Clifford A. Ochs

Free-Living and Particle-Associated Bacterioplankton in Large Rivers of the Mississippi River Basin Demonstrate Biogeographic Patterns

ABSTRACT The different drainage basins of large rivers such as the Mississippi River represent interesting systems in which to study patterns in freshwater microbial biogeography. Spatial variability in bacterioplankton communities in six major rivers (the Upper Mississippi, Missouri, Illinois, Ohio, Tennessee, and Arkansas) of the Mississippi River Basin was characterized using Ion Torrent 16S rRNA amplicon sequencing. When all systems were combined, particle-associated (>3 μm) bacterial assemblages were found to be different from free-living bacterioplankton in terms of overall community structure, partly because of differences in the proportional abundance of sequences affiliated with major bacterial lineages (Alphaproteobacteria, Cyanobacteria, and Planctomycetes). Both particle-associated and free-living communities ordinated by river system, a pattern that was apparent even after rare sequences or those affiliated with Cyanobacteria were removed from the analyses. Ordination of samples by river system correlated with environmental characteristics of each river, such as nutrient status and turbidity. Communities in the Upper Mississippi and the Missouri and in the Ohio and the Tennessee, pairs of rivers that join each other, contained similar taxa in terms of presence-absence data but differed in the proportional abundance of major lineages. The most common sequence types detected in particle-associated communities were picocyanobacteria in the Synechococcus/Prochlorococcus/Cyanobium (Syn/Pro) clade, while free-living communities also contained a high proportion of LD12 (SAR11/Pelagibacter)-like Alphaproteobacteria. This research shows that while different tributaries of large river systems such as the Mississippi River harbor distinct bacterioplankton communities, there is also microhabitat variation such as that between free-living and particle-associated assemblages.

Effect of UV-A irradiance on lipid accumulation in Nannochloropsis oculata.

Lipids produced by microalgae can be grouped into two categories, storage lipids and structural lipids. Storage lipids are mainly triglycerides (TGs) made up of saturated fatty acids; TGs can be transesterified to produce biodiesel. Structural lipids are made of polyunsaturated fatty acids (PUFAs), which are essential nutrients for aquatic animals and humans. The objectives of this study were: (1) to determine the effect of UV‐A at different levels of exposure on total lipid accumulation in Nannochloropsis oculata and check for reciprocity and (2) to study the interactive effect of UV‐A and nutrient concentration on lipid accumulation in N. oculata. Objective 1 was accomplished by testing the effects of a range of UV‐A irradiance (I), duration of exposure (T) and UV‐A doses (I × T) on lipid production by N. oculata. If the same doses have a similar effect, irrespective of I and T, reciprocity holds. UV‐A treatments significantly increased the chlorophyll‐specific lipid concentration of N. oculata cells, and we were unable to falsify that reciprocity holds. Objective 2 was addressed by a factorial bioassay experiment with manipulated nutrient and UV‐A levels. UV‐A and decreased nutrients had a synergistic effect on chlorophyll‐specific lipid concentration of N. oculata, resulting in higher lipid:chl ratios.

Spatial and temporal variation in phytoplankton community structure in a southeastern U.S. reservoir determined by HPLC and light microscopy

Spatial and temporal variation in phytoplankton community structure within a large flood-control reservoir (Sardis Reservoir, MS, USA) was investigated in relation to variation in physicochemical properties, location within the reservoir, hydraulic residence time (HRT), nutrient concentrations, temperature, and light conditions over a 14-month period. During periods of short HRT, phytoplankton communities throughout the reservoir were homogeneous in biomass, composition, and production. With a gradual increase in HRT from spring to summer, spatially heterogeneous phytoplankton communities developed along the longitudinal axis of the reservoir. During this period of longer HRT, diatoms and chlorophytes were a larger proportion of total phytoplankton biomass at shallow and more turbid locations near the head of the reservoir, whereas cyanobacteria were a larger proportion of the community at deeper and less turbid locations closer to the outflow. Seasonal succession of the phytoplankton community was represented by high abundance of diatoms in spring, increasing biomass of cyanobacteria through summer, and a secondary bloom of diatoms in fall. Species of Cyclotella, Asterionella, Nitzschia, and Ankistrodesmus were among the first colonizers in the early growing season, closely followed by Aulacoseira, whereas species of Staurastrum and Tetraedron appeared later in the spring. Species of Synedra, Crucigenia, Selenastrum, Scenedesmus, and Merismopedia occurred throughout the sampling period. As the diatoms started to decrease during mid-spring, cryptophytes increased, prior to dominance of species of Pseudanabaena in summer. Reservoir management of HRT, in combination with spatial variation in reservoir morphology and seasonal variation in temperature and riverine nutrient inputs, creates seasonally variable yet distinct spatial patterns in phytoplankton community biomass, composition, and production.

