Relationship between phosphate affinities and cell size and shape in various bacteria and phytoplankton

Abstract

Substrate affinity expresses the ability of an osmotroph organism to compete for a substrate at permanently low external concentrations and is thus a central parameter in conceptual and mathematical models of aquatic food webs. Assuming diffusion transport in the surrounding medium to be the limiting process at low external substrate concentrations, the theoretical maximum affinity (α max ) and its dependence on cell size and shape for a given osmotroph organism can be calculated from Fick's law of diffusion in combination with knowledge of the amount of substrate required to form a new cell. For a non-respired substrate, the actual affinity (α) can also be expressed as the biomass-specific turnover rate of the substrate, α = (TB) -1. Combining a measure of biomass (B), with determination of substrate turnover time (T), the affinity can thus be determined experimentally. We used this approach to compare measured with theoretical maximum affinities for phosphate in laboratory cultures of osmotrophic microorganisms. For bacteria and autotrophic flagellates, we found relatively good agreement between experimental and theoretical maximum values, suggesting that diffusion limitation is actually approached in P-limited cultures. Assuming P-free vacuoles, the theory predicts diatom affinities to exceed that of similarly sized flagellates. This prediction is consistent with our experimental observations. Previous reports of diatoms being unsuccessful under P-limited conditions may therefore need a more complex explanation than lack of competitive ability in diatoms.

Scaling Laws in Phytoplankton Nutrient Uptake Affinity

“…But increased diffusion will also increase affinity, and the notion of either being diffusion limited or transporter limited appears inappropriate. Experimental support for simultaneous diffusion and transporter limitation is presented in the study of Tambi et al (2009). Assuming diffusion limitation, they found relatively good agreement between experimentally measured and theoretically derived phosphate uptake affinities in bacteria and phytoplankton of different sizes.…”

“…Assuming diffusion limitation, they found relatively good agreement between experimentally measured and theoretically derived phosphate uptake affinities in bacteria and phytoplankton of different sizes. On the other hand, if a power function is fitted (by OLS) to their phytoplankton affinity measurements (Tambi et al, 2009, Table 2) vs. the ERS, a scaling exponent close to 2 is obtained (for the specific affinity, the exponent is −1). This suggests, that at least some the organisms in their study were imperfect sinks and that diffusion limitation is not a sole predictor for the observed uptake affinity.…”

“…If the cell volume is used, the unit becomes s −1 , i.e., the same as in growth rate. Specific affinity is often used as an index of competitive uptake ability for nutrients and therefore used in comparisons between species (e.g., Tambi et al, 2009). Unless otherwise stated, in the present study affinity refers to the quantity with unit µm 3 s −1 cell −1 .…”

“…This is particularly useful for the analyses of competitive relationships in oligotrophic conditions. Further, the scaling relationship between nutrient uptake affinity and organism size, often referred to as the master trait (Litchman and Klausmeier, 2008), is of considerable interest (e.g., Tambi et al, 2009;Edwards et al, 2012;Andersen et al, 2015). Encounters between microbial organisms and nutrient molecules are facilitated by molecular diffusion.…”

Seasonal phosphorus and carbon dynamics in a temperate shelf sea (Celtic Sea)

“…An alternative expression of the affinity of Pi-uptake is a biomass-specific turnover rate (1/P i turnover x POP), where the biomass is represented by POP and the units are proportional to the volume of water cleared of substrate per unit biomass per unit time (Thingstad and Rassoulzadegan, 1999;Tambi et al, 2009). For CCS, average values for November were 1.1 L pmol P -1 h -1 and were then 5-times higher in April (5.4 L pmol P -1 h -1 ) and 10-times higher in July (11.1 L pmol P -1 h -1 );…”