Evelyn B. Sherr

Significance of predation by protists in aquatic microbial food webs.

Predation in aquatic microbial food webs is dominated by phagotrophic protists, yet these microorganisms are still understudied compared to bacteria and phytoplankton. In pelagic ecosystems, predaceous protists are ubiquitous, range in size from 2 μm flagellates to >100 μm ciliates and dinoflagellates, and exhibit a wide array of feeding strategies. Their trophic states run the gamut from strictly phagotrophic, to mixotrophic: partly autotrophic and partly phagotrophic, to primarily autotrophic but capable of phagotrophy. Protists are a major source of mortality for both heterotrophic and autotrophic bacteria. They compete with herbivorous meso- and macro-zooplankton for all size classes of phytoplankton. Protist grazing may affect the rate of organic sinking flux from the euphotic zone. Protist excretions are an important source of remineralized nutrients, and of colloidal and dissolved trace metals such as iron, in aquatic systems. Work on predation by protists is being facilitated by methodological advances, e.g., molecular genetic analysis of protistan diversity and application of flow cytometry to study population growth and feeding rates. Examples of new research areas are studies of impact of protistan predation on the community structure of prey assemblages and of chemical communication between predator and prey in microbial food webs.

High bacterial production, uptake and concentrations of dissolved organic matter in the Central Arctic Ocean

Recent studies have indicated that biological activity is higher in the central Arctic Ocean than once thought. To assess the contribution of heterotrophic processes, we examined bacterial production and the uptake of specific components of the dissolved organic matter (DOM) pool during a trans-Arctic expedition. Bacterial production generally covaried with phytoplankton production and bacterial numbers; the highest values of all three parameters were in the more open waters of the Chukchi Sea and they were generally lower at the other stations which were covered by ice. Although bacterial production correlated with phytoplankton production, the ratio of bacterial to phytoplankton production (BP:PP) varied substantially. It was lowest (BP:PP<0.1) at stations with some open water and highest at ice-covered stations, sometimes exceeding 1, depending on how bacterial production was measured. These high BP:PP ratios imply that bacterial respiration and DOM fluxes are large. This bacterial activity could be accounted for by the uptake of dissolved free amino acids (DFAA) and glucose. Uptake of these DOM components was high because of rapid turnover and high concentrations, especially of DFAA. Concentrations of dissolved combined neutral carbohydrates (e.g. polysaccharides) were often also high compared to other environments. These data indicate that heterotrophic bacteria and the rest of the microbial loop are large and active components of the biological community in the Arctic Ocean in spite of perennially low temperatures.

Heterotrophic protists in the Central Arctic Ocean

Distribution, general composition and activity of heterotrophic protists, as well as the distribution of bacteria, were assessed in the upper water column of the central Arctic Ocean during the Arctic Ocean Section, July–September 1994. Bacterial biomass varied from 5 to > 25 mg C 1−1, with the highest values occurring in the Chukchi Sea. Protist biomass was highest (5–107 mg Cl−1) in the upper 50 m of the water column. Higher integrated (0–50 m) protist biomass values (average 910±250 mg C m−2, range 580–1370 mg C m−2) were found in the Chukchi Sea, compared to the central Arctic Ocean (average 480±320 mg C m−2, range 120–1120 mg C m−2). Heterotrophic dinoflagellates were more abundant than ciliates in the >20 μm size class at all stations. In the central Arctic Ocean, the <20 μm size class was numerically composed of dinoflagellates (16%), choanoflagellates (4%) and other flagellates (80%). Choanoflagellates were slightly more abundant in the Chukchi Sea (9% of cell numbers), but were a large component of the flagellate assemblage (55% of cell numbers) at only one station, in the Nansen Basin. Bacterivory estimated via uptake of added fluorescently labeled bacteria ranged from 1·2 × 103 to 46 × 103 bacteria ml−1 day−1; the highest rate was found at the station with a high choanoflagellate abundance. Observation of food vacuole contents showed that all size classes and taxonomic types of protists ingested phytoplankton. Choanoflagellates, and monads as small as 1·5 μm in size, ingested picoplanktonic eukaryotic phytoplankton, which were abundant (103−104 cells ml−1) in the upper 50 m. Larger protists ingested cryptomonads and diatoms, as well as pico-autotrophs. Clearance rates of 10–100 μm sized ciliates and dinoflagellates, based on the uptake of 1–5 μm fluorescent microspheres, were similar to rates reported for herbivorous protists in temperate waters. In terms of ecosystem carbon flow, we infer that phagotrophic protists in the Arctic Ocean are important consumers of phytoplankton and bacteria, and may represent a significant food resource for zooplankton.

