Research Interests:
Biological oceanography; physiology, ecology, and evolution of marine phytoplankton and bacteria; CO2 effects on oceanic productivity; trace metal nutrition and toxicity in the sea, biological isotope fractionation.

Phytoplankton - CO2 interactions

Though it is now apparent that marine phytoplankton contribute substantially to oceanic CO2 uptake through the export of particulate organic C to the deep sea (the biological carbon pump), very little is known about the response of phytoplankton to natural and anthropogenic fluctuations in atmospheric CO2 concentrations. Over the past six years, I have investigated the physiological mechanisms of inorganic C acquisition in marine phytoplankton and the effects of experimental CO2 manipulations on the cellular metabolism (e.g. enzyme expression, C isotope fractionation) and growth rates of model species in laboratory cultures. In addition, I have pursued extensive field studies in both the Atlantic and Pacific Oceans examining CO2 effects on the productivity and ecology of natural phytoplankton assemblages in situ. This work has documented the existence of active carbon concentrating mechanisms in marine diatom assemblages, and examined the relative importance of CO2 and HCO3- as sources of inorganic C for photosynthesis. Although the available data indicate that bulk primary production appears to be largely insensitive to changing CO2 concentrations, preliminary work suggests that CO2 may affect the species composition of marine phytoplankton assemblages. Future laboratory and field work shall examine the physiology and biochemistry of C assimilation in a variety of marine algal taxa (diatoms, prymnesiophytes, dinoflagellates, and cyanobacteria), and, more generally, the role of inorganic C in the ecology of oceanic primary producers.

Bacterial - trace metal interactions:

It is now firmly established that phytoplankton growth in large areas of the oceans is limited by the availability of Fe (and potentially other 'bio-active' trace metals). However, the extent to which marine heterotrophic bacteria may be subject to metal limitation or toxicity remains poorly known. In previous work, we showed that bacteria require large amounts of Fe for growth, and may indeed suffer Fe limitation. Such limitation could significantly affect the biological cycle of C in the oceans. In addition, trace metals may potential affect the biogeochemistry of several climatologically important greenhouse gases such as nitrous oxide whose metabolic production and consumption pathways in bacteria are trace-metal dependant. Research in this area will focus on examining the trace metal requirements of marine bacteria, and documenting the physiological and biochemical responses of these organisms to metal deficiency.

Tortell, Philippe D., Maria M. Maldonado, Neil M. Price (1996) The role of heterotrophic bacteria in iron-limited ocean ecosystems. Nature 383: 330-332.

Tortell, Philippe D., Greg H. Rau, François M.M. Morel (2000) Inorganic carbon acquisition in coastal Pacific phytoplankton communities. Limnology and Oceanography 45: 1485-1500.

Tortell, Philippe D. (2000) Evolutionary and ecological perspectives on inorganic carbon acquisition in phytoplankton. Limnology and Oceanography 45: 744-750.

Riebesell, Ulf, Ingrid Zondervan, Björn Rost, Philippe D. Tortell, Richard Zebe, François M.M. Morel (2000) Increased CO2 decreases marine planktonic calcification. Nature 407: 364-367.

Tortell, Philippe D., Giacomo R. DiTullio, Daniel M. Sigman, and François M.M. Morel - CO2 effects on species composition and nutrient ulilization in an Equatorial Pacific phytoplankton assemblage. Submitted to Marine Ecology Progress Series.