Changes in free amino acid concentrations during osmotic response in the intertidal copepod Tigriopus californicus

Interactive effects of temperature and salinity on metabolism and activity of the copepod Tigriopus californicus

In natural environments, two or more abiotic parameters often vary simultaneously, and interactions between co-varying parameters frequently result in unpredictable, non-additive biological responses. To better understand the mechanisms and consequences of interactions between multiple stressors, it is important to study their effects on not only fitness (survival and reproduction) but also performance and intermediary physiological processes. The splash-pool copepod Tigriopus californicus tolerates extremely variable abiotic conditions and exhibits a non-additive, antagonistic interaction resulting in higher survival when simultaneously exposed to high salinity and acute heat stress. Here, we investigated the response of T. californicus in activity and oxygen consumption under simultaneous manipulation of salinity and temperature to identify whether this interaction also arises in these sublethal measures of performance. Oxygen consumption and activity rates decreased with increasing assay salinity. Oxygen consumption also sharply increased in response to acute transfer to lower salinities, an effect that was absent upon transfer to higher salinities. Elevated temperature led to reduced rates of activity overall, resulting in no discernible impact of increased temperature on routine metabolic rates. This suggests that swimming activity has a non-negligible effect on the metabolic rates of copepods and must be accounted for in metabolic studies. Temperature also interacted with assay salinity to affect activity, and with acclimation salinity to affect routine metabolic rates upon acute salinity transfer, implying that the sublethal impacts of these co-varying factors are also not predictable from experiments that study them in isolation.

Multigenerational Life-History Responses to pH in Distinct Populations of the Copepod Tigriopus californicus

Intertidal zones are highly dynamic and harsh habitats: organisms that persist there must face many stressors, including drastic changes in seawater pH, which can be strongly influenced by biological processes. Coastal ecosystems are heterogeneous in space and time, and populations can be exposed to distinct selective pressures and evolve different capacities for acclimation to changes in pH. Tigriopus californicus is a harpacticoid copepod found in high-shore rock pools on the west coast of North America. It is a model system for studying population dynamics in diverse environments, but little is known about its responses to changes in seawater pH. I quantified the effects of pH on the survivorship, fecundity, and development of four T. californicus populations from San Juan Island, Washington, across three generations. For all populations and generations, copepod cultures had lower survivorship and delayed development under extended exposure to higher pH treatments (pH 7.5 and pH 8.0), whereas cultures maintained in lower pH (7.0) displayed stable population growth over time. Reciprocal transplants between treatments demonstrated that these pH effects were reversible. Life histories were distinct between populations, and there were differences in the magnitudes of pH effects on development and culture growth that persisted through multiple generations. These results suggest that T. californicus might not have the generalist physiology that might be expected of an intertidal species, and it could be adapted to lower average pH conditions than those that occur in adjacent open waters.

Elevated Salinity Rapidly Confers Cross-Tolerance to High Temperature in a Splash-Pool Copepod

Accurate forecasting of organismal responses to climate change requires a deep mechanistic understanding of how physiology responds to present-day variation in the physical environment. However, the road to physiological enlightenment is fraught with complications: predictable environmental fluctuations of any single factor are often accompanied by substantial stochastic variation and rare extreme events, and several factors may interact to affect physiology. Lacking sufficient knowledge of temporal patterns of co-variation in multiple environmental stressors, biologists struggle to design and implement realistic and relevant laboratory experiments. In this study, we directly address these issues, using measurements of the thermal tolerance of freshly collected animals and long-term field records of environmental conditions to explore how the splash-pool copepod Tigriopus californicus adjusts its physiology as its environment changes. Salinity and daily maximum temperature-two dominant environmental stressors experienced by T. californicus-are extraordinarily variable and unpredictable more than 2-3 days in advance. However, they substantially co-vary such that when temperature is high salinity is also likely to be high. Copepods appear to take advantage of this correlation: median lethal temperature of field-collected copepods increases by 7.5°C over a roughly 120 parts-per-thousand range of ambient salinity. Complementary laboratory experiments show that exposure to a single sublethal thermal event or to an abrupt shift in salinity also elicits rapid augmentation of heat tolerance via physiological plasticity, although the effect of salinity dwarfs that of temperature. These results suggest that T. californicus's physiology keeps pace with the rapid, unpredictable fluctuations of its hypervariable physical environment by responding to the cues provided by recent sublethal stress and, more importantly, by leveraging the mechanistic cross-talk between responses to salinity and heat stress.

