Kam W. Tang

Mechanisms and Rates of Bacterial Colonization of Sinking Aggregates

ABSTRACT Quantifying the rate at which bacteria colonize aggregates is a key to understanding microbial turnover of aggregates. We used encounter models based on random walk and advection-diffusion considerations to predict colonization rates from the bacterias motility patterns (swimming speed, tumbling frequency, and turn angles) and the hydrodynamic environment (stationary versus sinking aggregates). We then experimentally tested the models with 10 strains of bacteria isolated from marine particles: two strains were nonmotile; the rest were swimming at 20 to 60 μm s−1 with different tumbling frequency (0 to 2 s−1). The rates at which these bacteria colonized artificial aggregates (stationary and sinking) largely agreed with model predictions. We report several findings. (i) Motile bacteria rapidly colonize aggregates, whereas nonmotile bacteria do not. (ii) Flow enhances colonization rates. (iii) Tumbling strains colonize aggregates enriched with organic substrates faster than unenriched aggregates, while a nontumbling strain did not. (iv) Once on the aggregates, the bacteria may detach and typical residence time is about 3 h. Thus, there is a rapid exchange between attached and free bacteria. (v) With the motility patterns observed, freely swimming bacteria will encounter an aggregate in <1 day at typical upper-ocean aggregate concentrations. This is faster than even starving bacteria burn up their reserves, and bacteria may therefore rely solely on aggregates for food. (vi) The net result of colonization and detachment leads to a predicted equilibrium abundance of attached bacteria as a function of aggregate size, which is markedly different from field observations. This discrepancy suggests that inter- and intraspecific interactions among bacteria and between bacteria and their predators may be more important than colonization in governing the population dynamics of bacteria on natural aggregates.

Dynamics of microbial communities on marine snow aggregates: Colonization, growth, detachment, and grazing mortality of attached bacteria

ABSTRACT We studied the dynamics of microbial communities attached to model aggregates (4-mm-diameter agar spheres) and the component processes of colonization, detachment, growth, and grazing mortality. Agar spheres incubated in raw seawater were rapidly colonized by bacteria, followed by flagellates and ciliates. Colonization can be described as a diffusion process, and encounter volume rates were estimated at about 0.01 and 0.1 cm3 h−1 for bacteria and flagellates, respectively. After initial colonization, the abundances of flagellates and ciliates remained approximately constant at 103 to 104 and ∼102 cells sphere−1, respectively, whereas bacterial populations increased at a declining rate to >107 cells sphere−1. Attached microorganisms initially detached at high specific rates of ∼10−2 min−1, but the bacteria gradually became irreversibly attached to the spheres. Bacterial growth (0 to 2 day−1) was density dependent and declined hyperbolically when cell density exceeded a threshold. Bacterivorous flagellates grazed on the sphere surface at an average saturated rate of 15 bacteria flagellate−1 h−1. At low bacterial densities, the flagellate surface clearance rate was ∼5 × 10−7 cm2 min−1, but it declined hyperbolically with increasing bacterial density. Using the experimentally estimated process rates and integrating the component processes in a simple model reproduces the main features of the observed microbial population dynamics. Differences between observed and predicted population dynamics suggest, however, that other factors, e.g., antagonistic interactions between bacteria, are of importance in shaping marine snow microbial communities.

Bacterial Colonization of Particles: Growth and Interactions

ABSTRACT Marine particles in the ocean are exposed to diverse bacterial communities, and colonization and growth of attached bacteria are important processes in the degradation and transformation of the particles. In an earlier study, we showed that the initial colonization of model particles by individual bacterial strains isolated from marine aggregates was a function of attachment and detachment. In the present study, we have investigated how this colonization process was further affected by growth and interspecific interactions among the bacteria. Long-term incubation experiments showed that growth dominated over attachment and detachment after a few hours in controlling the bacterial population density on agar particles. In the absence of grazing mortality, this growth led to an equilibrium population density consistent with the theoretical limit due to oxygen diffusion. Interspecific interaction experiments showed that the presence of some bacterial strains (“residents”) on the agar particles either increased or decreased the colonization rate of other strains (“newcomers”). Comparison between an antibiotic-producing strain and its antibiotic-free mutant showed no inhibitory effect on the newcomers due to antibiotic production. On the contrary, hydrolytic activity of the antibiotic-producing strain appeared to benefit the newcomers and enhance their colonization rate. These results show that growth- and species-specific interactions have to be taken into account to adequately describe bacterial colonization of marine particles. Changes in colonization pattern due to such small-scale processes may have profound effects on the transformation and fluxes of particulate matter in the ocean.

