Frank J. Jochem

Ectoenzyme kinetics in Florida Bay : Implications for bacterial carbon source and nutrient status

Ectoenzyme kinetics [alkaline phosphatase, aminopeptidase (AM), lipase, α-glucosidase, and β-glucosidase] were determined over a seasonal cycle at four stations, Little Madeira Bay (northeast), Duck Key (east), Bob Allen Key (south-central), and Whipray Basin (north-central), which represent distinct regions of Florida Bay. Spatial and temporal variations in ectoenzyme kinetics were related to biotic and abiotic drivers in order to discern potential ectoenzyme substrate sources. Generally, ectoenzyme activities were higher in the central bay and lowest in the eastern bay. One pronounced exception was AM activity, which was highest at the mangrove fringe along the northeastern bay and reflected increased contribution of organic nitrogen inputs from upstream Everglades wetlands. When ectoenzymes were normalized to bacterial abundances, these trends dissipated and highest cell-specific activities were observed consistently in the south-central region. Relationships between ectoenzyme kinetics and environmental and biological parameters were complex, but three main spatially determined differences were discernable. Ectoenzyme kinetics were controlled by phosphorus availability in the eastern bay, by organic matter availability in the south-central bay, by microbial community composition and organic matter availability in northeastern bay (wetlands transition area), and by microbial community composition in the north-central bay. These differences in enzyme kinetics further support the hypothesis of distinct microbial communities in different regions of Florida Bay and provide insight into biogeochemical cycles and the microbial food web within Florida Bay.

Effects of Hydrological Flow Regime on Sediment-water Interface and Water Column Nitrogen Dynamics in a Great Lakes Coastal Wetland (Old Woman Creek, Lake Erie)

ABSTRACT Sediment-water interface nitrogen (N) transformations and water column ammonium cycling rates were measured along a stream to lake gradient at three sites within Old Woman Creek (OWC) and one near-shore Lake Erie site during two hydrological regimes: one with open flow to the lake after a rain event (July 2003), and another with a sand barrier blocking flow (July 2004). Net N2 effluxes in OWC at all times and at the near-shore Lake Erie site in July 2003 suggest that sediments are a N sink via denitrification. Observed dissimilatory nitrate reduction to ammonium (DNRA) may counteract some of this N removal, particularly when the creek mouth is closed. Upstream, a closed creek mouth led to higher sediment oxygen demand, net N2 flux, potential DNRA, and potential denitrification rates. The lake site exhibited lower rates of these processes with the creek mouth closed except denitrification potential, which was unchanged. Denitrification in OWC appeared to drive N limitation in the lower wet-land when the sand barrier was blocking flow to the lake. Higher potential versus in situ denitrification estimates imply that water column NO3− limits and drives denitrification in OWC. Water column to sediment regeneration ratios suggest that sediment recycling may drive primary production in the OWC interior when the creek mouth is closed and new N inputs from runoff are absent, but more data are needed to confirm these apparent trends. Overall, hydrological regime in OWC appeared to have a greater impact on sediment N processes than on water column cycling.

Estuarine Microbial Food Web Patterns in a Lake Erie Coastal Wetland

Composition and distribution of planktonic protists were examined relative to microbial food web dynamics (growth, grazing, and nitrogen cycling rates) at the Old Woman Creek (OWC) National Estuarine Research Reserve during an episodic storm event in July 2003. More than 150 protistan taxa were identified based on morphology. Species richness and microbial biomass measured via microscopy and flow cytometry increased along a stream–lake (Lake Erie) transect and peaked at the confluence. Water column ammonium (NH4+) uptake (0.06 to 1.82 μM N h–1) and regeneration (0.04 to 0.55 μM N h–1) rates, measured using 15NH4+ isotope dilution, followed the same pattern. Large light/dark NH4+ uptake differences were observed in the hypereutrophic OWC interior, but not at the phosphorus-limited Lake Erie site, reflecting the microbial community structural shift from net autotrophic to net heterotrophic. Despite this shift, microbial grazers (mostly choreotrich ciliates, taxon-specific growth rates up to 2.9 d–1) controlled nanophytoplankton and bacteria at all sites by consuming 76 to 110% and 56 to 97% of their daily production, respectively, in dilution experiments. Overall, distribution patterns and dynamics of microbial communities in OWC resemble those in marine estuaries, where plankton productivity increases along the river–sea gradient and reaches its maximum at the confluence.

