Antarctic Futures: An Assessment of Climate-Driven Changes in Ecosystem Structure, Function, and Service Provisioning in the Southern Ocean.

In this article, we analyze the impacts of climate change on Antarctic marine ecosystems. Observations demonstrate large-scale changes in the physical variables and circulation of the Southern Ocean driven by warming, stratospheric ozone depletion, and a positive Southern Annular Mode. Alterations in the physical environment are driving change through all levels of Antarctic marine food webs, which differ regionally. The distributions of key species, such as Antarctic krill, are also changing. Differential responses among predators reflect differences in species ecology. The impacts of climate change on Antarctic biodiversity will likely vary for different communities and depend on species range. Coastal communities and those of sub-Antarctic islands, especially range-restricted endemic communities, will likely suffer the greatest negative consequences of climate change. Simultaneously, ecosystem services in the Southern Ocean will likely increase. Such decoupling of ecosystem services and endemic species will require consideration in the management of human activities such as fishing in Antarctic marine ecosystems.

Bacterioplankton Dynamics in Estuarine Mesocosms: Effects of Tank Shape and Size.

Mesocosms provide a powerful tool for investigating bacterial dynamics at small scales; however, even these controlled studies are not exempt from spatial influences. Differences in mesocosm shape and size may have profound effects on the enclosed community since these features may influence the behavior of the system. Studies were conducted in mesocosms of varying dimensions (narrow, deep and wide, shallow tanks) and volumes (0.1, 1, and 10 m3) in an attempt to decipher effects attributable to changes in container size and shape. Both mesocosm volume and shape affected the course of bacterial growth following containment. Bacterial abundance and production were high in both groups of 0.1 m3 tanks and in the large, wide, shallow tanks with the greatest light supply at depth. Differences in bacterial growth between differently shaped tanks (i.e., differing wall area to volume ratios) were observed among equal volume enclosures, with faster growth in the wide/shallow tanks. Light availability, phytoplankton growth, and primary production differed among tanks, and bacterial growth and production were correlated with these properties. During high nutrient conditions, mesocosm volume and shape influenced bacterial growth, possibly because of periphyton growth on walls in small tanks and elevated light levels in wide/shallow tanks. These results suggest the importance of considering container dimensions when designing and interpreting mesocosm experiments and may allow deeper understanding of the fundamental processes underlying patterns observed in the real world.

Limitation of bacterial growth by dissolved organic matter and iron in the Southern ocean.

The importance of resource limitation in controlling bacterial growth in the high-nutrient, low-chlorophyll (HNLC) region of the Southern Ocean was experimentally determined during February and March 1998. Organic- and inorganic-nutrient enrichment experiments were performed between 42 degrees S and 55 degrees S along 141 degrees E. Bacterial abundance, mean cell volume, and [(3)H]thymidine and [(3)H]leucine incorporation were measured during 4- to 5-day incubations. Bacterial biomass, production, and rates of growth all responded to organic enrichments in three of the four experiments. These results indicate that bacterial growth was constrained primarily by the availability of dissolved organic matter. Bacterial growth in the subtropical front, subantarctic zone, and subantarctic front responded most favorably to additions of dissolved free amino acids or glucose plus ammonium. Bacterial growth in these regions may be limited by input of both organic matter and reduced nitrogen. Unlike similar experimental results in other HNLC regions (subarctic and equatorial Pacific), growth stimulation of bacteria in the Southern Ocean resulted in significant biomass accumulation, apparently by stimulating bacterial growth in excess of removal processes. Bacterial growth was relatively unchanged by additions of iron alone; however, additions of glucose plus iron resulted in substantial increases in rates of bacterial growth and biomass accumulation. These results imply that bacterial growth efficiency and nitrogen utilization may be partly constrained by iron availability in the HNLC Southern Ocean.

Regulation of bacterial abundance and production by substrate supply and bacterivory: A mesocosm study.

