Evol Ecol (2015) 29:551–563
DOI 10.1007/s10682-015-9760-0
ORIGINAL PAPER
Genotypic differences in phosphorus use physiology
in producers (Chlamydomonas reinhardtii)
and consumers (Daphnia pulex) interact to alter primary
and secondary production
Patrick R. Lind1 • Punidan D. Jeyasingh1
Received: 30 September 2014 / Accepted: 23 March 2015 / Published online: 28 March 2015
Ó Springer International Publishing Switzerland 2015
Abstract By considering the relative abundance of elements in trophic interactions,
ecological stoichiometry makes predictions about key ecological processes such as biomass production and consumer-driven nutrient recycling. Theoretical and empirical work
has focused on interspecific variation in elemental composition, and stoichiometric imbalances between resources and consumers in determining productivity, particularly at the
base of foodwebs. Recent work has found considerable intraspecific variation in elemental
composition. We know little about the ecological relevance of such variation, and whether
predictions of stoichiometric theory hold at the intraspecific level. Here, we used two
genotypes of a primary producer Chlamydomonas reinhardtii, and two genotypes of a
primary consumer Daphnia pulex, which are already known to vary considerably in their
phosphorus (P) use physiology, under conditions of P abundance and limitation, to explore
whether such intraspecific differences alter primary as well as secondary production.
Specifically, we tested whether there are intraspecific differences in the carbon: phosphorus
(C:P) stoichiometry of Chlamydomonas genotypes, whether such differences affect growth
and abundance of the two Daphnia genotypes, and whether the two Daphnia genotypes
had distinct effects on primary production and growth of the two Chlamydomonas genotypes. We found significant differences in C:P stoichiometry between the two Chlamydomonas genotypes in both P supply conditions. Such intraspecific differences altered the
growth of Daphnia genotypes, and affected the outcome of genotypic competition. Finally,
Daphnia genotype affected primary production, and interacted with P supply to distinctly
affect the growth of the two Chlamydomonas genotypes. Together, our results highlight the
potential ecological relevance of intraspecific differences in nutrient use physiology and
elemental composition, and the utility of ecological stoichiometry in understanding such
consequences.
Patrick R. Lind and Punidan D. Jeyasingh have contributed equally toward creating this manuscript.
& Patrick R. Lind
[email protected]
1
Department of Integrative Biology, Oklahoma State University, 501 Life Sciences West, Stillwater,
OK 74078, USA
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Keywords Consumer-driven nutrient cycling Ecological stoichiometry Eco-evolutionary dynamics Genetic variation Genotype by environment interactions Psr1
Introduction
Trophic interactions are key ecological processes that underlie ecosystem functions such as
the cycling of energy and materials. The framework of ecological stoichiometry (ES)
utilizes information on the elemental content of resources and consumers to make predictions about a wide array of ecological processes including nutrient recycling and biomass production at both the producer and consumer levels (Sterner et al. 1992; Elser and
Urabe 1999; Hall 2009; Sardans et al. 2011). For example, Elser et al. (1988) found that a
shift in the zooplankton community from phosphorus (P)-poor copepods to P-rich cladocerans altered the parameters limiting primary production, from N limitation to P limitation
via differential consumer-driven recycling. Differential P recycling to the algal community
not only alters the parameters limiting primary production, but also algal and zooplankton
community structure, and pelagic biogeochemistry (Sterner and Hessen 1994). It is clear
that species stoichiometry can illuminate some of the functional mechanisms that underlie
consumer-resource interactions, and their far-reaching population, community, and
ecosystem consequences (Sterner and Elser 2002; Cross et al. 2003; Moe et al. 2005; Hall
2009).
Most work in ecological stoichiometry has considered elemental content as a fixed,
species-level trait (see Jeyasingh and Weider 2007; Bertram et al. 2008; Jeyasingh et al.
2014). More recent work has discovered substantial intraspecific variation in elemental
content of consumers (El-Sabaawi et al. 2012a, b, Goos et al. 2014) and in the physiological rates at which key elements, such as P, are utilized and excreted (Jeyasingh et al.