Tolerance Patterns and Transcriptomic Response to Extreme and Fluctuating Salinities across Populations of the Intertidal Copepod Tigriopus californicus

AbstractPopulations that tolerate extreme environmental conditions with frequent fluctuations can give valuable insights into physiological limits and adaptation. In some estuarine and marine ecosystems, organisms must adapt to extreme and fluctuating salinities, but not much is known about how varying salinities impact local adaptation across a wide geographic range. We used eight geographically and genetically divergent populations of the intertidal copepod Tigriopus californicus to test whether northern populations have greater tolerance to low salinity stresses, as they experience greater precipitation and less evaporation. We used a common-garden experiment approach and exposed all populations to acute low (1 and 3 ppt) and high (110 and 130 ppt) salinities for 24 h and to a fluctuation between baseline salinity and moderate low (7 ppt) and high (80 ppt) salinities for over 49 h. We also performed RNA sequencing at several time points during the fluctuation between baseline and salinity of 7 ppt to understand the molecular basis of divergence between two populations with differing physiological responses. We present these novel findings: (1) acute low salinity conditions caused more deaths than high salinity; (2) molecular processes that elevate proline levels increased in salinity of 7 ppt, which contrasts with other physiological studies in T. californicus that mainly associated accumulation of proline with hyperosmotic stress; and (3) tolerance to a salinity fluctuation did not follow a latitudinal trend but was instead governed by a complex interplay of factors, including population and duration of salinity stress. This highlights the importance of including a wider variety of environmental conditions in empirical studies to understand local adaptation.

Phenotypic and transcriptomic responses to salinity stress across genetically and geographically divergent Tigriopus californicus populations

Species inhabiting the North American west coast intertidal must tolerate an extremely variable environment, with large fluctuations in both temperature and salinity. Uncovering the mechanisms for this tolerance is key to understanding species' persistence. We tested for differences in salinity tolerance between populations of Tigriopus californicus copepods from locations in northern (Bodega Reserve) and southern (San Diego) California known to differ in temperature, precipitation and humidity. We also tested for differences between populations in their transcriptomic responses to salinity. Although these two populations have ~20% mtDNA sequence divergence and differ strongly in other phenotypic traits, we observed similarities in their phenotypic and transcriptomic responses to low and high salinity stress. Salinity significantly affected respiration rate (increased under low salinity and reduced under high salinity), but we found no significant effect of population on respiration or a population by salinity interaction. Under high salinity, there was no population difference in knock-down response, but northern copepods had a smaller knock-down under low salinity stress, corroborating previous results for T. californicus. Northern and southern populations had a similar transcriptomic response to salinity based on a principle components analysis, although differential gene expression under high salinity stress was three times lower in the northern population compared to the southern population. Transcripts differentially regulated under salinity stress were enriched for "amino acid transport" and "ion transport" annotation categories, supporting previous work demonstrating that the accumulation of free amino acids is important for osmotic regulation in T. californicus.

Interpopulational variation in the cold tolerance of a broadly distributed marine copepod

Latitudinal trends in cold tolerance have been observed in many terrestrial ectotherms, but few studies have investigated interpopulational variation in the cold physiology of marine invertebrates. Here, the intertidal copepod Tigriopus californicus was used as a model system to study how local adaptation influences the cold tolerance of a broadly distributed marine crustacean. Among five populations spanning 18° in latitude, the following three metrics were used to compare cold tolerance: the temperature of chill-coma onset, the chill-coma recovery time and post-freezing recovery. In comparison to copepods from warmer southern latitudes, animals from northern populations exhibited lower chill-coma onset temperatures, shorter chill-coma recovery times and faster post-freezing recovery rates. Importantly, all three metrics showed a consistent latitudinal trend, suggesting that any single metric could be used equivalently in future studies investigating latitudinal variation in cold tolerance. Our results agree with previous studies showing that populations within a single species can display strong local adaptation to spatially varying climatic conditions. Thus, accounting for local adaptation in bioclimate models will be useful for understanding how broadly distributed species like T. californicus will respond to anthropogenic climate change.