Microbial methane production in oxygenated water column of an oligotrophic lake

The prevailing paradigm in aquatic science is that microbial methanogenesis happens primarily in anoxic environments. Here, we used multiple complementary approaches to show that microbial methane production could and did occur in the well-oxygenated water column of an oligotrophic lake (Lake Stechlin, Germany). Oversaturation of methane was repeatedly recorded in the well-oxygenated upper 10 m of the water column, and the methane maxima coincided with oxygen oversaturation at 6 m. Laboratory incubations of unamended epilimnetic lake water and inoculations of photoautotrophs with a lake-enrichment culture both led to methane production even in the presence of oxygen, and the production was not affected by the addition of inorganic phosphate or methylated compounds. Methane production was also detected by in-lake incubations of lake water, and the highest production rate was 1.8–2.4 nM⋅h−1 at 6 m, which could explain 33–44% of the observed ambient methane accumulation in the same month. Temporal and spatial uncoupling between methanogenesis and methanotrophy was supported by field and laboratory measurements, which also helped explain the oversaturation of methane in the upper water column. Potentially methanogenic Archaea were detected in situ in the oxygenated, methane-rich epilimnion, and their attachment to photoautotrophs might allow for anaerobic growth and direct transfer of substrates for methane production. Specific PCR on mRNA of the methyl coenzyme M reductase A gene revealed active methanogenesis. Microbial methane production in oxygenated water represents a hitherto overlooked source of methane and can be important for carbon cycling in the aquatic environments and water to air methane flux.

Bacteria dispersal by hitchhiking on zooplankton

Microorganisms and zooplankton are both important components of aquatic food webs. Although both inhabit the same environment, they are often regarded as separate functional units that are indirectly connected through nutrient cycling and trophic cascade. However, research on pathogenic and nonpathogenic bacteria has shown that direct association with zooplankton has significant influences on the bacterias physiology and ecology. We used stratified migration columns to study vertical dispersal of hitchhiking bacteria through migrating zooplankton across a density gradient that was otherwise impenetrable for bacteria in both upward and downward directions (conveyor-belt hypothesis). The strength of our experiments is to permit quantitative estimation of transport and release of associated bacteria: vertical migration of Daphnia magna yielded an average dispersal rate of 1.3 × 105·cells·Daphnia−1·migration cycle−1 for the lake bacterium Brevundimonas sp. Bidirectional vertical dispersal by migrating D. magna was also shown for two other bacterial species, albeit at lower rates. The prediction that diurnally migrating zooplankton acquire different attached bacterial communities from hypolimnion and epilimnion between day and night was subsequently confirmed in our field study. In mesotrophic Lake Nehmitz, D. hyalina showed pronounced diel vertical migration along with significant diurnal changes in attached bacterial community composition. These results confirm that hitchhiking on migrating animals can be an important mechanism for rapidly relocating microorganisms, including pathogens, allowing them to access otherwise inaccessible resources.

Limitation of zooplankton production: beyond stoichiometry

Stoichiometry is a commonly adopted concept in studying food-related limitation of zooplankton production in both marine and freshwater systems (Sterner 1990, Hessen 1992, Elser and Hassett 1994, Sterner and Hessen 1994, Hassett et al. 1997). Stoichiometric studies typically place substrates into three major elemental categories: carbon, nitrogen and phosphorus. By employing stoichiometric arguments, some researchers have concluded that production of zooplankton in the ocean is limited by nitrogen whereas freshwater zooplankton are limited by phosphorus (Hessen 1992, Elser and Hassett 1994). Some commonly quoted evidence includes: 1) The C:N or P:N ratio of in situ marine zooplankton biomass is lower than the C:N or P:N ratio of suspended particulate matter (Redfield et al. 1966, Elser and Hassett 1994); thus nitrogen is always in relatively shorter supply than carbon and phosphorus and becomes limiting (sensu Liebig). On the other hand, higher P:N ratio in freshwater zooplankton relative to particulate matter is interpreted as evidence of phosphorus limitation among freshwater zooplankton (Elser and Hassett 1994). 2) In laboratory studies, the gross-growth efficiency (growth/ingestion) in terms of carbon of marine calanoid copepods decreases with an increase in the C:N ratio (range 5-30) of food of monospecific algal diets, but the grossgrowth efficiency in terms of nitrogen remains constant over the same C:N ratio of food (see Fig. 11 in Checkley 1980 and Fig. 3 in Ki0rboe 1989). This has been interpreted as the animals maximizing the uptake of the limiting element for growth: nitrogen (Ki0rboe 1989; similar argument by Sterner and Hessen 1994). Other researchers have applied stoichiometric arguments implicitly by quoting the lower C:N ratio of protozoa, when compared to other diets, as evidence of their better nutritional quality for metazoan zooplankton (Stoecker and Capuzzo 1990, Gifford 1991, Sanders and Wickham 1993). Here we will argue that the concept of stoichiometry, although powerful and useful, is not sufficient for understanding food-related limitation of zooplankton production. We will discuss the confusion created by some assumptions in stoichiometric arguments and how stoichiometric theory can benefit from biochemical and physiological studies. One possible drawback of stoichiometric arguments is that they ignore the biochemical characteristics of specific groups of substrates (see debate among Brett 1993, Hessen 1993 and Urabe and Watanabe 1993). In nature, food substrates do not exist as individual elements, but always as compounds. Therefore, how an element is processed by the consumer is dictated by the biochemistry of the compounds in which that element is present. If we take biochemistry and physiology into consideration, nitrogen limitation in marine zooplankton is not always warranted by simple stoichiometric

