J. G. Field

Bio: J. G. Field is an academic researcher from University of Cape Town. The author has contributed to research in topics: Plankton & Bacterivore. The author has an hindex of 2, co-authored 2 publications receiving 4775 citations.

The Ecological Role of Water-Column Microbes in the Sea*

Abstract: Recently developed techniques for estimating bacterial biomass and productivity indicate that bacterial biomass in the sea is related to phytoplankton concentration and that bacteria utilise 10 to 50 % of carbon fixed by photosynthesis. Evidence is presented to suggest that numbers of free bacteria are controlled by nanoplankton~c heterotrophic flagellates which are ubiquitous in the marine water column. The flagellates in turn are preyed upon by microzooplankton. Heterotrophic flagellates and microzooplankton cover the same size range as the phytoplankton, thus providing the means for returning some energy from the 'microbial loop' to the conventional planktonic food chain.

The Role of Free Bacteria and Bactivory

Abstract: It is now recognized that bacteria and microheterotrophs play an important part in marine ecosystems (see Hobbie et al., 1972; Sieburth et al., 1977; Sieburth, 1979, 1983; Sorokin, 1979, 1981). This chapter is concerned with bacteria that live freely in the water column or on sand grains, as opposed to those on detritus particles, because detritus and detritivory are covered in another chapter of this volume. For the purposes of this chapter, bactivory is simply defined as feeding on such free bacteria.

Biogeochemical controls and feedbacks on ocean primary production

Abstract: Changes in oceanic primary production, linked to changes in the network of global biogeochemical cycles, have profoundly influenced the geochemistry of Earth for over 3 billion years. In the contemporary ocean, photosynthetic carbon fixation by marine phytoplankton leads to formation of approximately 45 gigatons of organic carbon per annum, of which 16 gigatons are exported to the ocean interior. Changes in the magnitude of total and export production can strongly influence atmospheric CO2 levels (and hence climate) on geological time scales, as well as set upper bounds for sustainable fisheries harvest. The two fluxes are critically dependent on geophysical processes that determine mixed-layer depth, nutrient fluxes to and within the ocean, and food-web structure. Because the average turnover time of phytoplankton carbon in the ocean is on the order of a week or less, total and export production are extremely sensitive to external forcing and consequently are seldom in steady state. Elucidating the biogeochemical controls and feedbacks on primary production is essential to understanding how oceanic biota responded to and affected natural climatic variability in the geological past, and will respond to anthropogenically influenced changes in coming decades. One of the most crucial feedbacks results from changes in radiative forcing on the hydrological cycle, which influences the aeolian iron flux and, in turn, affects nitrogen fixation and primary production in the oceans.

Marine viruses and their biogeochemical and ecological effects

Abstract: Viruses are the most common biological agents in the sea, typically numbering ten billion per litre. They probably infect all organisms, can undergo rapid decay and replenishment, and influence many biogeochemical and ecological processes, including nutrient cycling, system respiration, particle size-distributions and sinking rates, bacterial and algal biodiversity and species distributions, algal bloom control, dimethyl sulphide formation and genetic transfer. Newly developed fluorescence and molecular techniques leave the field poised to make significant advances towards evaluating and quantifying such effects.

Virioplankton: viruses in aquatic ecosystems

Abstract: The discovery that viruses may be the most abundant organisms in natural waters, surpassing the number of bacteria by an order of magnitude, has inspired a resurgence of interest in viruses in the aquatic environment. Surprisingly little was known of the interaction of viruses and their hosts in nature. In the decade since the reports of extraordinarily large virus populations were published, enumeration of viruses in aquatic environments has demonstrated that the virioplankton are dynamic components of the plankton, changing dramatically in number with geographical location and season. The evidence to date suggests that virioplankton communities are composed principally of bacteriophages and, to a lesser extent, eukaryotic algal viruses. The influence of viral infection and lysis on bacterial and phytoplankton host communities was measurable after new methods were developed and prior knowledge of bacteriophage biology was incorporated into concepts of parasite and host community interactions. The new methods have yielded data showing that viral infection can have a significant impact on bacteria and unicellular algae populations and supporting the hypothesis that viruses play a significant role in microbial food webs. Besides predation limiting bacteria and phytoplankton populations, the specific nature of virus-host interaction raises the intriguing possibility that viral infection influences the structure and diversity of aquatic microbial communities. Novel applications of molecular genetic techniques have provided good evidence that viral infection can significantly influence the composition and diversity of aquatic microbial communities.

The Ecology of Phytoplankton

Abstract: Communities of microscopic plant life, or phytoplankton, dominate the Earth's aquatic ecosystems. This important new book by Colin Reynolds covers the adaptations, physiology and population dynamics of phytoplankton communities in lakes and rivers and oceans. It provides basic information on composition, morphology and physiology of the main phyletic groups represented in marine and freshwater systems and in addition reviews recent advances in community ecology, developing an appreciation of assembly processes, co-existence and competition, disturbance and diversity. Although focussed on one group of organisms, the book develops many concepts relevant to ecology in the broadest sense, and as such will appeal to graduate students and researchers in ecology, limnology and oceanography.

Protein content and protein synthesis rates of planktonic marine bacteria

Abstract: Bacterial carbon production is an important parameter in understanding the flows of carbon and energy in aquatic ecosystems, but has been difficult to measure. Present methods are based on measuring the rate of cell production, and thus require a knowledge of cellular carbon content of the growing bacteria to convert cell production into carbon production. We have examined the possibility that protein synthesis rate of pelagic bacteria might serve as the basis for directly estimating bacterial carbon production. We measured bacterial protein content and protein production of pelagic bacteria. Bacterial protein content was measured as amino acids by high performance liquid chromatography of cell hydrolysates of bacterial assemblages of mean diameters from 0.026 to 0.4 km. Cellular protein:volume (w/v) in the largest bacteria was 15.2 '10 (similar to cultured Escherichia coli] but increased with decreasing cell size to 46.5 % in 0.026 pm bacteria. Protein per bacterium was correlated with cell volume by the power function y = 8 8 . ~ 2 ~ ' (r2 = 0.67; p C 0.01; n = 25) . An inventory of major bacterial macromolecular pools revealed that cell protein:dry weight and cell protein:carbon were essentially constant (63 % and 54 %. respectively) for the entire cell size range although cell protein:volume increased with decreasing cell size. Thus, the smaller cells in the size range were rich in carbon and dry weight and poor in water compared with larger cells. We established the experimental conditions for estimating protein synthesis on the basis of 3H leucine incorporation by bacteria, and determined the necessary parameters (including the intracellular isotope dilution by HPLC) for converting 3~ leucine incorporation into protein synthesis rate. In samples from Scripps Institution of Oceanography pier the intracellular isotope dilution was only 2-fold. In a field study in Southern California Bight bacterial protein production and %I-thymidine incorporation methods yielded comparable rates of bacterial production. Bacterial protein production method was an order of magnitude more sensitive and yielded bacterial carbon production directly without the need to know the cell size of the part of the assemblage in growth state.