Microbial Ecology Changes in Morphology and Elemental Composition of Vibrio splendidus Along a Gradient from Carbon-limited to Phosphate-limited Growth Trond Løvdal 1 , Evy F. Skjoldal 1 , Mikal Heldal 1 , Svein Norland 1 and T. Frede Thingstad 1 Department of Biology, University of Bergen, Jahnebakken 5 PO Box 7800 N-5020, Bergen, Norway Received: 12 February 2007 / Revised: 17 March 2007 / Accepted: 14 April 2007 / Online publication: 8 June 2007 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 concen- trations 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 cul- tures with free glucose in the media were significantly larger than cells in glucose-depleted cultures (PG0.0001). We found functional allometry between cellular C-, N-, and P content (in femtograms) and V (in cubic micro- meters) in V. splendidus (C=224V 0.89 , N=52.5V 0.80 , P=2V 0.65 ); 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 theoret- ical 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. Introduction Modern tools of molecular biology have started to give insight into species composition and diversity of natural bacterial communities. We need answers to the old question of what physiological mechanisms and what life strategies control success of a particular species in a given environment, to better understand the internal dynamics between ecologically functional units within the bacterioplankton Bblack box^. The need for labora- tory experiments to elucidate the theory that relates cell size to mass transfer was pointed out 10 years ago by Karp-Boss et al.[11] because the size range examined was very narrow and limited to small-celled bacteria. Small, spherical cells are traditionally believed to be more efficient in their uptake of nutrients because of their large surface-to-volume ratio [cf. 21]. However, based on nutrient diffusion theory, a more precise formulation would be that it is the Bsurface/cell requirement of limiting element^, rather than the Bsurface/volume^ ratio, that is important [10, 11, 40]. At low external concentrations, the rate-limiting step for uptake will be diffusive transport toward the cell. The expression for maximum diffusive transport toward a cell can be expressed as the product of three terms G  D  S, where G is the conductance of the cell, D is the molecular diffusion constant for the substrate in water, and S the substrate concentration at arbitrarily large distance from the cell. Defining an organism’s specific affinity for a substrate as the volume cleared of substrate per unit biomass per unit time, the ability of an osmotroph to compete at permanently low substrate concentrations is given by its maximum specific affinity (a max ). Assuming that the cell is diffusion-limited, i.e., that the cell’s uptake system is so efficient (and the bulk nutrient concentration so low) that all substrate molecules hitting the cell surface are captured, it is Correspondence to: Trond Løvdal; E-mail: trond.lovdal@start.no DOI: 10.1007/s00248-007-9262-x & Volume 55, 152–161 (2008) & * Springer Science + Business Media, LLC 2007 152