Daily bacterial abundance and production, heterotrophic nanoflagellates (HNAN) abundance, chlorophyll, and NH4 (+) concentrations were measured in four indoor 400-liter tanks over 13 days to study the role of heterotrophic bacterioplankton in NH4 (-) cycling and to identify the succession of top-down and bottom-up processes in regulating bacterial biomass and production. Ammonium (NH4 (+)) was added to these four tanks daily whenever its concentration in tanks was < 4 µM. Tanks 3 and 4 (treatment tanks) also received 4 µM of glucose daily till the end of experiment. Lower NH4 (-) concentrations and higher bacterial specific growth rate and production observed in the treatment tanks indicated that bacteria might take up NH4+ with the addition of labile organic carbon. Bacterial biomass was controlled by substrate supply and HNAN grazing from day 7 to day 13, when phytoplankton declined. Bacterial size distribution patterns were determined primarily by substrate supply, with HNAN grazing playing a less important role. Certain variabilities existed between the control (and the treatment) tanks. These inconsistencies could be due to differences in time of expression of given variables. However, the total amounts of bacterial biomass accumulated in the four tanks were very similar. The inconsistency in timing of expression of variables was probably due to different initial conditions in each tank. The ecological meanings of the inconsistency in timing and overall consistency were discussed.

Modeling the microbial food web.

Models of the microbial food web have their origin in the debate over the importance of bacteria as an energetic subsidy for higher trophic levels leading to harvestable fisheries. Conceptualization of the microbial food web preceded numerical models by 10-15 years. Pomeroy's work was central to both efforts. Elements necessary for informative and comprehensive models of microbial loops in plankton communities include coupled carbon and nitrogen flows utilizing a size-based approach to structuring and parameterizing the food web. Realistic formulation of nitrogen flows requires recognition that both nitrogenous and nonnitrogenous organic matter are important substrates for bacteria. Nitrogen regeneration driven by simple mass-specific excretion constants seems to overestimate the role of bacteria in the regeneration process. Quantitative assessment of the link-sink question, in which the original loop models are grounded, requires sophisticated analysis of size-based trophic structures. The effects of recycling complicate calculation of the link between bacteria or dissolved organic matter and mesozooplankton, and indirect effects show that the link might be much stronger than simple analyses have suggested. Examples drawn from a series of oceanic mixed layer plankton models are used to illustrate some of these points. Single-size class models related to traditional P-Z-N approaches are incapable of simulating bacterial biomass cycles in some locations (e.g., Bermuda) but appear to be adequate for more strongly seasonal regimes at higher latitudes.

Bacterioplankton cell growth and macromolecular synthesis in seawater cultures during the North Atlantic Spring Phytoplankton Bloom, May, 1989.

We performed a series of seawater culture experiments on surface mixed layer samples during the spring phytoplankton bloom in the North Atlantic Ocean. Diluted (20% unfiltered + 80% 0.22 µm filtered) and untreated "whole" seawater samples were incubated up to 40 hour and sampled periodically for cell numbers, biovolume, and incorporation of (3)H-thymidine and -leucine. Abundance and biovolume increased exponentially at similar rates in diluted and whole samples, suggesting that removal by bacteriovores was low compared with growth. The exponential increase in biovolume was due to increases in cell numbers and mean cell volume. Generation times (i.e., 0.693/µ) averaged 36-53 hour in these surface (10 m) samples. Ninety percent of the tritiated thymidine incorporation (TTI) into cold trichloroacetic acid-insoluble cell fractions was recovered after extraction with NaOH and phenolchloroform, indicating that catabolism of thymidine and its appearance in RNA or protein was very low. The percentage of thymidine recovered in DNA did not change over the 40 hour of incubation and was the same as in water column samples. Rates of thymidine and leucine incorporation also increased exponentially. Incorporation rates tended to increase more rapidly than cell numbers or biovolume, though the differences were not significantly different, due to the small number of samples and variability over the time courses. Differential rates of increase in cellular properties during growth might indicate a lack of coupling between incorporation and production over time scales of hours-days. This in turn may reflect unbalanced growth of bacterial assemblages, which is an adaptation to variable conditions in the upper ocean in this season. Nonequality of rate constants for cells and incorporation yields conversion factors that are either higher or lower than would be calculated from balanced growth (i.e., rates of increase in numbers and incorporation rates equal), depending on the calculation approach chosen. An alternative approach to calculating conversion factors (the modified derivative approach) is proposed, which is insensitive to differential rates of increase of abundance and incorporation.