2009; Roy Chowdhury et al. 2014; Frisch et al. 2014). Such variation is known to alter the
relative abundance of consumer genotypes under contrasting P supply regimens (Weider
et al. 2005; Jeyasingh and Weider 2005; Jeyasingh et al. 2009). Thus, it is likely that
intraspecific variation in somatic stoichiometry, and in the physiological kinetics of elements can have unique ecological effects, and may also potentially become visible to
selection (Jeyasingh et al. 2014).
To date, however, we know little about how genotypic differences in producers and
consumers in key stoichiometrically-relevant traits interact to affect important ecological
processes, including how genotypic differences in the processing of P affect the rate of
primary and secondary production. We posit that genetically based differences in P content
and/or utilization in resource genotypes will affect the growth and frequency of consumer
genotypes differing in P content and/or utilization. Moreover, we predict that differential P
recycling of consumer genotypes will, in turn, alter the frequency of consumer genotypes
varying in P content and/or utilization. Here, we explored these interactions in the
Daphnia-Chlamydomonas consumer-resource system.
Specifically, we used two genotypes of the green alga Chlamydomonas reinhardtii as a
resource for two genotypes of the microcrustacean primary consumer, Daphnia pulex. The
mutant Chlamydomonas genotype (MT), CC-4267, lacks a functional Phosphate Starvation Response (psr1) gene, and subsequently is unable to acclimatize to P limiting conditions compared to the wildtype (WT), CC-1690. Consequently, psr1 mutants are less
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efficient in utilizing P (Shimogawara et al. 1999). Two genotypes of Daphnia, Genotypes 1
and 2 (G1, G2), respectively, were chosen as consumers because G1 is known to be less
efficient in P use, assimilating less and recycling more of what it acquires, compared to G2
(Roy Chowdhury et al. 2014), which underlies differential growth (Jeyasingh and Weider
2005) and genotypic frequency (Weider et al. 2005) of these genotypes under high and low
phosphorus supply conditions.
Using these experimental organisms we tested the following predictions to better understand the role of intraspecific differences in P use at both producer and consumer
trophic levels in determining primary and secondary production in microcosms: (i) C:P
stoichiometry of Chlamydomonas genotypes will exhibit genotype 9 environment interactions under two P supply conditions, (ii) such variation in C:P among algal genotypes
interact with Daphnia genotypes to affect Daphnia growth, (iii) growth differences due to
P supply, genotype (Chlamydomonas and Daphnia), and their interactions should determine the relative abundance of either Daphnia genotype in intraspecific competition trials,
and (iv) Daphnia genotypes should have distinct effects on rates of primary production due
to differences in P recycling, and such differences in consumer driven P recycling should
have unique effects on the growth of the two Chlamydomonas genotypes.
Methods
Experimental organisms
We purchased two genotypes of Chlamydomonas reinhardtii (Chlamydomonas Resource
Center; http://chlamycollection.org/), wild (CC-1690) and mutant (CC-4267) types, each
expressing a different response during phosphorus limitation (Shimogawara et al. 1999;
Pröschold et al. 2005). The wildtype is the same genotype used in the sequencing of the
Chlamydomonas Genome Project (Pröschold et al. 2005). The mutant genotype lacks a
functional psr1 gene. This gene is critical in the physiological responses to low environmental P supply. Mutations in psr1 have been shown to reduce growth rate, photosynthetic
rate, and other responses in Chlamydomonas under condition of low P supply (Shimogawara et al. 1999; Wykoff et al. 1999). Upon receipt, we grew both genotypes in 1L
continuous flow chemostats using COMBO (Kilham et al. 1998) media at 20 °C with an
18:6 light:dark cycle.
