Feasibility of Fe Autoradiography as Performed on N(2)-Fixing Anabaena spp. Populations and Associated Bacteria.

55Fe emits low-energy X rays and Auger electrons by electron capture decay. Auger electrons are useful for autoradiographic examination of 55Fe incorporation among microbial communities. Attainable resolution, in terms of silver grain deposition, is excellent and comparable to 3H. Two known Fe-demanding processes, photosynthetic CO2 fixation and N2 fixation, were examined by autoradiography of Anabaena populations. During photosynthetically active (illuminated) N2-fixing periods, biological incorporation of 55FeCl3 by vegetative cells and heterocysts was evident. When N2 fixation was suppressed by NH4+ additions, heterocysts revealed no incorporation of 55Fe. Conversely, when N2-fixing Anabaena filaments were placed in darkness, 55Fe incorporation decreased in vegetative cells, whereas heterocysts showed sustained rates of 55Fe incorporation. Bacteria actively incorporated 55Fe under both light and dark conditions. The chelated (by Na2-ethylenediaminetetraacetate) form of 55FeCl3 was more readily incorporated than the nonchelated form. Furthermore, abiotic adsorption of 55Fe to filters and nonliving particles proved lower when chelated 55Fe was used in experiments. 55Fe autoradiography is useful for observing the fate and cellular distribution of various forms of Fe among aquatic microbial communities

Controlling cyanobacterial harmful blooms in freshwater ecosystems

Cyanobacteria's long evolutionary history has enabled them to adapt to geochemical and climatic changes, and more recent human and climatic modifications of aquatic ecosystems, including nutrient over-enrichment, hydrologic modifications, and global warming. Harmful (toxic, hypoxia-generating, food web altering) cyanobacterial bloom (CyanoHAB) genera are controlled by the synergistic effects of nutrient (nitrogen and phosphorus) supplies, light, temperature, water residence/flushing times, and biotic interactions. Accordingly, mitigation strategies are focused on manipulating these dynamic factors. Strategies based on physical, chemical (algaecide) and biological manipulations can be effective in reducing CyanoHABs. However, these strategies should invariably be accompanied by nutrient (both nitrogen and phosphorus in most cases) input reductions to ensure long-term success and sustainability. While the applicability and feasibility of various controls and management approaches is focused on freshwater ecosystems, they will also be applicable to estuarine and coastal ecosystems. In order to ensure long-term control of CyanoHABs, these strategies should be adaptive to climatic variability and change, because nutrient-CyanoHAB thresholds will likely be altered in a climatically more-extreme world

Contemporaneous nitrogen fixation and denitrification in intertidal microbial mats: rapid response to runoff events

We examined the contemporaneous responses of N2 fixation and denitrification to inorganic nitrogen-enriched runoff in 2 intertidal microbial mats in Tomales Bay (California, USA). Prior to runoff, N2 fixation rates averaged 3 mmol N m-2 d-1. Denitrification rates were lower, corresponding to 0 to 25 % of N2 fixation rates. After the initiation of runoff, N2 fixation rates decreased, approaching zero at both sites. In contrast, denitrification rates increased by an order of magnitude. We developed a simple model to examine the magnitude of N removal from creek water by mat denitrification before, during and after runoff. Model results show that during peak runoff, N removal was limited by the residence time of creek water in the intertidal region. As runoff volumes decreased and residence times increased, N removal became limited by the supply of organic matter. Our results illustrated the rapid metabolic adaptation of microbial mats to altered N fluxes. Changes occurring in the mat N cycling after the initiation of runoff led to 2 fundamental ecological changes: (1) mats became sinks for rather than sources of fixed N and (2) denitrification became limited by the availability of organic carbon rather than nitrate

Eukaryotic phytoplankton community spatiotemporal dynamics as identified through gene expression within a eutrophic estuary