Evidence of genetic selection for growth in new recruits of a marine fish

Abstract A cohort of Diplodus sargus, a coastal marine fish abundant in the Mediterranean Sea, has been surveyed from its settlement following the pelagic larval stage up to 4 months of age, when the juveniles are moving to adult habitats in order to assess selective processes. We followed the mortality by looking at the decrease in population abundance and, simultaneously, the genetic structure using allozymes and the growth associated with each genotype to test for a relationship between genotype and phenotype. The recruitment survey demonstrated that 80% of individuals arrived within a single night and that they show very similar age providing a discrete pulse of new recruits that we followed for changes in survival and allele frequencies. After 4 months, there was a total mortality of 80.8%, with the disappearance of 181 of 224 fish that initially colonized the rocky barrier. The decrease in number followed a logarithmic model with a maximum decrease in the early period (first 30 days). The model derived from the 4 months of data demonstrates that most of the mortality in the cohort occurs over the first 120 days following settlement and the model predicted a final abundance of 10 individuals after 1 year. Within the same period of 4 months, we observed significant decrease in multilocus heterozygosity. Such a decrease in heterozygosity partly resulted from a purge of the Pgm-80* allele. Together with this major change in a natural population, an aquarium experiment demonstrated that individuals with Pgm-80* alleles show significantly lower growth than other new recruits. We propose that the decrease in frequency of Pgm-80* in the natural environment is the result of targeted predation that eliminates smaller individuals and therefore individuals bearing Pgm-80*. The potential metabolic effect as well as a scenario that could lead to the maintenance of polymorphism is discussed.

Phototrophic utilization of taurine by the purple nonsulfur bacteria Rhodopseudomonas palustris and Rhodobacter sphaeroides

Taurine metabolism by two phototrophically grown purple nonsulfur bacteria enrichment isolates has been examined.Rhodopseudomonas palustris(strain Tau1) grows with taurine as a sole electron donor, sulfur and nitrogen source during photoautotrophic growth.Rhodobacter sphaeroides(strain Tau3) grows on the compound as sole electron donor, sulfur and nitrogen source, and partial carbon source, in the presence of CO2during photoheterotrophic growth. Both organisms utilize an inducible taurine–pyruvate aminotransferase and a sulfoacetaldehyde acetyltransferase. The products of this metabolism are bisulfite and acetyl phosphate. Bisulfite ultimately was oxidized to sulfate, but this was not an adequate source of electrons for photometabolism. Experiments using either [U-14C]taurine or14CO2demonstrated thatRb. sphaeroidesTau3 assimilated the carbon from approximately equimolar amounts of taurine and exogenous CO2. The taurine-carbon assimilation was not diminished by excess non-radioactive bicarbonate. Malate synthase (but not isocitrate lyase) was induced in these taurine-grown cells. It is concluded that assimilation of taurine carbon occurs through an intermediate other than CO2. Similar labelling experiments withRp. palustrisTau1 determined that taurine is utilized only as an electron donor for the reduction of CO2, which contributes all the cell carbon. Photoautotrophic metabolism was confirmed in this organism by the absence of either malate synthase or isocitrate lyase in taurine+CO2-grown cells. Culture collection strains of these two bacteria did not utilize taurine in these fashions.

Characterization of the glutamate dehydrogenase gene and its regulation in a euryhaline copepod

Glutamate dehydrogenase (GDH) plays a key role in the metabolism of free amino acids (FAA) in crustaceans and other metazoans. Glutamate synthesized by GDH via reductive amination is the amino group donor for alanine synthesis and the precursor required for proline synthesis. Since both proline and alanine are important intracellular osmolytes in many marine invertebrates, GDH has been widely implicated as playing a central role in response to hyperosmotic stress in these organisms. We have isolated the gene encoding a GDH homolog from the euryhaline copepod Tigriopus californicus and examined the regulation of GDH under salinity stress. The gene encodes a protein of 557 residues with 76% amino acid identity with Drosophila melanogaster GDH. The gene encodes an N-terminal mitochondrial signal sequence peptide. Only a single intron of 71 bp was found in the GDH gene in T. californicus when genomic sequences and cDNA sequences were compared. The levels of GDH mRNA do not increase during hyperosmotic stress in this copepod. The effects of salt and hyperosmotic stress on GDH enzyme activity were also investigated. GDH activities decrease with increasing NaCl concentrations in in vitro enzyme assays, while live animals exposed to hyperosmotic stress showed no change in GDH enzyme activities. Combined, these results indicate that GDH transcription and enzyme activity do not appear to function in the regulation of alanine and proline accumulation during hyperosmotic stress in T. californicus. The manner in which this important physiological process is regulated remains unknown.