The two Daphnia pulex genotypes, G1 and G2, were isolated from a pond in northwest
Iowa (Weider et al. 2004) and propagated parthenogenetically in the laboratory for several
years. We selected these genotypes for their differences in P use efficiency with G1
exhibiting lower efficiency than G2 (Roy Chowdhury et al. 2014). Under conditions of
high P availability, G1 grows much faster than G2; however, while under diminished P
supply, the reverse is true (Weider et al. 2005; Jeyasingh and Weider 2005). We raised
each genotype in 1L jars in nitrogen (N) and P free COMBO, feeding them a diet of
1 mgC L-1 of Scenedesmus acutus algae cultured in high P conditions. These jars were
stored in a growth chamber set at a temperature of 20 °C with an 18:6 light:dark cycle.
Prior to the experiments, we removed gravid females from the culture jars and placed them
individually in 100 mL jars of N/P free COMBO. The resulting neonates were used for the
experiments.
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Does phosphorus supply alter the stoichiometry of Chlamydomonas
genotypes?
For both genotypes of Chlamydomonas, we set up continuous flow chemostats of high
(50 lmol L-1) and low P (5 lmol L-1) media (Kilham et al. 1998) at 20 °C and constant
light (*120 lmol m-2 s-1). In order to determine algal stoichiometry, we filtered algae
from experimental chemostats onto pre-combusted (550 °C for 2 h) and pre-weighed GF/C
filters (Whatman, Maidstone, UK), and dried at 60 °C for 72 h. C-content was determined
using an automated CHNOS analyzer (Elementar VarioMICRO, NJ, USA). Total P content
was quantified by a modified sulfuric acid digestion method (APHA 1992) and verified
using a spinach standard (NIST 1570a). We then converted C and P content by mass into
molar C:P for analyses.
Does Chlamydomonas genotype and stoichiometry affect the growth
of Daphnia genotypes?
In order to test whether intraspecific differences in prey stoichiometry elicit noticeable
differences in growth of consumer genotypes, we collected twenty neonates from both
Daphnia genotypes within 24 h of their birth and divided them among four experimental
treatments: (i) wildtype Chlamydomonas grown in high P conditions (WH), (ii) wildtype
grown in low P media (WL), (iii) mutant grown in high P (MH), and (iv) mutant grown in a
low P environment (ML). Each individual Daphnia was placed in 100 mL of N/P free
COMBO and fed 1 mgC L-1 day-1 of Chlamydomonas from the respective genotype and
P treatment groups mentioned above with media changed before each feeding. We replicated each treatment five times. To measure Daphnia length, we recorded the distance
from the top of the head to the base of the tail spine (Ingle 1936) and defined growth as the
change in length from birth to day five [a reliable proxy for mass gain in these two
genotypes (Jeyasingh and Weider 2005; Jeyasingh 2007)].
Does Chlamydomonas genotype and stoichiometry affect competition
between Daphnia genotypes?
To determine whether growth differences between Daphnia genotypes driven by Chlamydomonas genotype and P supply alters their ability to compete for resources, we
performed intraspecific competition trials. We placed six \24 h-old daphniids from both
G1 and G2 (12 individuals total) in 500 mL jars of N/P-free COMBO media. These jars
were fed 1 mgC L-1 day-1 of WH, MH, WL, and ML Chlamydomonas. Daphnia were
carefully transferred to a new jar with fresh media and algae every third day. After
4 weeks, we gently stirred the jars to evenly distribute daphniids in the water column and
took a 10 % sample from each jar. We counted the number of Daphnia in the sample and
genotyped every individual in the sample using allozyme electrophoreses at the Pgi locus
(Hebert and Beaton 1989).
Does Daphnia genotype affect the growth of Chlamydomonas genotypes?
We designed the final experiment to test whether differing consumer phosphorus use
efficiencies (and thus, P excretion) alters the growth of producer genotypes. G1 and G2
Daphnia genotypes of similar size, which had previously been starved for 3 h to reduce gut
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content, were divided into the same four experimental groups as the previous experiment
(i.e. WH, MH, WL, ML at 1 mgC L-1 day-1). We placed five daphniids of a single
genotype in each 50 mL jar with N/P-free COMBO, accounting for one experimental unit.
Each treatment was replicated five times.
