Pumped Up by the Cold: Elemental Quotas and Stoichiometry of Cold-Water Diatoms

Michael W Lomas,Jeffrey W Krause,Steven E Baer,Sydney Acton

doi:10.3389/fmars.2019.00286

Abstract

Phytoplankton cellular elemental quotas are important for evaluating the coupling of nutrient cycles and assessing nutrient limitation in the ocean. While cell element quotas have been shown to vary in response to growth conditions (e.g., light, nutrient supply), an overriding trait is cell size and biovolume. Allometric relationships are lacking data for phytoplankton grown under polar temperature conditions (e.g., <5oC); limited field data using single-cell stoichiometry measurements have shown polar diatoms may have significantly different allometry versus low-latitude counterparts. In this study, 11 strains of diatoms isolated from sub/polar waters (both Arctic and Antarctic) were grown in batch growth mode. Cellular carbon (C), nitrogen (N), phosphorus (P), silica (Si), cell number and biovolume were measured during mid-exponential and early stationary growth phases. An allometric log-log relationship was observed between cell element quotas and biovolume, although the strain Odontella aurita was consistently an outlier. Including O. aurita resulted in regression slopes for C, N, P and Si that were lower than values from the reviewed literature of temperate organisms; excluding O. aurita resulted in regression slopes that were more similar to published values for each element. However, the intercepts of the elemental quotas in the allometric relationships ranged from ~5-fold to ~100-fold greater than published values depending upon the element and the growth phase, meaning that the elemental density is significantly higher for diatoms grown at cold temperatures, although the physiological mechanism(s) cannot be resolved in this study. Cellular ratios of C, N, and P were consistent with prior research, but clearly showed the importance of taxonomic variability as all strains of the genus Thalassiosira behaved more similarly to each other than to the other diatom strains tested. Si:C ratios, in contrast, for non-Thalassiosira strains were greater and approached values commonly observed for iron-limited cells. These results show that a different set of allometric scaling equations is needed when considering sub/polar diatoms relative to temperate and tropical diatoms, which will impact regional and global model parameterization. In addition, these results highlight the role of phytoplankton diversity on biogeochemistry, even within the closely aligned families of diatoms.

Highlights

The ecological and biogeochemical functions of aquatic systems are intertwined through the traits of the biological organisms that inhabit them (e.g., Margalef, 1978)
The results from this study show that diatoms grown at polar temperatures (2◦C) follow an allometric relationship similar to that of warmer water diatoms, they differ substantially in their elemental density, resulting in a significant relationship offset for each of the macronutrient elements
Biogeochemical models commonly use fixed stoichiometry to convert from a measured property to the model-currency unit (e.g., C, N) for diatoms (Chai et al., 2002) which can propagate error if diatom C or N is determined using standard allometry

Summary

Introduction

The ecological and biogeochemical functions of aquatic systems are intertwined through the traits of the biological organisms that inhabit them (e.g., Margalef, 1978). Microbial populations are incredibly diverse, even more so when one considers the results of metagenomics studies that suggest there are thousands of species yet to be identified and their biological traits characterized (e.g., Venter et al, 2004). Given the immense and poorly understood repository of species diversity among the plankton, many models simulate diversity by using functional groups as state variables, where groups are differentiated by properties which can be ground truthed with field measurements (e.g., particle size). Such an approach has been useful in global models which differentiate various classes of phytoplankton (Follows et al, 2007). Long considered a master variable from which other traits would scale (Elton, 1927), can provide a representation of plankton diversity even if it has not been experimentally quantified; this field is called allometry

Methods

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Discussion

Conclusion