Ivor R. Elrifi

STEADY-STATE LUXURY CONSUMPTION AND THE CONCEPT OF OPTIMUM NUTRIENT RATIOS: A STUDY WITH PHOSPHATE AND NITRATE LIMITED SELENASTRUM MINUTUM (CHLOROPHYTA)1

Selenastrum minutum (Naeg.) Collins was grown over a wide range of growth rates under phosphate or nitrate limitation with non‐limiting nutrients added to great excess. This resulted in saturated luxury consumption. The relationships between growth rate and cell quota for the limiting nutrients were well described by the Droop relationship. The observed variability in N cell quota under N limitation as reflected in kQ·Qmax−1*, was similar in magnitude to previously reported values but kQ·Qmax−1* for P under P limitation was greater than previously reported for other species. These results were evaluated in light of the optimum ratio hypothesis. Our findings support previous work suggesting that the use of a single optimum ratio (kQi·KQj−1) is inappropriate for dealing with a species growing under steady‐state nutrient limitation. Under these conditions the optimum ratio should be viewed as a growth rate dependent variable. Two approaches for testing the growth rate dependency of optimum ratios are proposed.

PREDICTING THE KINETICS OF DISSOLVED INORGANIC CARBON LIMITED GROWTH FROM THE SHORT-TERM KINETICS OF PHOTOSYNTHESIS IN SYNECHOCOCCUS LEOPOLIENSIS (CYANOPHYTA)1

The blue‐green alga (Cyanobacterium) Synechococcus leopoliensis (Racib.) Komarek was grown in dissolved inorganic carbon [DIC]‐limited chemostats over the entire range of growth rates. At each growth rate, the kinetics of photosynthesis with respect to [DIC] and the maximal rate of photosynthesis (Pmax) were determined. The half‐saturation constant for [DIC]‐limited photosynthesis (K1/2DIC) for cells growing below 1.7 d−1 was constant (4.7 μM) whereas for growth rates between 1.7 d−1 and 2.1 d−1 (μmax) the kinetics of photosynthesis were multiphasic with an apparent K1/2DIC between 1.5–2.0 mM. Pmax increased in a linear fashion with growth rate for growth rates below 1.7 d−1. No trend in Pmax was apparent for growth rates greater than 1.7 d−1. These kinetic parameters were used to predict a growth rate versus [DIC] relationship. Results show that the Monod relationship is a physiologically valid expression of growth as a function of [DIC] provided (K1/2DIC) remains constant. The major change in (K1/2DIC) as μ approaches μmax results in the conclusion that two separate and distinct Monod equations must be used to describe growth as a function of DIC over the entire growth range. These results point to a major discontinuity in the μ vs. [DIC] curve at 1.7 d−1 which corresponds to the change from high to low affinity photosynthetic kinetics. We believe these results account for the previously described deficiencies of the Monod equation in describing [DIC]‐limited algal growth.

Inexpensive, Computer-Automated HPLC for Ion Exchange Separation and Quantification of Amino Acids in Physiological Fluids

Abstract An inexpensive, computer-automated HPLC for separation and quantification of amino acids in physiological fluids is described. The system offers fully automated equipment control, data collection, processing and storage capabilities. The component nature of the system and the software flexibility permit extensive system modification, accomodating a wide variety of different separatory procedures which are not possible with many dedicated amino acid analysers. The system uses a lithium-based ion exchange column with post-column o-phthalaldehyde derivatization. A time of 128 minutes, including column regeneration, is required for separation of all amino acids through to arginine. The advantages of post-column derivatization over pre-column derivatization methods for post-separatory amino acid techniques are discussed. Accurate quantification of radiolabel in amino acids is demonstrated.