Phosphorus Supply Shifts the Quotas of Multiple Elements in Algae and Daphnia: Ionomic Basis of Stoichiometric Constraints
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Ecology Letters, (2020) 23: 1064–1072 doi: 10.1111/ele.13505 LETTER Phosphorus supply shifts the quotas of multiple elements in algae and Daphnia: ionomic basis of stoichiometric constraints Abstract Punidan D. Jeyasingh,1* The growth rate hypothesis posits that the rate of protein synthesis is constrained by phosphorus Jared M. Goos,1,2 (P) supply. P scarcity invokes differential expression of genes involved in processing of most if not Patrick R. Lind,1,3 all elements encompassing an individual (the ionome). Whether such ionome-wide adjustments to Priyanka Roy Chowdhury1,4 and P supply impact growth and trophic interactions remains unclear. We quantified the ionomes of a Ryan E. Sherman1,5 resource-consumer pair in contrasting P supply conditions. Consumer growth penalty was driven by not only P imbalance between trophic levels but also imbalances in other elements, reflecting The peer review history for this arti- complex physiological adjustments made by both the resource and the consumer. Mitigating such cle is available at https://publons. imbalances requires energy and should impact the efficiency at which assimilated nutrients are com/publon/10.1111/ele.13505 converted to biomass. Correlated shifts in the handling of multiple elements, and variation in the supplies of such elements could underlie vast heterogeneity in the rates at which organisms and ecosystems accrue biomass as a function of P supply. Keywords Biomass production, consumer–resource interactions, ecological stoichiometry, ionomics, growth rate hypothesis, nutrient quotas, zooplankton. Ecology Letters (2020) 23: 1064–1072 Commonly referred to as the cell quota model, Droop’s model INTRODUCTION is widely used in ecology (Sterner & Elser 2002). The rate at which organisms produce new biomass is central The growth rate hypothesis (GRH; Elser et al. 2003) illumi- to understanding population growth and the fluxes of energy nated key cellular mechanisms underlying the relationship and materials in ecosystems (Odum 1968). Moreover, growth between growth and P quota because ribosomal RNA rate is a central parameter in models describing organismal fit- (rRNA) is the dominant cellular sink for P, accounting for c. ness because it is under direct selection or is strongly corre- 50% of total P (see Fig. 1c in Elser et al. 2003) in unicellular lated with several other fitness-relevant traits (Stearns 1992). and small (< 1 mg dry mass) multicellular organisms (Gillooly The synthesis of proteins is the primary anabolic process et al. 2005). The GRH predicts three key positive associations underlying growth (Milo & Phillips 2015; Kafri et al. 2016), between: P quota and rRNA, rRNA and protein synthesis, and protein concentration in a cell is involved in triggering and protein synthesis and growth. Because the GRH arises cell division (e.g. Cookson et al. 2010). These basic cellular from fundamental biochemical processes, it is useful in under- processes underlie the growth of organisms and populations, standing large scale ecological patterns such as the relation- and the productivity of ecosystems. ship between P supply and ecosystem productivity Approaches based on temperature, energy or nutrients have (Vollenweider 1968; Smith 1982). Nevertheless, there is con- shaped the study of biomass production at various scales (re- siderable variation in such log–log relationships, and studies viewed in Ratkowsky et al. 1982; van der Ploeg et al. 1999). have found no support for one or more of the tripartite GRH Such seminal efforts resulted in the identification of key linkages (e.g. Flynn et al. 2010; Sherman et al. 2017). extrinsic parameters that have large impacts on biomass pro- The GRH focusses on key material resources (i.e. N, P) duction (e.g. Monod 1949). The supply of phosphorus (P) is required for protein synthesis in situations where energy often important in constraining the rate of biomass produc- required for peptide anabolism is not limiting (Loladze & tion because it is more abundant in biomass than in the inor- Elser 2011), arguably due to the initial focus of ecological sto- ganic environment (Smil 2000). When extrinsic supply of P is ichiometry on factors other than energy in impacting ecosys- low, cells expend energy in sequestering P from the environ- tem productivity (Sterner et al. 1992a). Nevertheless, even ment, or recycling/reallocating internal P stores, ultimately when there is replete supply of nitrogen and phosphorus, compromising rates of biomass production (Droop 1973). growth cannot proceed without sufficient energy for 1Department of Integrative Biology, Oklahoma State University, Stillwater, 4Department of Biology, Keene State College, Keene, NH, USA OK, USA 5Biology Program, MacMurray College, Jacksonville, IL, USA 2BioFire Diagnostics, Salt Lake City, UT, USA *Correspondence: E-mail: [email protected] 3Department of Biological Sciences, University of Arkansas, Fayetteville, AR, USA © 2020 John Wiley & Sons Ltd/CNRS Letter Ionomic consequences of