After 2 days, we removed Daphnia from their jars, and filtered the water through a
0.2 lM GF/C filter to remove any particulate matter. In order to determine the effect of
Daphnia genotype on algal growth, we added a 1 mL volume of slurry containing a
homogenous number of cells of the same Chlamydomonas genotype by P level as the
filtrate to each 50 mL jar. We took the new solution and placed it on a lit shaker table to
reduce settling at 20 °C under constant light. We used the change in light absorbency, a
simple, yet reliable measure of algal growth (Sivakumar and Rajendran 2013), of each
sample from inoculation until 48 h had elapsed to assess growth.
Statistical analyses
To test the interactive effects of genotype and phosphorus level on Chlamydomonas C:P in
our first experiment, we used a two-way analysis of variance (ANOVA) and Tukey’s
honest significant difference (HSD) post hoc analysis to confirm differences. The interactive effects of Daphnia genotype, Chlamydomonas genotype, and phosphorus level on
Daphnia growth were tested using a three-way ANOVA. A two-way ANOVA was performed to test the effects of Chlamydomonas genotype and phosphorus level on the
genotypic ratio of Daphnia. Finally, a three-way ANOVA was performed to test the
effects of P supply, Daphnia genotype, and Chlamydomonas genotype on algal growth.
We performed Levene’s Test for equality of variance on all treatments for each experiment and found no significant variation within any of the experiments. However, in
order to confirm that differences in variation did not influence the results of the ANOVA’s,
we performed Welch’s Test for all experiments. We report only ANOVA results because
the results of the two tests were similar. All tests were performed using IBM SPSS
Statistics Version 22.
800
C:P Ratio in Chlamydomonas
Fig. 1 Mean molar C:P ratios of
each algal genotype 9 P
treatment with 95 % confidence
intervals shown. Treatments are
divided by wildtype (W) or
mutant (M) Chlamydomonas
genotype and by high (H) or low
(L) phosphorus level. Letters
indicate significantly
homogenous groups based on
Tukey’s HSD (p \ 0.05)
B
700
600
500
C
400
300
200
AC
A
100
0
WH
MH
WL
ML
Treatment
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Results
Differences in the stoichiometry of Chlamydomonas genotypes in response
to phosphorus supply
Phosphorus supply invoked strong, genotype-specific responses in the stoichiometry of
Chlamydomonas (Fig. 1, Table 1a, ‘‘Appendix’’). In the high P treatments molar C:P was
found to be approximately 110 and 190 for wildtype and mutant Chlamydomonas genotypes, respectively (Fig. 1; Table 1a). In the low P treatments, the same genotypes yielded
mean ratios of 620 and 330.
Table 1 Results of separate ANOVAs showing significant factors found to affect Chlamydomonas C:P (a),
Daphnia growth (b), relative abundance of the two Daphnia genotypes (c), and growth of the two Chlamydomonas genotypes (d)
F
df
p
(a)
Chlamydomonas C:P
Genotype
7.318
1,12
0.027
P Level
68.158
1,12
<0.001
Genotype 9 P level
22.116
1,12
0.002
0.19
(b)
Daphnia growth
Algae genotype
1.7534
1,32
Daphnia genotype
1.5411
1,32
0.22
107.0205
1,32
<0.001
P treatment
Algae genotype 9 daphnia genotype
Algae genotype 9 P treatment
4.2808
1,32
0.046
16.4452
1,32
<0.001
Daphnia genotype 9 P treatment
0.1096
1,32
0.74
Algae genotype 9 daphnia genotype 9 P treatment
0.1096
1,32
0.74
(c)
Daphnia ratio
Algae genotype
P treatment
Algae genotype 9 P treatment
17.654
100.25
24.175
1,12
0.0012
1,12
<0.001
1,12
<0.001
(d)
Absorbance
Algae genotype
0.1316
1,32
0.72
Daphnia genotype
4.2247
1,32
0.048
P treatment
138.9676
1,32
<0.001
Algae genotype 9 daphnia genotype
22.8086
1,32
<0.001
Algae genotype 9 P treatment
<0.001
38.0228
1,32
Daphnia genotype 9 P treatment
0.2339
1,32
0.63
Algae genotype 9 daphnia genotype 9 P treatment
0.1791
1,32
0.68
Bold values indicate significance at p \ 0.05
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Phosphorus supply and Chlamydomonas genotype interact to alter the growth
of Daphnia genotypes
We found significant interactions between Chlamydomonas genotype and phosphorus
treatment, and Chlamydomonas genotype and Daphnia genotype in affecting secondary
production (Fig. 2; Table 1b). Both Daphnia genotypes exhibited significantly decreased
growth rate under a diet of wildtype Chlamydomonas cultured under low phosphorus
conditions, while no significant differences in growth were found in high P conditions
(Fig. 2a). Growth of the G1 Daphnia was significantly slower when fed the Chlamydomonas wildtype, compared to the mutant strain under low phosphorus conditions, while
the G2 Daphnia genotype did not exhibit such interactions (Fig. 2b).
