Mikal Heldal

Elemental Composition (C, N, P) and Cell Volume of Exponentially Growing and Nutrient-Limited Bacterioplankton

ABSTRACT Marine bacterioplankton were isolated and grown in batch cultures until their growth became limited by organic carbon (C), nitrogen (N), or phosphorus (P). Samples were taken from the cultures at both the exponential and stationary phases. The elemental composition of individual bacterial cells was analyzed by X-ray microanalysis with an electron microscope. The cell size was also measured. The elemental content was highest in exponentially growing cells (149 ± 8 fg of C cell−1, 35 ± 2 fg of N cell−1, and 12 ± 1 fg of P cell−1; average of all isolates ± standard error). The lowest C content was found in C-limited cells (39 ± 3 fg of C cell−1), the lowest N content in C- and P-limited cells (12 ± 1 and 12 ± 2 fg of N cell−1, respectively), and the lowest P content in P-limited cells (2.3 ± 0.6 fg of P cell−1). The atomic C:N ratios varied among treatments between 3.8 ± 0.1 and 9.5 ± 1.0 (average ± standard error), the C:P ratios between 35 ± 2 and 178 ± 28, and the N:P ratios between 6.7 ± 0.3 and 18 ± 3. The carbon-volume ratios showed large variation among isolates due to different types of nutrient limitation (from 51± 4 to 241 ± 38 fg of C μm−1; average of individual isolates and treatments ± standard error). The results show that different growth conditions and differences in the bacterial community may explain some of the variability of previously reported elemental and carbon-volume ratios.

The importance of microbiological activity in the alteration of natural basaltic glass

Abstract The textural development of palagonite may differ profoundly depending on whether alteration occurred in the outermost 6–7 mm thick light-exposed surface zone of deposits, or elsewhere. In the former case, a pit-textured development of the parent basaltic glass develops as a consequence of local establishment of a highly alkaline micro-environment (pH > 9) for which the light-dependent cryptoendolithic cyanobacteria are considered most likely to be responsible. A highly porous, sponge-textured variety of palagonite, frequently defining zoned layers, contains abundant examples of bacteria. The shape and size of the pores combined with the geochemical development strongly suggest that bacteria have played an important role in the development of this texture.

Global-scale processes with a nanoscale drive: the role of marine viruses

Viruses, the smallest and most numerous of all biotic agents, represent the planets largest pool of genetic diversity. The sheer abundance of oceanic viruses results in ~1029 viral infections per day, causing the release of 108–109 tonnes of carbon per day from the biological pool (Suttle, 2007). Still, how and to what extent virus-mediated nanoscale processes are linked to global-scale biodiversity and biogeochemistry is poorly defined.

VIRUSES AND THE MICROBIAL LOOP

The abundance of viral-like particles in marine ecosystems ranges from <104 ml−1 to >108 ml−1. Their distribution in time and space parallels that of other biological parameters such as bacterial abundance and chlorophyll a. There is a lack of consensus between methods used to assess viral activity, i.e., rate of change in viral abundance (increase or decrease). The highest rates, 10–100 days−1, are observed in experiments with short sampling intervals (0.2–2 h), while lower rates, on the order of 1 day−1, are observed in experiments with longer sampling intervals (days). Few studies have been carried out, but viruses appear, at least in some cases, to have a significant impact on carbon and nutrient flow in microbial food webs. Viruses have also been demonstrated to exert a species specific control of both bacteria and phytoplankton populations in natural waters.