Activity and Phylogenetic Diversity of Bacterial Cells with High and Low Nucleic Acid Content and Electron Transport System Activity in an Upwelling Ecosystem

ABSTRACT We evaluated whether bacteria with higher cell-specific nucleic acid content (HNA) or an active electron transport system, i.e., positive for reduction of 5-cyano-2,3-ditolyl tetrazolium chloride (CTC), were responsible for the bulk of bacterioplankton metabolic activity. We also examined whether the phylogenetic diversity of HNA and CTC-positive cells differed from the diversity of Bacteria with low nucleic acid content (LNA). Bacterial assemblages were sampled both in eutrophic shelf waters and in mesotrophic offshore waters in the Oregon coastal upwelling region. Cytometrically sorted HNA, LNA, and CTC-positive cells were assayed for their cell-specific [3H]leucine incorporation rates. Phylogenetic diversity in sorted non-radioactively labeled samples was assayed using denaturing gradient gel electrophoresis (DGGE) of PCR-amplified 16S rRNA genes. Cell-specific rates of leucine incorporation of HNA and CTC-positive cells were on average only slightly greater than the cell-specific rates of LNA cells. HNA cells accounted for most bacterioplankton substrate incorporation due to high abundances, while the low abundances of CTC-positive cells resulted in only a small contribution by these cells to total bacterial activity. The proportion of the total bacterial leucine incorporation attributable to LNA cells was higher in offshore regions than in shelf waters. Sequence data obtained from DGGE bands showed broadly similar phylogenetic diversity across HNA, LNA, and CTC-positive cells, with between-sample and between-region variability in the distribution of phylotypes. Our results suggest that LNA bacteria are not substantially different from HNA bacteria in either cell-specific rates of substrate incorporation or phylogenetic composition and that they can be significant contributors to bacterial metabolism in the sea.

Trophic interactions within pelagic microbial communities: Indications of feedback regulation of carbon flow

Future considerations of carbon-energy flows within pelagic food webs should include internal, biotic feedback controls, in addition to abiotic forcing functions, in the regulation of these flows. Over the past two decades, research on microbial communities of pelagic ecosystems has yielded data suggestive of cybernetic-like regulation operating within these communities. As presently conceived, phagotrophic protozoa have a pivotal role in such regulation as a consequence of their rapid growth, grazing, and nutrient regenerative capabilities. Feedback controls within microbial food webs may have significant effects on distal portions of pelagic ecosystems, including the fate of organic detritus and metazoan production.

Subtidal food webs in a georgia estuary: δ13C analysis

Stable carbon isotope compositions were analyzed for estuarine fauna collected in tidal creeks draining salt marshes dominated by either Spartina alterniflora Loisel. (δ13C of −12.5%.) or Juncus roemerianus Scheele (δ13C of −26%.). Mean δ13C values of subtidal invertebrates and fishes were −17.8%. and −17.5%. in the Spartina creek and −20.1%. and −20.0%. respectively in the two Juncus creeks. The intermediate δ13C values of the fauna implied an alternate carbon input to the food web, most probably benthic algae (δ13C of −16 to −18%.) and phytoplankton (δ13C of −20 to −22%.). In the Spartina marsh creek, animals with δ13C values less negative than −17%., e.g. mummichogs, mullet, and mud crabs, were most closely linked to the Spartina detritus/benthic algal food web, while species with δ13C values more negative than −19%., e.g. brown shrimp, menhaden, and oysters, were apparently more dependent on the phytoplankton food web. The data suggested that 1. (1) marsh plant carbon is a component of subtidal food webs, 2. (2) phytoplankton carbon is a more important food source for subtidal animals than for intertidal marsh fauna, 3. (3) the subtidal food web is structured such that individual invertebrate and fish species show varying degrees of dependence on the detritus/benthic algae-based food web of the marsh vs. the phytoplankton-based food web of the tidal creek.

