Svein Norland

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.

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.

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.

Changes in Morphology and Elemental Composition of Vibrio splendidus Along a Gradient from Carbon-limited to Phosphate-limited Growth

We examined morphology, elemental composition (C, N, P), and orthophosphate-uptake efficiency in the marine heterotrophic bacterium Vibrio splendidus grown in continuous cultures. Eight chemostats were arranged along a gradient of increasing glucose concentrations in the reservoirs, shifting the limiting factor from glucose to phosphate. The content of carbon, nitrogen, and phosphorus was measured in individual cells by x-ray microanalysis using a transmission electron microscope (TEM). Cell volumes (V) were estimated from length and width measurements of unfixed, air-dried cells in TEM. There was a transition from coccoid cells in C-limited cultures toward rod-shaped cells in P-limited cultures. Cells in P-limited cultures with free glucose in the media were significantly larger than cells in glucose-depleted cultures (P < 0.0001). We found functional allometry between cellular C-, N-, and P content (in femtograms) and V (in cubic micrometers) in V. splendidus (C = 224 × V0.89, N = 52.5 × V0.80, P = 2 × V0.65); i.e., larger bacteria had less elemental C, N, and P per V than smaller cells, and also less P relative to C. Biomass-specific affinity for orthophosphate uptake in large P-limited V. splendidus approached theoretical maxima predicted for uptake limited by molecular diffusion toward the cells. Comparing these theoretical values to respective values for the smaller, coccoid, C-limited V. splendidus indicated, contrary to the traditional view, that large size did not represent a trade-off when competing for the non-C-limiting nutrients.

The elemental composition of bacteria: A signature of growth conditions?

Abstract X-ray microanalysis combined with Transmission Electron Microscopy (TEM) has been employed to measure cell quotas of P:C, N:C and O:C of individual cells from cultures and natural microbial communities. The relevance of such data for evaluation of nutrient availability and growth is discussed according to the Droop model. Using minimum subsistent quotas of 0.031 (P:C) and 0.17 (N:C) and μmax of 1 h−1 we have been able to identify some of the criteria for nutrient limited growth, but energy/carbon limitation will interfere with interpretation of these data. An additional signature for growth limitation may be the O:C ratio for which preliminary data indicate values of >0.40 for growing cells and

Emiliania huxleyi. Chemical composition and size of coccoliths from enclosure experiments and a Norwegian fjord

Abstract Single Emiliania huxleyi (Lohmann) Hay et Mohler, type A, coccoliths harvested from mesocosm enclosures (June 1991) with different nutrient regimes, and the Norwegian fjord, Samnangerfjorden (October 1992), were analyzed with- X-ray microanalysis in a Transmission Electron Microscope (TEM). The average molar Ca : C ratios of the free coccoliths varied within a range of 0.65–1.0, dependent on growth condition of E. huxleyi. Free coccoliths sampled from enclosures with high production or density of E. huxleyi, had Ca : C ratios near 1. Coccoliths sampled from Samnangerfjorden, and enclosures with low production or cell density the Ca : C ratios were close to 0.7. The excess of carbon in the coccoliths compared to CaC03 is probably due to organic matter, mainly carbohydrates. It is concluded that the amount of organic matter associated with coccoliths of E. huxleyi, is dependent on the growth conditions, and carbohydrate may protect the coccoliths from dissolution. A higher Ca : C ratio (0.8 versus ...

Mg2+ as an indicator of nutritional status in marine bacteria.

Cells maintain an osmotic pressure essential for growth and division, using organic compatible solutes and inorganic ions. Mg2+, which is the most abundant divalent cation in living cells, has not been considered an osmotically important solute. Here we show that under carbon limitation or dormancy native marine bacterial communities have a high cellular concentration of Mg2+ (370–940 mM) and a low cellular concentration of Na+ (50–170 mM). With input of organic carbon, the average cellular concentration of Mg2+ decreased 6–12-fold, whereas that of Na+ increased ca 3–4-fold. The concentration of chlorine, which was in the range of 330–1200 mM, and was the only inorganic counterion of quantitative significance, balanced and followed changes in the concentration of Mg2++Na+. In an osmotically stable environment, like seawater, any major shift in bacterial osmolyte composition should be related to shifts in growth conditions, and replacing organic compatible solutes with inorganic solutes is presumably a favorable strategy when growing in carbon-limited condition. A high concentration of Mg2+ in cells may also serve to protect and stabilize macromolecules during periods of non-growth and dormancy. Our results suggest that Mg2+ has a major role as osmolyte in marine bacteria, and that the [Mg2+]/[Na+] ratio is related to its physiological condition and nutritional status. Bacterial degradation is a main sink for dissolved organic carbon in the ocean, and understanding the mechanisms limiting bacterial activity is therefore essential for understanding the oceanic C-cycle. The [Mg2+]/[Na+]-ratio in cells may provide a physiological proxy for the transitions between C-limited and mineral nutrient-limited bacterial growth in the oceans surface layer.

Elemental composition of natural populations of key microbial groups in Atlantic waters

Intracellular carbon (C), nitrogen (N) and phosphorus (P) content of marine phytoplankton and bacterioplankton can vary according to cell requirements or physiological acclimation to growth under nutrient limited conditions. Although such variation in macronutrient content is well known for cultured organisms, there is a dearth of data from natural populations that reside under a range of environmental conditions. Here, we compare C, N and P content of Synechococcus, Prochlorococcus, low nucleic acid (LNA) content bacterioplankton and small plastidic protists inhabiting surface waters of the North and South subtropical gyres and the Equatorial Region of the Atlantic Ocean. While intracellular C:N ratios ranged between 3.5 and 6, i.e. below the Redfield ratio of 6.6, all the C:P and N:P ratios were up to 10 times higher than the corresponding Redfield ratio of 106 and 16, respectively, reaching and in some cases exceeding maximum values reported in the literature. Similar C:P or N:P ratios in areas with different concentrations of inorganic phosphorus suggests that this is not just a response to the prevailing environmental conditions but an indication of the extremely low P content of these oceanic microbes.

Correlation of dispersant effectiveness and toxicity of oil dispersants towards the alga Chlamydomonas reinhardti

Abstract Using synchronous cultures of the unicellular green alga Chlamydomonas reinhardti, the toxicities of mixtures of Ekofisk crude oil and oil dispersants were measured. Sixteen so-called concentrates and 10 solvent-based dispersants were tested. The dispersing effectiveness of these compounds with respect to the Ekofisk crude oil was also measured. The concentrates were tested undiluted as well as diluted using algal growth medium (2‰ salinity) and artificial sea water (33‰ salinity) as dispersing liquid. The solvent-based compounds were tested in algal medium. For all compounds we found significant correlations between their toxicity and their effectiveness in dispersing the Ekofisk oil, such that the more effective the compound, the more toxic it was.

Toxic responses of the green alga Dunaliella bioculata (Chlorophycea, Volvocales) to selected oxidised hydrocarbons

Abstract Hydroperoxides, alcohols, acids, ketones, quinones and epoxides represent classes of compounds likely to be formed as a result of photodecomposition of crude oil. Selected representatives of these classes of compounds were tested for toxicity on the green alga Dunaliella bioculata . Hydroperoxides and quinones were found to be extremely toxic, with 50%-effect concentration values from 0·01 to 0·7 ppm. No clear correlation between chemical structure and toxic mode of action could be established. The alga showed two major responses, depending on the compound tested. One group of compounds had mainly an effect on the growth rate while the other caused an immediate mortality and a following disruption of the dead cells. This latter effect was seen as an initial reduction in biomass, whereas the growth rate was unaffected.