanomaterial alters biomass, activity and composition of stream biofilm communities

Nano-TiO2 is an engineered nanomaterial that is found in a wide range of commercial products. Production of nano-TiO2 has increased rapidly over the last several decades, raising concerns about release of this material into the environment. Domestic wastewater is one route through which this material enters the environment, resulting in chronic inputs of low concentrations of nano-TiO2 to surface waters. The goal of this study was to assess impacts of chronic addition of an environmentally realistic concentration of nano-TiO2 on microbial biofilms, which play a foundational role in lotic ecosystems. Microbial consortia were collected from natural benthic habitats, cultivated in stream mesocosms, and exposed to daily additions of commercial nano-TiO2 pigment at a concentration that has been measured in treated wastewater effluent (30 μg L−1). After 22 weeks treated streams showed significantly lower amounts of biofilm (biofilm mass, bacterial and algal cell densities), lower biofilm metabolic activity (respiration and photosynthesis) and altered algal and bacterial community composition (the latter based on high-throughput sequencing of 16S rRNA genes) as compared to control streams. Specific changes in bacterial communities resulting from nano-TiO2 additions included increases in relative abundance of taxa linked to resistance to reactive oxygen species (Novosphingobium and Achromobacter) and decreases in relative abundance of taxa linked to nitrogen fixation (Pleurocapsa and Azoarcus) and denitrification (Paracoccus). These data provide one of the first demonstrations that low-level, chronic release of nano-TiO2 to streams can have significant, negative effects on structure and function of benthic communities and the critical ecosystem services they provide.

Comparing Acute Effects of a Nano-TiO2 Pigment on Cosmopolitan Freshwater Phototrophic Microbes Using High-Throughput Screening

Production of titanium-dioxide nanomaterials (nano-TiO2) is increasing, leading to potential risks associated with unintended release of these materials into aquatic ecosystems. We investigated the acute effects of nano-TiO2 on metabolic activity and viability of algae and cyanobacteria using high-throughput screening. The responses of three diatoms (Surirella angusta, Cocconeis placentula, Achnanthidium lanceolatum), one green alga (Scenedesmus quadricauda), and three cyanobacteria (Microcystis aeruginosa, Gloeocapsa sp., Synechococcus cedrorum) to short-term exposure (15 to 60 min) to a common nano-TiO2 pigment (PW6; average crystallite size 81.5 nm) with simulated solar illumination were assessed. Five concentrations of nano-TiO2 (0.5, 2.5, 5, 10, and 25 mg L-1) were tested and a fluorescent reporter (fluorescein diacetate) was used to assess metabolic activity. Algae were sensitive to nano-TiO2, with all showing decreased metabolic activity after 30-min exposure to the lowest tested concentration. Microscopic observation of algae revealed increased abundance of dead cells with nano-TiO2 exposure. Cyanobacteria were less sensitive to nano-TiO2 than algae, with Gloeocapsa showing no significant decrease in activity with nano-TiO2 exposure and Synechococcus showing an increase in activity. These results suggest that nanomaterial contamination has the potential to alter the distribution of phototrophic microbial taxa within freshwater ecosystems. The higher resistance of cyanobacteria could have significant implications as cyanobacteria represent a less nutritious food source for higher trophic levels and some cyanobacteria can produce toxins and contribute to harmful algal blooms.

Heterogeneity in algal—grazer associations in a small montane spring

Reciprocal relationships between benrhic algae and their grazers are well documenred at relatively small spatial scales. For example, macroinvertebrates vary in grazing efficiency and algae differ in susceptibility to ingestion based on growth form or on their position within vertically stratified biofilms (see STEINMAN 1996). Experimenrs have illustrated that the effects of grazing vary with grazer densiry (COLLETTI et al. 1987, H!LL & .KNIGHT 1988), that exclusion of some groups of herbivores can dramatically alter algal assemblages (McAULIFFE 1984, PüWER et al. 1988) and that variation in nutrienrs, light and currenr can modifY these effects, often interactively (RosEMOND 1993, PoFF & WARD 1995, WELLNITZ et al. 1996). Algal biomass and species composition may, in turn, strongly influence macroinvertebrate distribution and behavior (KüHLER 1985, RrcHARDS & MINSHALL 1988). Fewer studies have examined how algallgrazer relationships documented at small scales, with one to two types of grazers, are manifested at larger spatial scales in natural streams, where resource supply and grazing pressure can vary spatially and with season, and grazer assemblages are often diverse. Here, we examine whether discernable associations between macroinvertebrates and microalgae were apparent when measured at a broad spatial scale in a grazer-rich, first-order monrane stream. W e also wished to determine the colonization time needed for biotic characteristics of communities developing on previously sarnpled substrata to converge with those on permanently inundated cobbles. This information was necessary because we anticipated that baseline sampling in our study stream would eventually deplete the stock of cobbles needed to monitor seasonal and inter-annual variation in epilithon. We used macroinvertebrate and algal data collected from two sets of stream cobbles, one not previously sampled and the second consisting of cobbles previously collected from our study reaches, denuded of biota and reintroduced. Because we sampled from two proximate, physicochemically similar stream reaches, we also assessed spatial variation in algal/ macroinvertebrate associations.