Relative abundance of Daphnia genotypes is impacted by Chlamydomonas
genotype and phosphorus supply
Phosphorus supply had a significant interaction with Chlamydomonas genotype to influence the outcome of competition between the Daphnia genotypes (Table 1c). Under high P
supply conditions G1 was more frequent, while G2 was more frequent in low P conditions
(Fig. 3a). In addition, Chlamydomonas genotype significantly affected the outcome of
competition in high P conditions, with G1 becoming more frequent when feeding on the
wildtype Chlamydomonas genotype compared to the mutant (Fig. 3b).
Daphnia genotype and phosphorus supply interact to affect the growth
of Chlamydomonas genotypes
Daphnia growth per day (mm)
There were significant interactions affecting algal growth between both Chlamydomonas
genotype and Daphnia genotype and P treatment (Fig. 4; Table 1d). Growth of the two
Chlamydomonas genotypes was significantly different in high P treatments only when
grown in media that previously contained the G1 Daphnia genotype, and no such differences were found when grown in media that previously contained the G2 Daphnia
0.25
a
Wild
b
WH
MH
Mutant
0.2
WL
ML
0.15
0.1
0.05
0
High
Low
Phosphorus level
G1
G2
Daphnia genotype
Fig. 2 Mean growth rate per day of Daphnia by Chlamydomonas genotype in each P treatment (a), and
differences in Daphnia growth rates per treatment by Daphnia genotype (b). Treatments are divided by
wildtype (W) or mutant (M) Chlamydomonas genotype and by high (H) or low (L) phosphorus level. 95 %
confidence intervals shown. Mean values are offset on the x-axis for visualization purposes only
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70
a
G1
60
G2
Daphnia per liter
Fig. 3 Mean number of
individuals per genotype per liter
in Daphnia competition trials (a),
and ratio of G1 Daphnia to total
sampled (b). Treatments are
divided by wildtype (W) or
mutant (M) Chlamydomonas
genotype and by high (H) or low
(L) phosphorus level. 95 %
confidence intervals shown.
Letters indicate significantly
homogenous groups based on
Tukey’s HSD (p \ 0.05)
Evol Ecol (2015) 29:551–563
50
40
30
20
10
0
0.9
b
0.8
A
B
G1:Total
0.7
0.6
0.5
0.4
C
C
0.3
0.2
0.1
0
WH
MH
WL
ML
Treatment
0.04
Change in absorbance (nm)
Fig. 4 Mean change in
absorbency of each
Chlamydomonas treatment by
Daphnia genotype over 48 h.
Treatments are divided by
wildtype (W) or mutant
(M) Chlamydomonas genotype
and by high (H) or low
(L) phosphorus level. 95 %
confidence intervals shown.
Mean values are offset on the
x-axis for visualization purposes
only
WH
MH
WL
ML
0.035
0.03
0.025
0.02
0.015
0.01
0.005
0
G1
G2
Daphnia genotype
genotype. However, in the low P treatments, growth of mutant Chlamydomonas was
significantly greater in media that previously contained the G2 Daphnia genotype compared to the wildtype, while no such differences were observed between Chlamydomonas
genotypes in low P media previously containing the G1 Daphnia genotype.