Zooplankton and aggregates as refuge for aquatic bacteria: protection from UV, heat and ozone stresses used for water treatment

Aggregates and zooplankton may provide refuge for aquatic bacteria against external hazards. The ability of attached bacteria to survive and recover from stressors commonly used for water treatment was tested in the laboratory. Without zooplankton or aggregates, both UV and ozone significantly reduced abundance of free-living bacteria in both freshwater and marine medium. The presence of zooplankton carcasses and aggregates, however, allowed some of the attached bacteria to survive and recover quickly within 3 days. Heat exposure was the least effective as both free-living and attached bacteria were able to recover quickly. Selective survival of bacterial phylotypes led to large changes in bacterial community composition after stress exposures, and some of the bacteria that recovered belonged to groups with known pathogens. This study demonstrates that zooplankton and aggregates protected various aquatic bacteria from external stressors, and organic remains generated from zooplankton and aggregates after stress exposure even enabled the surviving bacteria to quickly regrow and subsequently be released into the surrounding water. Hence, water disinfection treatments that overlooked the potential persistence of bacteria associated with organisms and aggregates may not be effective in preventing the spread of undesirable bacteria.

Effects of chromium, copper and nickel on survival and feeding behaviour of Metapenaeus ensis larvae and postlarvae (Decapoda: Penaeidae)

The effects of heavy metals—copper, chromium, and nickel—on the survival and feeding behaviour of the early developmental stages of the penaeid shrimp, Metapenaeus ensis, were studied in the laboratory. High variability in mortality was observed among animals from different spawners. Copper was more toxic to M. ensis than chromium or nickel. The 48-h LC50, values of copper were lower than those of chromium and nickel for all developmental stages tested. Tolerance of the heavy metals increased with age. From protozoea III to 3-day postlarvae, increase in the 48-h LC50 value ranged from less than three-fold for chromium to about 30-fold for copper. Gut fullness (chlorophyll + phaeopigment) of protozoea III feeding on Chaetoceros gracilis was reduced by a 2-h exposure to copper at 0·25 mg litre1. In contrast, gut fullness of mysis III was not affected even after a 24-h exposure to chromium, copper and nickel at concentrations close to the 48-h LC50. Postlarvae exposed to chromium, copper and nickel at concentrations close to the 48-h LC50 for 24 h consumed significantly fewer Artemia nauplii. Predation rate was also reduced after a 24-h exposure to nickel at concentration close to the 48-h LC25.

Effects of food on bacterial community composition associated with the copepod Acartia tonsa Dana

The estuarine copepod Acartia tonsa naturally carried diverse strains of bacteria on its body. The bacterial community composition (BCC) remained very conservative even when the copepod was fed different axenic algal species, indicating that the food per se did not much affect BCC associated with the copepod. In xenic algal treatments, however, copepod-associated BCC differed with each alga fed, even though the same bacterial source was used to inoculate the algae. In addition, starved copepods taken at the same location but at different times significantly differed in their BCC. Algal species composition and copepod life history therefore serve to regulate BCC associated with copepods, and spatial and temporal variations in algal species composition and copepod origin would alter bacteria–copepod interactions.

COLONY SIZE OF PHAEOCYSTIS ANTARCTICA (PRYMNESIOPHYCEAE) AS INFLUENCED BY ZOOPLANKTON GRAZERS 1

The haptophyte Phaeocystis antarctica G. Karst. is a dominant phytoplankton species in the Ross Sea, Antarctica, and exists as solitary cells and mucilaginous colonies that differ by several orders of magnitude in size. Recent studies with Phaeocystis globosa suggest that colony formation and enlargement are defense mechanisms against small grazers. To test if a similar grazer‐induced morphological response exists in P. antarctica, we conducted incubation experiments during the austral summer using natural P. antarctica and zooplankton assemblages. Dialysis bags that allowed exchange of dissolved chemicals were used to separate P. antarctica and zooplankton during incubations. Geometric mean colony size decreased by 35% in the control, but increased by 30% in the presence of grazers (even without physical contact) over the 15 d incubation. The estimated colonial‐to‐solitary cell carbon ratio was significantly higher in the grazing treatment. These results suggest that P. antarctica colonies would grow larger in the presence of indigenous zooplankton and skew the carbon partitioning significantly toward the colonial phase. While these observations show that the colony size of P. antarctica was affected by a chemical signal related to grazers, the detailed nature and ecological significance of this signal remain unknown.