phosphorus limitation 1065 transcription of mRNA, and translation of proteins (e.g. Bier major ions (e.g. Kramer et al. 1986). Consequently, cells allo- 1999). The synthesis of RNA and protein utilises about 35% cate considerable energy to maintain trace metal homeostasis of available energy in cells (Buttgereit & Brand 1995) which is (Lodish et al. 2000; Balamurugan & Schaffner 2006; Worms the basis for a rule of thumb stating that incorporation of one et al. 2006), indicating a trade-off in energy allocated to the carbon atom into biomass requires c. 1 ATP (Milo & Phillips predominant energy sink, protein anabolism. We posit that 2015). Ion pumps utilise the next highest amount of energy (c. such trade-offs in the energy quotient allocated to peptide 20%; Buttgereit & Brand 1995) to maintain optimal electro- anabolism has a large role to play in the efficiency at which chemical gradients across the cell membrane. Although these assimilated N and P are used for generation of new biomass, values are approximations based on a few observations, they potentially underlying substantial variation in P use efficien- indicate that the extrinsic supply of the c. 20 elements repre- cies among genotypes (e.g. Jeyasingh et al. 2009), species (e.g. sented in biomass (Williams & Frausto da Silva 2005) can Seidendorf et al. 2010) and ecosystems (e.g. Kelley et al. substantially impact energy budgets. 2018). Moreover, extrinsic supply of an element alters the process- We quantified changes in the ionome of the green alga Sce- ing of multiple other elements inside a cell or organism. Tran- nedesmus obliquus to inorganic P supply, and fed it to Daphnia scriptomic studies capturing changes in the expression of all pulicaria genotypes and observed growth and ionomic genes in a genome in response to change in a single element responses. Specifically, we measured the ionomic response of (e.g. P) find that genes unrelated to the processing of P are algae to two P-supply conditions (LoP, 5 µmol LÀ1; HiP, differentially expressed in autotrophs (e.g. Mission et al. 2005) 50 µmol LÀ1) and predicted that concentrations of all ele- as well as heterotrophs (e.g. Jeyasingh et al. 2011). Such ments should decrease under P limitation. We fed LoP and responses impact the processing of multiple other elements HiP Scenedesmus to four genotypes of Daphnia pulicaria and and associated energy demands (e.g. Malinouski et al. 2014). measured their growth and ionomic responses and predicted Understanding the biochemical and genetic mechanisms of that daphniid growth will be slower in LoP conditions, and such correlated shifts in all elements encompassing an individ- that the concentration of all elements in the Daphnia ionome ual (i.e. the ionome), and the relevance of shifts to the expres- will be lower. Because an element in a biological system never sion of key traits such as growth rate is the central goal of the acts independently of other elements, univariate analyses of field of ionomics (Salt et al. 2008). each element in the ionome or even dimension reduction mul- Aforementioned observations indicate that information (e.g. tivariate analyses could potentially miss pivotal biological extrinsic supply, cellular quotas) on the entire suite of ele- information (Baxter 2015). As such, we identified suites of ele- ments in a system should have much ecological relevance (e.g. ments using the nutrient balance approach of Parent et al. Jeyasingh et al. 2014, 2017; Kaspari & Powers 2016; Penuelas~ (2013), and tested for treatment effects (i.e. P supply for algae et al., 2019 who refer to the ionome as the ‘elementome’). and Daphnia, and P supply and genotype for Daphnia). However, the majority of ionomic data is on autotrophs such Finally, to understand how ionome-wide changes in response as algae (e.g. Cunningham & John 2017) and land plants (re- to P supply in both algae and Daphnia impact the balance of viewed in Huang & Salt 2016). Although similar data are elements between trophic levels, and its consequences for becoming available for a few other taxa (e.g. arthropods, Daphnia growth, we constructed trophic stoichiometric ratios Goos et al. 2017; fish, Rudman et al. 2019; amphibians, Prater (TSR; Filipiak & Weiner 2014) which is an index of elemental et al., 2019), the majority of such work do not characterise imbalance between resource and consumer as a function of ionomic responses to changes in elemental supply. Unlike carbon, for each of the other (12) elements measured. We pre- autotrophs, the responses of metazoan consumer ionomes to dicted that multidimensional TSRs will be sensitive to P treat- changes in inorganic nutrient supply include associated ment and that TSRs for all elements will be negatively ionomic changes in diet (Jeyasingh & Pulkkinen 2019). correlated with growth of Daphnia. Ionomic responses of higher trophic levels to changes in key elements such as P have yet to be characterised and should have much to contribute towards understanding patterns in MATERIALS AND METHODS higher order ecology, such as variation in ecosystem produc- Study organisms and experimental setup tivity unexplained by classical limiting factors (e.g.