Resistance to co-occurring phages enables marine synechococcus communities to coexist with cyanophages abundant in seawater.

Recent reports documenting very high viral abundances in seawater have led to increased interest in the role of viruses in aquatic environments and a resurgence of the hypothesis that viruses are significant agents of bacterial mortality. Synechococcus spp., small unicellular cyanobacteria that are important primary producers at the base of the marine food web, were used to assess this hypothesis. We isolated a diverse group of Synechococcus phages that at times reach titers of between 10 and 10 cyanophages per ml in both inshore and offshore waters. However, despite their diversity and abundance, we present evidence in support of the hypothesis that lytic phages have a negligible effect in regulating the densities of marine Synechococcus populations. Our results indicate that these bacterial communities are dominated by cells resistant to their co-occurring phages and that these viruses are maintained by scavenging on the relatively rare sensitive cells in these communities.

A cyanobacterium capable of swimming motility.

A novel cyanobacterium capable of swimming motility was isolated in pure culture from several locations in the Atlantic Ocean. It is a small unicellular form, assignable to the genus Synechococcus, that is capable of swimming through liquids at speeds of 25 micrometers per second. Light microscopy revealed that the motile cells display many features characteristic of bacterial flagellar motility. However, electron microscopy failed to reveal flagella and shearing did not arrest motility, indicating that the cyanobacterium may be propelled by a novel mechanism.

Production of NO(2) and N(2)O by Nitrifying Bacteria at Reduced Concentrations of Oxygen.

Pure cultures of the marine ammonium-oxidizing bacterium Nitrosomonas sp. were grown in the laboratory at oxygen partial pressures between 0.005 and 0.2 atm (0.18 to 7 mg/liter). Low oxygen conditions induced a marked decrease in the rate for production of NO(2), from 3.6 x 10 to 0.5 x 10 mmol of NO(2) per cell per day. In contrast, evolution of N(2)O increased from 1 x 10 to 4.3 x 10 mmol of N per cell per day. The yield of N(2)O relative to NO(2) increased from 0.3% to nearly 10% (moles of N in N(2)O per mole of NO(2)) as the oxygen level was reduced, although bacterial growth rates changed by less than 30%. Nitrifying bacteria from the genera Nitrosomonas, Nitrosolobus, Nitrosospira, and Nitrosococcus exhibited similar yields of N(2)O at atmospheric oxygen levels. Nitrite-oxidizing bacteria (Nitrobacter sp.) and the dinoflagellate Exuviaella sp. did not produce detectable quantities of N(2)O during growth. The results support the view that nitrification is an important source of N(2)O in the environment.

Determination of bacterial number and biomass in the marine environment.

Three techniques for the measurement of bacterial numbers and biomass in the marine environment are described. Two are direct methods for counting bacteria. The first employs an epifluorescence microscope to view bacteria that have been concentrated on membrane filters and stained with acridine orange. The second uses a transmission electron microscope for observing replicas of bacteria that are concentrated on membrane filters. The other technique uses Limulus amebocyte lysate, an aqueous extract from the amebocytes of the horseshoe crab, Limulus polyphemus, to quantitate lipopolysaccharide (LPS) in seawater samples. The biomass of gram-negative (LPS containing) bacteria was shown to be related to the LPS content of the samples. A factor of 6.35 was determined for converting LPS to bacterial carbon.

Fine structure of the cytomembranes of Nitrosocystis oceanus.

Thin-sectioned, negatively stained, and freeze-etched preparations of Nitrosocystis oceanus cytomembranes were compared. The cytomembranes in freeze-etched cells were covered with 80- to 120-A particles. When cells were disrupted and differentially centrifuged, various membrane and particle fractions were obtained. Negatively stained membrane fragments from the pellet centrifuged at 3,000 x g showed 70- to 80-A stalked particles, whereas those from the pellet centrifuged at 39,000 x g exhibited a crystalline array of subunits with a 30- to 40-A periodicity. High-speed supernatant and pellet fractions centrifuged at greater than 39,000 x g contained 40- to 120-A free particles but no membranes. In chemically fixed cells, 40-A particles were found embedded in the matrix of membranes. Results suggest that the larger 80- to 120-A particles are enzyme complexes, whereas the smaller 30- to 40-A particles represent a structural protein or a lipoprotein of the membrane.