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Discussion
We found that differences in the physiology of phosphorus use within a species can have
significant impacts across trophic levels, affecting the flow of energy and nutrients in a
simple consumer-resource system. Specifically, we found that phosphorus supply and
genetic differences in C:P stoichiometry of the resource (Fig. 1) affects the growth of
consumers (Fig. 2), and the outcome of competition between consumer genotypes (Fig. 3).
In addition, consumer genotype affected primary production (Fig. 4). Together, these results show that intraspecific differences can be important in trophic interactions, and that
existing (interspecific) stoichiometric models (Sterner and Elser 2002) are useful in understanding the ecological relevance of such differences.
We selected the mutant Chlamydomonas genotype due to its lower ability to uptake
phosphorus under limiting conditions (Shimogawara et al. 1999; Wykoff et al. 1999).
However, after analyzing C:P ratios, we found that the mutant had a lower ratio in the low
P treatment than the wild genotype (Fig. 1). Because inactivation of psr1 not only causes a
reduction in P scavenging ability but also results in the down regulation of photosynthesis
under P limitation (Shimogawara et al. 1999), lower C:P of the mutant in the low P
treatment is mostly likely a function of both P sequestration as well as C fixation. While we
cannot rule out the potential for other changes in the mutant type to have affected its
physiology, these results demonstrate substantial intraspecific differences in producer
stoichiometry.
As the second experiment shows, genotypic differences in producer stoichiometry in
response to resource availability affected consumer growth rates (Fig. 2). As expected, P
supply, when coupled with Chlamydomonas genotype, had a major effect on Daphnia
growth. Moreover, we observed the lowest Daphnia growth rates (Fig. 2a) when daphniids
were fed wildtype Chlamydomonas under low P supply conditions, which also had higher
C:P compared to mutants (Fig. 1). This difference in Daphnia growth due to Chlamydomonas genotype was particularly apparent in the P inefficient G1 Daphnia genotype
(Fig. 2b). These results suggest that differences in the efficiency of P use between these
genotypes (Roy Chowdhury et al. 2014) may be an important parameter determining
growth.
Intraspecific competition experiments between the two Daphnia genotypes under the
four dietary conditions indicated that genotypic growth rate had no apparent relationship
with the density of each genotype. Although there were no differences in growth between
the two Daphnia genotypes in high P conditions (Fig. 2b), G1 was more abundant than G2
in competition jars (Fig. 3a). Weider et al. (2005) reported similar outcomes of intraspecific competition between the same two D. pulex genotypes under a diet of a single
genotype of Scenedesmus acutus differing in C:P stoichiometry. These results are consistent with previous studies that found juvenile growth rate was not useful in explaining
the outcome of genotypic competition under contrasting stoichiometrically-explicit conditions (Weider et al. 2008). It is likely that differences in other key life history traits (e.g.,
age and size and maturity, lifetime fecundity, and lifespan) between the two genotypes may
underlie observed competition results. For example, under conditions of low nutrient
quality, some Daphnia have been observed to alter the allocation of resources between
somatic growth and reproduction (Weers and Gulati 1997; Becker and Boersma 2003;
Ravet and Brett 2006). These Daphnia sacrifice growth in favor of investment in reproduction. A similar shift in resource use could have contributed to the findings of this
study (see Jeyasingh and Weider 2005).
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Regardless, it is clear that C:P stoichiometry of Chlamydonomas (Fig. 1) was negatively
related to the frequency of the G1 Daphnia genotype (Fig. 3b). Previous work has shown
that G1 is inefficient in retaining assimilated P compared to G2, while the reverse is true in
the case of C (Roy Chowdhury et al. 2014). Because a high C:P diet is not only P limiting,
but also C replete, consumers can mitigate such stoichiometric imbalances not only by
efficient P use, but also inefficient C use (e.g., Jeyasingh 2007). Thus, physiological
variation in C and P use among consumer genotypes could interact with resource C:P
stoichiometry to determine the outcome of intraspecific competition (Jeyasingh et al.