Community respiration/production and bacterial activity in the upper water column of the central Arctic Ocean

Community metabolism (respiration and production) and bacterial activity were assessed in the upper water column of the central Arctic Ocean during the SHEBA/JOIS ice camp experiment, October 1997–September 1998. In the upper 50 m, decrease in integrated dissolved oxygen (DO) stocks over a period of 124 d in mid-winter suggested a respiration rate of B3.3 nM O2 h � 1 and a carbon demand of B4.5 gC m � 2 . Increase in 0–50 m integrated stocks of DO during summer implied a net community production of B20 gC m � 2 . Community respiration rates were directly measured via rate of decrease in DO in whole seawater during 72-h dark incubation experiments. Incubation-based respiration rates were on average 3-fold lower during winter (11.0710.6 nM O2 h � 1 ) compared to summer (35.3724.8 nM O2 h � 1 ). Bacterial heterotrophic activity responded strongly, without noticeable lag, to phytoplankton growth. Rate of leucine incorporation by bacteria (a proxy for protein synthesis and cell growth) increased B10-fold, and the cell-specific rate of leucine incorporation B5-fold, from winter to summer. Rates of production of bacterial biomass in the upper 50 m were, however, low compared to other oceanic regions, averaging 0.5270.47 ngC l � 1 h � 1 during winter and 5.173.1 ngC l � 1 h � 1 during summer. Total carbon demand based on respiration experiments averaged 2.472.3 mgC m � 3 d � 1 in winter and 7.875.5 mgC m � 3 d � 1 in summer. Estimated bacterial carbon demand based on bacterial productivity and an assumed 10% gross growth efficiency was much lower, averaging about 0.1270.12 mgC m � 3 d � 1 in winter and 1.370.7 mgC m � 3 d � 1 in summer. Our estimates of bacterial activity during summer were an order of magnitude less than rates reported from a summer 1994 study in the central Arctic Ocean, implying significant inter-annual variability of microbial processes in this region. r 2003 Elsevier Science Ltd. All rights reserved.

Enumeration of Heterotrophic microprotozoa by epifluorescence microscopy

The fluorochrome fluorescein isothiocyanate (FITC) binds to the amino groups of protein, and when used with epifluorescence microscopy provides a clear image of formalin preserved nanoplankton cells filtered from natural waters. The red autofluorescence of chlorophyll is not masked by the green fluorescence of FITC, so that autotrophic and heterotrophic plankton can be distinguished. The FITC direct count method can be used to determine in situ population size and growth rates of heterotrophic microprotozoa in aquatic systems. In Georgia estuarine and nearshore waters, we found between 0·3 × 103 and 4 × 103 colorless microprotozoa ml−1; most of the cells were less than 5 μm diameter. Rates of population increase measured at two estuarine sites corresponded to biomass production rates of 0·3 and 0·5 mg protozoan C m−3 h−1.

Planktonic microbes: Tiny cells at the base of the ocean's food webs.

Phytoplankton in the size range 5-100 μm was originally thought to be the primary source of food for most life in the sea. However, smaller planktonic microbes, down to 0.2 μm in size, have been the focus of intensive investigation by marine scientists during the past two decades. These microbes attain high abundance and biomass in all parts of the world ocean. They include non-photosynthesizing bacteria, at least two types of photosynthesizing prokaryotes, and eukaryotic phototrophs. The new information has resulted in a greatly revised concept of how pelagic ecosystems in both marine and freshwater environments function. The original idea of a basically linear food chain from diatoms to copepods to fish has given way to an extremely complex model of trophic interactions within a microbial food web, which supports metazoan food webs via biomass production of both heterotrophic and autotrophic cells.

Trophic Behaviour and Related Community Feeding Activities of Heterotrophic Marine Protists

As a consequence of their evolutionary diversity in form and function the protista of aquatic ecosystems exhibit a great variety of complex trophodynamic interactions, which confounds attempts to produce gross estimates of their in situ feeding rates. It is our view that studies of trophic behaviour at species and taxon level and quantification of community ingestion rates, are needed to develop an understanding of how food webs operate. New behavioral observations (e.g. veil/pallium feeding in dinoflagellates, Gaines and Taylor 1984, Jacobson and Anderson 1986, or small, aloricate planktonic ciliates feeding on bacteria, Sherr and Sherr 1987) suggest possible directions for future quantitative studies.