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We tested the hypothesis that nutrient sequestration and carbon flow through heterotrophic bacteria is relatively highest in oligotrophic systems and decreases with trophic state in 17 reservoirs in Texas and Ohio, U.S.A. The percentage of particulate carbon (C), nitrogen (N), and phosphorus (P) in bacteria (< 1 [micro]m) was highest in oligotrophic reservoirs and decreased with chlorophyll a, our measure of trophic state. Patterns of nutrient sequestration in bacteria were very similar to Minnesota (U.S.A.) natural lakes. Bacterial production (BPr) and primary production (PPr) ranged from 13 [micro]g C [L.sup.-1] d 1 to 172 [micro]g C [L.sup.-1] [d.sup.-1] and from 41 [micro]g C [L.sup.-1] [d.sup.-1] to 1695 [micro]g C [L.sup.-1] [d.sup.-1], respectively, and the BPr:PPr ratio decreased with trophic state. Inclusion of variables such as dissolved organic carbon (DOC), DOC:soluble reactive phosphorus, and DOC:dissolved inorganic nitrogen did not improve predictions of BPr : PPr over that based solely upon chlorophyll a. BPr : PPr was better predicted by DOC: particulate carbon (PC) and mixing depth ([Z.sub.mix]) than by chlorophyll a. The relationship between BPr : PPr and DOC : PC was primarily driven by the relationship between BPr : PPr and PC, which was a surrogate for trophic state. Unexpectedly, the strength of [Z.sub.mix] as a predictor BPr:PPr did not primarily reflect differences in the light environment. Our results support the hypothesis that the relative importance of heterotrophic bacteria is highest in oligotrophic systems and decreases with trophic state, but provide limited support for proposed mechanisms of this pattern. Patterns in microbial nutrient dynamics are similar in reservoirs and natural lakes.

To date no study exists that directly addresses changes in dynamics of heterotrophic bacteria in surface waters in relation to partial pressure of C[O.sub.2] (pC[O.sub.2]). Therefore, we studied the effect of changes in pC[O.sub.2] on bacterial abundance and activities by using mesocosms with different pC[O.sub.2] levels (~190, ~370, and ~700 ppmV, representing past, present-day, and future atmospheric pC[O.sub.2], respectively). Abundance of total bacteria did not differ with increasing pC[O.sub.2] throughout the whole study period, whereas bacterial protein production (BPP) was highest at highest pC[O.sub.2]. This effect was even more pronounced for cell-specific production rates, especially those of attached bacteria, which were up to 25 times higher than those of free bacteria. During the breakdown of the bloom, however, the abundance of both flee and attached bacteria was significantly increased with pC[O.sub.2], Differences in bacterial growth rate ([micro]) were smaller than those of BPR but both [micro] and BPP of attached bacteria were elevated under high pC[O.sub.2]. Averages of total protease as well as [alpha]- and [beta]-glucosidase activities were highest at elevated pC[O.sub.2] levels, but a statistically significant dependence on pC[O.sub.2] was only evident for protease activity. There is a measurable but indirect effect of changes in pC[O.sub.2] on bacterial activities that are mainly linked to phytoplankton and presumably particle dynamics.

There is now clear evidence for major differences in heterotrophic bacterial composition between freshwater and marine ecosystems. A fundamental question that remains unresolved is whether the compositional succession occurs by a gradual replacement of the major phylotypes, or whether there are drastic compositional shifts in discrete areas along the gradient from one system to another. The aim of this study is to examine the change in the phylogenetic composition of the free-living bacterioplankton across the salinity gradient in both the Choptank and the Pocomoke Rivers, both subestuaries of the Chesapeake Bay, using fluorescence in sim hybridization (FISH). The proportion of free-living cells that could be detected using FISH varied widely (3 to 80%), and the majority of cells detected belonged to bacteria, whereas archaea represented less than 3%. The distribution of the different members of bacteria exhibited a clear pattern along the salinity gradient, with the dominance of [alpha]-proteobacteria in the lower, saltwater regions and of [beta]-proteobacteria in the upper freshwater regions. The cytophaga-flavobacterium cluster prevailed in the turbidity maximum located in the middle estuary, and the [gamma]-proteobacteria showed sporadic peaks along the transect that may have been related to local events. The replacement of [alpha]- for [beta]-proteobacteria along the salinity gradient was not gradual but rather occurred rapidly within the turbidity maximum region of the estuaries where the fresh and salt waters mix. The pattern of phylogenetic succession was linked to the development of the turbidity maximum, which is related to rainfall and the ensuing hydrological conditions. There is indication that the phylogenetic succession is accompanied by strong physiological changes in the bacterial assemblage, expressed as a decline in bacterial growth efficiency and community production. The transition appears not to be simply the result of conservative mixing of riverine and estuarine bacterial assemblages, but rather appears to involve cell inactivation and/or death.

Study of the degradability of Phaeocystis mucus carbohydrates by heterotrophic bacteria is discussed. The study used partially purified mucus as a substrate in oxic and anoxic enrichments of marine bacteria. The degradable of the important part of the biomass of the organism is of help for understanding impact of Phaeocystis blooms on the marine carbon cycle. Breakdown of the mucopolysaccharides, during which the carbohydrate composition of the residual fraction did not change, slowed greatly after about 15 days. Incomplete degradation was the result of release of inhibitors produced during breakdown.

Mesozooplankton as a factor in the microbial food web and its direct/indirect trophic interactions in the oligotrophic open ocean are discussed as studied in the subtropical North Pacific using cruise grazing experiments with different concentration treatments of the mixed mesozooplankton community. Increasing the concentration of mesozooplankton raised growth rates of heterotrophic bacteria, and mesozooplankton fed diverse microzooplankton and nanoheterotrophs and their predation indirectly raised net phytoplankton/heterotroph growth rates

Bacterioplankton production in large temperate rivers appears to be carbon limited, and especially limited by saccharides. Abundant sugars and peptides are associated with the highest productivity. The microbial loop appears to be the main trophic path in summer. However, the trophic base of production may change during spring and autumn from autochthonous to allochthonous sources.