Particle-associated and cell-free extracellular enzyme activity in relation to nutrient status of large tributaries of the Lower Mississippi River

Microbial extracellular enzyme activity is responsible for much of the carbon and nutrient cycling in freshwaters, and logically there should a relationship between the chemical properties of a system and its enzymatic profile. To evaluate this concept, we surveyed extracellular enzyme activity in five large rivers (the Upper Mississippi, Missouri, Ohio, Tennessee, and Arkansas) of the Mississippi River Basin, one of the world’s largest river systems. The rivers drain areas of different climate, physiography, and land use, and differ in their physicochemical properties. Despite differences in nutrient concentrations, there were no consistent differences in enzyme activity between the five rivers, with as much variation in activity between sites on the same river as among different rivers. Enzymatic profiles were dominated by leucine aminopeptidase, phosphatase, and β-glucosidase, and appreciable enzymatic activity was still present following the removal of particles (3-micron filtration) or cells (0.22-micron filtration). The proportion of particle- or cell- associated enzymatic activity contributing to overall activity varied between enzymes, being higher for β-glucosidase, leucine aminopeptidase, and N-acetyl-β-D-glucosaminidase than for sulfatase or cellobiohydrolase. Dissolved elemental stoichiometry suggested that bacterioplankton in all rivers were limited by C overall, with P also being more limiting than N. While regional-scale patterns in enzyme activity in large rivers may indicate anthropogenic influences, this study demonstrates that finer-scale variation, such as between sites on the same river, or between particles and free-living bacterioplankton, may be equally as important.

Observations of silver carp (Hypophthalmichthys molitrix) planktivory in a floodplain lake of the Lower Mississippi River basin.

ABSTRACT The invasive silver carp (Hypophthalmichthys molitrix) has become pervasive in much of the Mississippi River, its tributaries, and in connected lakes and wetlands. As an increasingly abundant planktivore, it competes directly for food with native fishes. Its greatest impact may be in connected backwater lakes and wetlands, which due to their high primary production serve as critical sites for feeding and growth of many fishes. To assess the impact that silver carp may have on one such system, we examined the composition of plankton samples and of alimentary tract (gut) contents of collected from an oxbow lake in Mississippi, Forest Home Chute. Through an occasional connection to the Mississippi River, Forest Home Chute was invaded by silver in winter 2005, after which the river and lake became disconnected for about two years. In the water-column, the most common types of phytoplankton were euglenoid algae, cyanobacteria, and diatoms. The vast majority of zooplankton was rotifers with densities sometimes exceeding 7,000 organisms per liter. Very high concentrations of phytoplankton in the gut, relative to in the water-column, indicate substantial consumption of phytoplankton production. In October 2006, euglenoid phytoplankters were a much greater, and cyanobacteria a much lesser, proportion of prey in the fish gut compared to their proportions in the water-column. In December, however, there was no evidence of selective consumption by the silver carp population. Some of the phytoplankters observed in the lowest portion of the gut, including pinnate diatoms and euglenoid algae, were motile, indicating they had survived transit through the 5 to 7-m long gut tract. There was no evidence of rotifer survival of gut passage. By its high consumption of plankton, possible selective planktivory, and differential digestion of consumed phytoplankton and zooplankton, the silver may be altering the food web structure of these important connected lakes.

