Showing papers by "Mikal Heldal published in 2008"

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

Abstract: Viruses, the smallest and most numerous of all biotic agents, represent the planet's 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.

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

Abstract: An experiment in which degradable organic carbon was added to Arctic waters has come up with the counterintuitive finding that in certain conditions, the addition of carbon actually reduces total organic carbon in the ecosystem. The outcome depends on the species interactions within the food web and how they are affected by nutrient supply. When mineral nutrients are limiting, organic carbon accumulates in the system, while in a system limited by organic carbon, marine bacteria are able to outcompete phytoplankton, with the net result a reduction in total organic carbon. This result highlight how descriptions of present and future states of the oceanic carbon cycle depend upon an understanding of the stoichiometric coupling between carbon and growth-limiting mineral nutrients within the marine food web. 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 world1. 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.

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

Abstract: 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.