Optimal N:P ratios of growth media: quantification of nutrient-replete growth rates in five ion hyperspace for Chlorella vulgaris (Dinophyceae) and Peridinium cinctum (Dinophyceae).

Authors: Evens, Terence; Niedz, Randall

Submitted to: European Journal of Phycology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/7/2010
Publication Date: 4/15/2010
Citation: Evens, T.J., Niedz, R.P. 2010. Optimal N:P ratios of growth media: quantification of nutrient-replete growth rates in five ion hyperspace for Chlorella vulgaris (Chlorophyceae) and Peridinium cinctum (Dinophyceae). European Journal of Phycology. 45(3):247-257.

Interpretive Summary: This study represents an investigation into nutrient/ion effects on the ecology, physiology and biochemistry of aquatic biota endemic to South Florida nursery/greenhouse retention ponds in order to better understand how management practices associated with floral and nursery operations may impact on-site and off-site receiving waters and drainage. We have attempted to better define the fundamental interactions between nutrients, bulk solution ions and planktonic microalgae. To this end we quantified the main effects and interactions of several primary nutrient and bulk solution ions using a novel experimental protocol developed in our laboratory, which allows us to explore several questions that have been at the forefront of aquatic ecology for years: 1) what is the 'optimal' nitrate:phosphate (N:P) ratio for any given alga, 2) how does the ionic mileu affect primary nutrition, and 3) can we characterize the ion-based niche space of algae at a degree of complexity that approximates 'real-world' conditions? This study shows that: 1) There is no truly universal 'optimal' N:P ratio for any alga; 2) Maximal growth rates can be achieved at N:P ratios that vary over several orders of magnitude. The ionic makeup, both ion proportions and total amounts, of the medium is the primary determinant of growth rate when nitrate and phosphate are present in non-limiting concentrations; and 3) The approach utilized in this study is the first successful multi-dimensional characterization of the nutrient-/ion-based fundamental niche of any alga.

Technical Abstract: In this study our principal goal was to quantify the main effects and interactions of several primary nutrient and bulk solution ions. The total ion concentration range chosen spans fresh to brackish waters (1-30 milliMolar) and explores most of the hypervolume delineated by the five ion/concentration vectors. While there may be very few ‘natural’ waters where an alga may encounter 29.5 milliMolar nitrate, there are two compelling reasons for exploring these ‘unnatural’ regions of the design space: 1) We can explore physico-biological questions such as, “are negatively-charged ion effects anion-specific or ion-specific?”, i.e. are bulk solution effects on microalgal growth rates the same regardless of the anion under consideration? and 2) It is mathematically favorable to minimize constraints on the experimental design space. Both Chlorella vulgaris and Peridinium cinctum exhibited ion-specific, as opposed to cation/anion-specific, responses, but only at the lowest total ion concentrations. These ion-specific responses were more pronounced in Peridinium cinctum and resulted in this alga growing in relatively small regions of the ion hypervolume that appear to be inaccessible to Chlorella vulgaris. Ion-specificity was replaced by a more general anion-/cation-specificity at total ion concentrations > 15 milliMolar for both algae. While there was significant overlap between the hypervolumes where each alga exhibited positive growth, Chlorella vulgaris grew much faster than Peridinium cinctum over most of this common ion-space. It is clear that the question of ‘optimal’ media N:P ratios is purely contextual. At 1 milliMolar total ion concentration Peridinium cinctum maximal growth rates can be achieved at a variety of N:P ratios, from 0.10-13.00, if 0.30 < [sodium] < 0.75 (milliMolar) and [potassium] + [chloride] = 0.00. At 30 milliMolar total ion concentration Peridinium cinctum maximal growth rates can be achieved at nitrate:phosphate ratios of 0.15-6.00 if 0.65 < [sodium] < 0.90 (milliMolar) and [potassium] + [chloride] = 0.00. Thus, maximal growth rates can be achieved with media nitrate:phosphate ratios that vary over an order of magnitude if the bulk solution conditions are amenable. This means that from a nutrient-replete growth perspective, there is no truly ‘optimal’ nitrate:phosphate ratio; there is only an optimal range of nitrate:phosphate ratios for any given medium composition. However, there are regions within the ionic hypervolume where each alga will grow well and these regions are a multivariate function of the proportion, type and concentration of each ion/nutrient present. With the conceptual approach and experimental methodologies outlined in this paper we are now able to map these regions in Euclidean space and describe them mathematically. Thus, we can quantify not only the main effects of each ion of interest, but also the interactions between them. An experiment designed from this hypervolume perspective can transcend questions of whether the ionic proportions/concentrations of artificial media are ‘ecologically relevant’, because all of the possible combinations of ions are encompassed within the experimental design hypervolume. Accordingly, the ionic composition of any given water body/medium can be reduced to a set of coordinates in the Euclidean space delineated by this type of experimental design. There is a great deal to be learned from examining biological responses to a broad range of conditions. We have no reason to expect that the medium from which an organism is isolated is optimal for that organism’s growth. This type of information is important for considering ecological issues such as competition and community dynamics within dynamic ecosystems, and for more practical matters such as mass culture of algae for biodiesel production.