Counterintuitive carbon-to-nutrient coupling in an Arctic pelagic ecosystem

Predicting the ocean’s role in the global carbon cycle requires an understanding of the stoichiometric coupling between carbon and growth-limiting elements in biogeochemical processes. A recent addition to such knowledge is that the carbon/nitrogen ratio of inorganic consumption and release of dissolved organic matter may increase in a high-CO2 world. This will, however, yield a negative feedback on atmospheric CO2 only if the extra organic material escapes mineralization within the photic zone. Here we show, in the context of an Arctic pelagic ecosystem, how the fate and effects of added degradable organic carbon depend critically on the state of the microbial food web. When bacterial growth rate was limited by mineral nutrients, extra organic carbon accumulated in the system. When bacteria were limited by organic carbon, however, addition of labile dissolved organic carbon reduced phytoplankton biomass and activity and also the rate at which total organic carbon accumulated, explained as the result of stimulated bacterial competition for mineral nutrients. This counterintuitive ‘more organic carbon gives less organic carbon’ effect was particularly pronounced in diatom-dominated systems where the carbon/mineral nutrient ratio in phytoplankton production was high. Our results highlight how descriptions of present and future states of the oceanic carbon cycle require detailed understanding of the stoichiometric coupling between carbon and growth-limiting mineral nutrients in both autotrophic and heterotrophic processes.

Occurrence of virus-like particles in the Dead Sea

Abstract Electron-microscopic examination of water samples from the hypersaline Dead Sea showed the presence of high numbers of virus-like particles. Between 0.9 and 7.3 × 107 virus-like particles ml−1 were enumerated in October 1994 in the upper 20 m of the water column during the decline of a bloom of halophilic Archaea. Virus-like particles outnumbered bacteria by a factor of 0.9–9.5 (average 4.4). A variety of viral morphologies were detected, the most often encountered being spindle-shaped, followed by polyhedral and tailed phages. In addition, other types of particles were frequently found, such as unidentified algal scales, and virus-sized star-shaped particles. Water samples collected during 1995 contained low numbers of both bacteria and virus-like particles (1.9–2.6 × 106 and 0.8–4.6 × 107 ml−1 in April 1995), with viral numbers sharply declining afterwards (less than 104 ml−1 in November 1995–January 1996). It is suggested that viruses may play a major role in the decline of halophilic archaeal communities in the Dead Sea, an environment in which protozoa and other predators are absent.

ISOLATION AND CHARACTERIZATION OF A VIRUS INFECTING PHAEOCYSTIS POUCHETII (PRYMNESIOPHYCEAE)1

A virus infecting the haptophyte Phaeocystis pouchetii (Hariot) Lagerheim was isolated from Norwegian coastal waters in May 1995 at the end of a bloom of this phytoplankter. The virus was specific for P. pouchetii because it did not lyse 10 strains of P. globosa Scherffel, Phaeocystis sp., and P. antarctica Karsten. It was a double‐stranded DNA virus, and the viral particle was a polyhedron with a diameter of 130–160 nm. The virus had a main polypeptide of about 59 kDa and at least five minor polypeptides between 30 and 50 kDa. The latent period of the virus when propagated in cultures of P. pouchetii was 12–18 h, and the time required for complete lysis of the cultures was about 48 h. The burst size was estimated to be 350–600 viral particles per lysed cell.

Are viruses important partners in pelagic fend webs

Viruses have been assumed to play a rather negligible role as partners in microbial food web dynamics. However, recent discoveries suggest that the rate of virally induced lysis of marine microbial populations may be significant. This, in turn, may have important consequences for the developing conceptual framework of the microbial food web.

On the relation between dry matter and volume of bacteria.

Dry matter and volumes of 337 individual bacterial cells with volumes in the range 0.01–7μm3 from different origins were measured. An allometric relation was established between dry matter and volume, such that smaller bacteria tended to have a higher dry matter to volume ratio than larger bacteria. The results are compared to results from similar work on algae. The implications for the use of conversion factors are discussed.

Viral control of Emiliania huxleyi blooms

Virus and virus-like particles (VLP) have been observed in all major algal classes. Few host-virus systems of microalgae have until now been brought into culture and extensively studied. For Emiliania huxleyi we have been able to describe viral infection during blooms in mesocosms and in landlocked fjords. Evidence of viral lysis of E. huxleyi during blooms in the North Sea has also been obtained. We have also developed a plaque assay for E. huxleyi virus with which we have been able to isolate a virus that could be propagated in the laboratory. This virus isolate lost its virulence possibly due to defective interfering particles (DI). The sizes of viruses related to E. huxleyi indicate two major groups, one with a particle diameter of 180 nm and one with a head diameter of 140 nm.