Bacterioplankton: a sink for carbon in a coastal marine plankton community.

Recent determinations of high production rates (up to 30 percent of primary production in surface waters) implicate free-living marine bacterioplankton as a link in a "microbial loop" that supplements phytoplankton as food for herbivores. An enclosed water column of 300 cubic meters was used to test the microbial loop hypothesis by following the fate of carbon-14-labeled bacterioplankton for over 50 days. Only 2 percent of the label initially fixed from carbon-14-labeled glucose by bacteria was present in larger organisms after 13 days, at which time about 20 percent of the total label added remained in the particulate fraction. Most of the label appeared to pass directly from particles smaller than 1 micrometer (heterotrophic bacterioplankton and some bacteriovores) to respired labeled carbon dioxide or to regenerated dissolved organic carbon-14. Secondary (and, by implication, primary) production by organisms smaller than 1 micrometer may not be an important food source in marine food chains. Bacterioplankton can be a sink for carbon in planktonic food webs and may serve principally as agents of nutrient regeneration rather than as food.

Do continental shelves export organic matter?

It has been suggested that biological production and consumption of organic matter is not balanced in coastal marine ecosystems1,2. If, as suggested, 90% of the phytoplankton produced during the spring bloom period were exported, excess organic carbon would be sequestered on the continental slope below the permanent thermocline. Here we summarize the shelf-edge exchange processes (SEEP) experiment, designed to test the export hypothesis. The absence of a positive imbalance in the organic carbon budget, reinforced by modest sediment deposition and biomass on the continental slope, led us to reject the concept. Only a small fraction of continental shelf phytodetritus is exported; that not consumed in the spring is for the most part used on the continental shelf during the ensuing stratified season. The original hypothesis failed to recognize the contribution of pelagic microbial consumption and the lag in coupling between seasonal production and consumption processes.

Estimates of bacterial growth from changes in uptake rates and biomass.

Rates of nucleic acid synthesis have been used to examine microbiol growth in natural waters. These rates are calculated from the incorporation of [3H]adenine and [3H]thymidine for RNA and DNA syntheses, respectively. Several additional biochemical parameters must be measured or taken from the literature to estimate growth rates from the incorporation of the tritiated compounds. We propose a simple method of estimating a conversion factor which obviates measuring these biochemical parameters. The change in bacterial abundance and incorporation rates of [3H]thymidine was measured in samples from three environments. The incorporation of exogenous [3H]thymidine was closely coupled with growth and cell division as estimated from the increase in bacterial biomass. Analysis of the changes in incorporation rates and initial bacterial abundance yielded a conversion factor for calculating bacterial production rates from incorporation rates. Furthermore, the growth rate of only those bacteria incorporating the compound can be estimated. The data analysis and experimental design can be used to estimate the proportion of nondividing cells and to examine changes in cell volumes.

Production and vertical flux of attached bacteria in the hudson river plume of the new york bight as studied with floating sediment traps.