2009). Specifically, these results indicate that the competitive advantage of G2 in high C:P
conditions may not only be due to efficient P use, but also inefficient C use (Roy
Chowdhury et al. 2014). On the other hand, under low C:P conditions, more efficient C use
and inefficient P use by G1 may be advantageous. Furthermore, our results indicate that
such intraspecific differences in consumer nutrient use physiology can interact with even
slight differences in C:P among producer genotypes to affect the outcome of competition.
Changes in the abundance of specific consumer genotypes differing in nutrient use
should affect the rate at which nutrients are resupplied to producers. We found that when
grown in media that previously contained the G1 Daphnia genotype, the wildtype Chlamydomonas cultured in high P conditions grew faster than the mutant, while no differences were found when grown in media that previously contained G2 Daphnia (Fig. 4).
This difference could be partially explained by increased P release by G1 compared with
G2 under high P conditions (Roy Chowdhury et al. 2014). These results indicate that
intraspecific differences in P use by the consumer can feedback to affect the growth of
producer genotypes. However, in the low P treatments, we found that algae raised in
media that had previously contained G2 Daphnia had higher growth rates, even though the
amount of P available should be substantially less than in the G1 treatments, indicating
that although differences in P use physiology are an important factor in determining
growth rates, other mechanisms are also at play. Many elements are required for optimal
growth, and interactive effects between P limitation and other elements, particularly trace
metals, have been shown to occur in Daphnia resulting in altered stoichiometry (Roy
Chowdhury 2014). These changes in consumer stoichiometry could have led to the
changes observed in this study, but further work needs to be conducted to elucidate these
interactions.
This study on only two genotypes of producers and consumers has obvious limitations.
Whether the degree of standing genetic variation in P-use physiology and associated
ecological implications, as explored here, are relevant in natural populations remains an
important frontier. Nevertheless, these results highlight the importance of considering
intraspecific variation in nutrient use physiology while parameterizing stoichiometric
models, and applying predictions of such models to understand patterns in the field.
These experiments indicate that the genetic differences in nutrient physiology can
drive differential flux of P between the producers and consumers, with important ecological consequences such as biomass production, and evolutionary consequences such
as changes in genotypic frequency. For example, it is possible that P-efficient Daphnia
and Chlamydomonas genotypes will be more prevalent in low P (oligotrophic) conditions, reinforcing low P supply. As eutrophication proceeds, P inefficient genotypes
could become abundant, thereby increasing the amount of soluble P available. Although
this scenario is hypothetical, Frisch et al. (2014) found a corresponding change in
Daphnia P use before and during elevated P levels (i.e. eutrophication). This Daphnia
population showed striking shifts in population genetic structure before and during
changes in the amount of P in the lake. Moreover, physiological assays on genotypes
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resurrected from eggs laid before eutrophication were found to be more P efficient than
extant descendants. Such microevolutionary shifts in consumer physiology could have,
as examined in this study, altered the amount of P resupplied to the algae, with subsequent effects on algal stoichiometry, and rates of primary and secondary production
(Elser and Urabe 1999). Although more work is needed, these results support the notion
that nutrient content and use are ideal traits to study eco-evolutionary dynamics. As
such, attention to elemental traits should be useful in understanding the ecological
relevance of genetic variation, and evolutionary change (Jeyasingh et al. 2014), both of
which are central challenges in contemporary evolutionary ecology (e.g., Bolnick et al.
2011; Schoener 2011).
Acknowledgments This work was supported by NSF Grant No. 0924401 to PDJ. We thank JM Goos, KD
Gustafson, two anonymous reviewers, and MD Hall for comments that improved this manuscript.
Appendix
See Fig. 5.
Carbon (%)
50
a
40
30
20
10
0
Nitrogen (%)
6
b
4
2
0
1.5
Phosphorus (%)
Fig. 5 Mean percent carbon (a),
nitrogen (b), and phosphorus
(c) found in Chlamydomonas.
Treatments are divided by
wildtype (W) or mutant
(M) Chlamydomonas genotype
and by high (H) or low
(L) phosphorus level. 95 %
confidence intervals shown
c
1
0.5
0
WH
MH
WL
ML
Treatment
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