The Population Dynamics of Freshwater Armored Dinoflagellates in a Small Lake in Mississippi

ABSTRACT We investigated the temporal and spatial changes in population density of five dinoflagellates in Boondoggle Lake, a shallow, nutrient-poor lake in northern Mississippi. Dinoflagellate density and physicochemical conditions were assessed at three depths over a period of 16 months. We identified five Peridinium species (P. deflandrei, P. volzii, P. wisconsinense, P. limbatum and P. inconspicuum) and one Peridiniopsis species (P. polonicium). With the exception of P. inconspicuurn, none of these species has previously been reported in Mississippi. P. defandrei dominated the summer bloom (90% of the total dinoflagellate population) and reached a maximum population density of 2.75 X 105 cells/L. P. wisconsinense and P. polonicium also had maximum densities in the summer. The increase of these species coincided with the onset of temperature and oxygen stratification, and their decrease coincided with lake mixing. P. volzii and P. limbatum reached maximum abundances in the spring and were less abundant than the summer species. Declines in these species were observed at the onset of stratification, with increases again after mixing.

Patterns of variation in diversity of the Mississippi river microbiome over 1,300 kilometers

We examined the downriver patterns of variation in taxonomic diversity of the Mississippi River bacterioplankton microbiome along 1,300 river kilometers, or approximately one third the total length of the river. The study section included portions of the Upper, Middle, and Lower Mississippi River, confluences with five tributaries draining distinct sub-basins, river cities, and extended stretches without major inputs to the Mississippi. The composition and proportional abundance of dominant bacterial phyla was distinct for free-living and particle-associated cells, and constant along the entire reach, except for a substantial but transient disturbance near the city of Memphis, Tennessee. At a finer scale of taxonomic resolution (operational taxonomic units, OTUs), however, there were notable patterns in downriver variation in bacterial community alpha diversity (richness within a site) and beta diversity (variation in composition among sites). There was a strong and steady increase downriver in alpha diversity of OTUs on suspended particles, suggesting an increase in particle niche heterogeneity, and/or particle colonization. Relatively large shifts in beta diversity of free-living and particle-associated communities occurred following major tributary confluences and transiently at Memphis, while in long stretches between these points diversity typically varied more gradually. We conclude that the Mississippi River possesses a bacterioplankton microbiome distinct in diversity from other large river microbiomes in the Mississippi River Basin, that at major river confluences or urban point sources its OTU diversity may shift abruptly and substantially, presumably by immigration of distinct external microbiomes, but that where environmental conditions are more stable along the downriver gradient, microbiome diversity tends to vary gradually, presumably by a process of successional change in community composition.

Spatial Patterns in Autotrophic Picoplankton Abundance in a Reservoir Examined Using Microcosm Experiments

Autotrophic picoplankton (APP) are critically important in the production of organic matter in plankton food webs, A number of obvious environmental factors, including light, temperature, nutrient concentrations, and predation, have been identified as influencing APP abundance and growth rates. However, few experiments have investigated more than one factor at a time for their relationships to APP abundance or growth rate. We conducted a series of single and multi-factor experiments to examine the relative importance of several biotic and abiotic variables, and their interactions, for their effects on APP abundance and growth rates. Experiments were conducted using APP samples collected during summer from the riverine and lacustrine zones of a southeastern U.S. reservoir. Growth rates of APP responded rapidly to nutrient addition, but the magnitude of the response was dependent on location of collection, and on the presence of grazers and other phytoplankton. Growth rates of APP from the riverine zone were limited by nitrogen, whereas lacustrine zone APP responded positively to both phosphorus and nitrogen addition. Regulation of APP population size by heterotrophic nanoflagellates appeared to be most effective when APP growth rates were relatively slow. APP cell chlorophyll concentrations varied inversely with the percentage of surface light in the mixed layer of the reservoir. We suggest that due to spatial differences in the rate and depth of vertical mixing, lacustrine zone APP receive less light on average, or light at less frequent intervals, than APP in the more turbid but shallower riverine zone. Longitudinal declines in both nutrient and light availability during summer may reduce APP growth rates and population size in the lacustrine zone compared to the more productive riverine zone of this reservoir.