Over the span of a year, we investigated the interactions between biotic and abiotic factors within the eutrophic Neuse River Estuary (NRE). Through metatranscriptomic sequencing in combination with water quality measurements, we show that there are different metabolic strategies deployed along the NRE. In the upper estuary, taxonomically resolved phytoplankton groups express more transcripts of genes for synthesis of cellular components and carbon metabolism whereas in the lower estuary, transcripts allocated to nutrient metabolism and transport were more highly expressed. Metabolisms for polysaccharide synthesis and transportation were elevated in the lower estuary and could be reflective of unbalanced growth and/or interactions with their surrounding microbial consortia. Our results indicate phytoplankton have high metabolic activity, suggestive of increased growth rates in the upper estuary and display patterns reflective of nutrient limitation in the lower estuary. Among all the environmental parameters varying along the NRE, nitrogen availability is found to be the main driving factor for the observed spatial divergence

Phytoplankton composition in a eutrophic estuary: Comparison of multiple taxonomic approaches and influence of environmental factors

To assess the comparability between taxonomic identification methods for phytoplankton, multiple approaches were used to characterize phytoplankton community composition within the Neuse River Estuary (NRE), North Carolina. Small subunit 18S rRNA gene sequencing and accessory pigment analysis displayed similar trends, indicating chlorophytes were the dominant microalgal group during most of the year, whereas results from microscopic cell counts, biovolume analysis and metatranscriptomics suggested diatom and dinoflagellate-dominated communities. Spatial environmental gradients drove variation in taxonomic composition due to preferences for specific environmental conditions among different microalgal groups. Cryptophytes were a greater proportion of the phytoplankton community within high nutrient, fresher environments whereas diatoms and dinoflagellates dominated higher salinity sections of the estuary. This study provides a detailed examination of phytoplankton communities associated with environmental gradients present in the NRE. The high level of taxonomic resolution offered by DNA sequencing (i.e., species to sub-species level) provides a better understanding of population dynamics at the base of estuarine food webs

Regulation of estuarine primary production by watershed rainfall and river flow

Enhanced phytoplankton production and algal blooms, symptoms of eutrophication, are frequently caused by elevated nutrient loading, usually as nitrogen, to coastal waters. This nitrogen is derived primarily from anthropogenic sources (urban, industrial, and agricultural) but is delivered to coastal waters through meteorological and hydrological means. We utilized a 4 yr monthly data set to investigate the effect of these upstream physical forces upon primary productivity of the Neuse River Estuary (North Carolina, USA), a large temperate coastal plain estuary. Our results indicate that the magnitude of estuarine primary production and the periodicity of algal blooms can be directly related to variations in upper watershed rainfall and its subsequent regulation of downstream river flow. Future changes in precipitation patterns for coastal regions may thus lead to substantial alterations in coastal primary productivity rates and patterns

The role of nutrient loading and eutrophication in estuarine ecology.

Eutrophication is a process that can be defined as an increase in the rate of supply of organic matter (OM) to an ecosystem. We provide a general overview of the major features driving estuarine eutrophication and outline some of the consequences of that process. The main chemical constituent of OM is carbon (C), and therefore rates of eutrophication are expressed in units of C per area per unit time. OM occurs in both particulate and dissolved forms. Allochthonous OM originates outside the estuary, whereas autochthonous OM is generated within the system, mostly by primary producers or by benthic regeneration of OM. The supply rates of limiting nutrients regulate phytoplankton productivity that contributes to inputs of autochthonous OM. The trophic status of an estuary is often based on eutrophication rates and can be categorized as oligotrophic (<100 g C m(-2) y(-1), mesotrophic (100-300 g C m(-2) y(-1), eutrophic (300-500 g C m(-2) y(-1), or hypertrophic (>500 g C m(-2) y(-1). Ecosystem responses to eutrophication depend on both export rates (flushing, microbially mediated losses through respiration, and denitrification) and recycling/regeneration rates within the estuary. The mitigation of the effects of eutrophication involves the regulation of inorganic nutrient (primarily N and P) inputs into receiving waters. Appropriately scaled and parameterized nutrient and hydrologic controls are the only realistic options for controlling phytoplankton blooms, algal toxicity, and other symptoms of eutrophication in estuarine ecosystems