INTENSE GRAZING AND PREY‐DEPENDENT GROWTH OF PFIESTERIA PISCICIDA (DINOPHYCEAE)1

Grazing and growth of Pfiesteria piscicida (Pfiest) were investigated using batch and cyclostat cultures with Rhodomonas sp. (Rhod) as prey. Observed maximum growth rates (1.4 d−1) and population densities (2 × 105 cells·mL−1) corresponded to values predicted by Monod functions (1.76 d−1; 1.4 × 105 cells·mL−1). In batch cultures under a range of prey‐to‐predator ratios (0.1:1 to 180:1) and prey concentrations (1000–71,000 cells·mL−1), Rhodomonas sp. was always depleted rapidly and P. piscicida concentrations increased briefly. The rate of Rhodomonas sp. depletion and the magnitude of P. piscicida population maxima depended on the prey‐to‐predator ratio and prey concentration. Starvation resulted in cell cycle arrest at G1 and G2+M and ultimately the demise of both P. piscicida and Rhodomonas sp. populations, demonstrating the dependence of P. piscicida on the supply of appropriate prey. The depletion of Rhodomonas sp. populations could be attributed directly to grazing, because P. piscicida did not exert detectable inhibitory effects on the growth of Rhodomonas sp. but grazed intensely, with maximum grazing rates>10 Rhod·Pfiest−1·d−1 and with no apparent threshold prey abundance for grazing. The results suggest that 1) the abundance of appropriate prey may be an important factor regulating P. piscicida abundance in nature, 2) P. piscicida may control prey population, and 3) high growth and grazing potentials of P. piscicida along with cell cycle arrest may confer survival advantages.

On the seasonal occurrence of autotrophic naked nanoflagellates in Kiel Bight, western Baltic

Abstract From April to October 1986 the occurrence of autotrophic, mainly naked nanoflagellates was surveyed qualitatively and semi-quantitatively at four stations in Kiel Bight and Kiel Fjord by use of the Dilution Culture technique and light-microscopy of live preparations. In April, the flagellate community was characterized by species mostly >10μm (Cryptomonas pseudobaltica, Katodinium rotundatum, Pseudopedinella pyriforme, Heterosigma akashiwo). After a bloom of Dictyocha speculum in May, mainly smaller species occurred during summer (Dicrateria/Imantonia sp., Nephroselmis pyriformis, Ochromonas minima and others). From late August onwards Prorocentrum minimum occurred and formed a ‘red tide’ in Kiel Fjord. Based on their seasonal occurrence, temperature ranges are discussed for each species.

Short-term physiologic effects of mechanical flow sorting and the becton-dickinson cell concentrator in cultures of the marine phytoflagellata : Emiliania huxleyi and Micromonas pusilla

In contrast to large, high‐efficiency cytometers, mechanically sorting benchtop instruments provide a feasible alternative for shipboard cell sorting of oceanic microbial communities. However, sorting efficiency of these instruments is constrained by their maximum sorting rate of approximately 300 cells/s and by constant dilution of sorted samples by sheath flow. These factors often render too low sorted cell concentrations for postsorting experiments of oceanic phytoplankton populations of low natural abundance. A Cell Concentrator module has been marketed to overcome these dilution effects. Postsorting experiments also have to consider potential physiologic effects of cell sorting. Short‐term physiologic effects on phytoplankton photosynthetic rates and esterase activities by mechanical flow sorting and cell concentration and on the efficiency of the Cell Concentrator module are evaluated.