Steady-state bioassay approach applied to phosphorus-limited continuous cultures: A growth study of the marine chlorophyte Dunaliella salina

Scott R. Grant, Paul K. Bienfang and Edward A. Laws

Limnol. Oceanogr., 58(1), 2013, 314-324 | DOI: 10.4319/lo.2013.58.1.0314

ABSTRACT: Phosphorus-limited growth and phosphate uptake kinetics of the marine phytoplankter Dunaliella salina were studied using continuous-culture techniques. Determination of residual phosphate concentrations in the cultures were made using a steady-state bioassay approach in which short-term phosphate uptake rates as measured by 33P-radiotracer were balanced with the turnover of phosphorus in the culture. This allowed the determination of low-nanomolar ambient phosphate concentrations (8–95 nmol P L−1) at seven unique specific growth rates, which agreed closely with an alternative kinetics bioassay phosphate determination. The relationship of specific growth rate to ambient phosphate concentration for the D. salina cultures was well-described by the Monod growth equation with μmax = 1.8 d−1 and Kμ = 28 nmol P L−1. Phosphate uptake kinetics were evaluated at three growth rates and adhered to Michaelis–Menten kinetics, with evidence of growth-rate–dependent variability of uptake affinities (1.9–3.0 L µmol−1 h−1) and half-saturation constants (1.85–3.8 µmol P L−1). Maximum uptake rates were constant across growth rates when measured in either carbon-specific (220 µmol P mol C−1 min−1), volume-specific (7 amol P µm−3 min−1), cell-specific (1 fmol P cell−1 min−1), or phosphorus-specific units (7 h−1). This represents a phosphate uptake rate capacity between 200-fold and 600-fold greater than the requirements for growth. Finally, we compared the results from our study with previous studies across four phytoplankton species and found that the Monod half-saturation constant for phosphorus-limited growth (Kμ) is well-predicted by the theoretical ratio of the maximum specific growth rate to the phosphorus-specific uptake affinity.

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