We investigated the growth and vertical flux of attached bacteria with floating sediment traps in the Hudson River Plume of the New York Bight during the spring diatom blooms. Traps were floated at the base of the mixed layer (ca. 10 m) for 1-day periods. After recovery, we measured bacterial abundance and rates of [methyl-H]thymidine incorporation in the trap samples. The vertical flux of attached bacteria was estimated with a model formulated to distinguish between bacterial accumulation in traps due to in situ growth and that due to vertical flux. Attached bacterial flux ranged from 0.6 x 10 to 2.0 x 10 cells m day, and attached bacterial settling rates of 0.1 to 1.0 m day were observed during periods of vertical particulate organic carbon flux ranging from 254 to 1,267 mg of C m day. In situ growth of bacteria in sediment traps was unimportant as a source of bacterial increase when compared with vertical flux during our study. The vertical flux of attached bacteria removed 3 to 67% of the total daily bacterial production from the water column. Particulate organic carbon is not significantly mineralized by attached bacteria during its descent to the sea floor in the plume area during this period, when water temperature and grazing rates are at their annual minima.

Ecology of bacterial communities in the schistosomiasis vector snailBiomphalaria glabrata.

The internal colony-forming bacterial flora of the schistosome intermediate host snailBiomphalaria glabrata (Say) has been characterized in ca. 500 individual snails from Puerto Rico, Guadeloupe, and St. Lucia, and from laboratory aquaria. Freshly captured wild snails harbor 2-40×10(6) CFU·g(-1), and healthy aquarium snails harbor 4-16×10(7) CFU·g(-1), whereas moribund individuals have 4-10 times as many bacteria as healthy individuals from the same habitats.Pseudomonas spp. are the most common predominant bacteria in normal snails, whereasAcinetobacter, Aeromonas, andMoraxella spp. predominate in moribund snails. External bacterial populations in water appear to have little effect on the composition and size of the flora in any snail. In addition to normal (healthy) and moribund snails, a third group of snails has been distinguished on the basis of internal bacterial density and predominating genera. These "high-density" snails may have undergone stresses and may harbor opportunistic pathogens. The microfloras of wild and laboratory-reared snails can be altered and stimulated to increase in density by crowding the snails or treating them with antibiotics.

Experimental pathogenicity of Vibrio parahaemolyticus for the schistosome-bearing snail Biomphalaria glabrata.

The bacterium Vibrio parahaemolyticus was found to be pathogenic for the schistosome intermediate host Biomphalaria glabrata (Say). When administered topically, a nonenteritis-associated strain of the bacterium had an LD50 (median lethal dose) of 6.8 x 10(7) cells per snail. A 5% trichloroacetic acid (TCA) extract from V. parahaemolyticus was found to kill B. glabrata. Sublethal effects of V. parahaemolyticus include shell deterioration and increased heart rate. Both albino aquarium populations and naturally occurring Puerto Rican wild populations of B. glabrata are susceptible to V. parahaemolyticus. This bacterium provides a useful model for the study of pathogens and biological control of schistosome vector snails, since it causes significant mortality and is recognized as a pathogen of other invertebrates.

Bacterial flora of the schistosome vector snail Biomphalaria glabrata.

The aerobic heterotrophic bacterial flora in over 200 individuals from 10 wild populations and 3 laboratory colonies of the schistosome vector snail Biomphalaria glabrata was examined. Internal bacterial densities were inversely proportional to snail size and were higher in stressed and laboratory-reared snails. The numerically predominant bacterial genera in individual snails included Pseudomonas, Acinetobacter, Aeromonas, Vibrio, and several members of the Enterobacteriaceae. Enterobacteriaceae seldom predominated in laboratory colonies. Our data suggest that Vibrio extorquens and a Pasteurella sp. tend to predominate in high-bacterial-density snails. These snails may be compromised and may harbor opportunistic snail pathogens.

Observations on naturally and artificially diseased tropical corals: A scanning electron microscope study.

Scanning electron microscopic (SEM) observations of naturally and artificially diseased corals reveal that the disease is characterized by a filamentous matrix of cyanobacterial andBeggiatoa filaments. Spiral bacteria are commonly embedded in the matrix. The artificial disease is not manifested as the characteristic "black line disease" and does not contain filaments of cyanobacteria. This suggests that cyanobacteria are necessary for the black line phenomenon. The colorless, sulfide-oxidizing bacteriumBeggiatoa, however, is always associated with the disease.