Effects of developmental acclimation on adult salinity tolerance in the freshwater-invading copepod Eurytemora affinis

Invasive species are commonly thought to have broad tolerances that enable them to colonize new habitats, but this assumption has rarely been tested. In particular, the relative importance of acclimation (plasticity) and adaptation for invasion success are poorly understood. This study examined effects of short-term and developmental acclimation on adult salinity tolerance in the copepod Eurytemora affinis. This microcrustacean occurs in estuarine and salt marsh habitats but has invaded freshwater habitats within the past century. Effects of short-term acclimation were determined by comparing adult survival in response to acute versus gradual salinity change to low salinity (fresh water). Effects of developmental acclimation on adult tolerance were determined using a split-brood 4 x 2 factorial experimental design for one brackish-water population from Edgartown Great Pond, Massachusetts. Twenty full-sib clutches were split and reared at four salinities (fresh, 5, 10, and 27 practical salinity units [PSU]). On reaching adulthood, clutches from three of the salinity treatments (no survivors at fresh) were split into low- (fresh) and high- (40 PSU) salinity stress treatments, at which survival was measured for 24 h. Short-term acclimation of adults did not appear to have a long-term affect on low-salinity tolerance, given that gradual transfers to fresh water enhanced survival relative to acute transfers in the short term (after 7 h) but not over a longer period of 8 d. Developmental acclimation had contrasting effects on low- versus high-salinity tolerance. Namely, rearing salinity had a significant effect on tolerance of high-salinity (40 PSU) stress but no significant effect on tolerance of low-salinity (freshwater) stress. In addition, there was a significant effect of clutch on survival under freshwater conditions, indicating a genetic component to low-salinity tolerance but no significant clutch effect in response to high salinity. While developmental acclimation might enhance survival at higher salinities, the minimal effect of acclimation and significant effect of clutch on low-salinity tolerance suggest the importance of natural selection during freshwater invasion events.

Allele frequency shifts in response to climate change and physiological consequences of allozyme variation in a montane insect

Rapid changes in climate may impose strong selective pressures on organisms. Evolutionary responses to climate change have been observed in natural populations, yet no example has been documented for a metabolic enzyme locus. Furthermore, few studies have linked physiological responses to stress with allozyme genotypic variation. We quantified changes in allele frequency between 1988 and 1996 at three allozyme loci (isocitrate dehydrogenase, Idh; phosphoglucose isomerase, Pgi; and phosphoglucomutase, Pgm) for the leaf beetle Chrysomela aeneicollis in the Bishop Creek region of the Sierra Nevada of California (2900-3300 m). Beetles often experience high daytime (> 32 degrees C) and extremely low nighttime (< -5 degrees C) temperatures during summer. Bishop Creek weather station data indicated that conditions were unusually dry before 1988, and that conditions were cool and wet during the years preceding the 1996 collection. We found directional changes in allele frequency at Pgi (11% increase in the Pgi-1 allele), but not at Idh or Pgm. We also found that physiological response to thermal extremes depended on Pgi genotype. Pgi 1-1 individuals induced expression of a 70-kD heat shock protein (HSP) at lower temperatures than 1-4 or 4-4 individuals, and 1-1 individuals expressed higher levels of HSP70 after laboratory exposure to temperatures routinely experienced in nature. Survival after nighttime laboratory exposure to subzero temperatures depended on gender, previous exposure to cold, and Pgi genotype. Females expressed higher levels of HSP70 than males after exposure to heat, and recovery by female Pgi 1-1 homozygotes after exposure to cold (-5 degrees C) was significantly better than 1-4 or 4-4 genotypes. These data suggest that the cooler climate of the mid-1990s may have caused an increase in frequency of the Pgi-1 allele, due to a more robust physiological response to cold by Pgi 1-1 and 1-4 genotypes.

Proline biosynthesis genes and their regulation under salinity stress in the euryhaline copepod Tigriopus californicus

Diverse organisms regulate concentrations of intracellular organic osmolytes in response to changes in environmental salinity or desiccation. In marine crustaceans, accumulation of high concentrations of proline is a dominant component of response to hyperosmotic stress. In the euryhaline copepod Tigriopus californicus, synthesis of proline from its metabolic precursor glutamate is tightly regulated by changes in environmental salinity. Here, for the first time in a marine invertebrate, the genes responsible for this pathway have been cloned and characterized. The two proteins display the sequence features of homologous enzymes identified from other eukaryotes. One of the cloned genes, delta1-pyrroline-5-carboxylase reductase (P5CR), is demonstrated to have the reductase enzyme activity when expressed in proline-auxotroph bacteria, while the second, delta1-pyrroline-5-carboxylase synthase (P5CS), does not rescue proline-auxotroph bacteria. In contrast to results from higher plants, neither levels of P5CS nor P5CR mRNAs increase in response to salinity stress in T. californicus. Hence, regulation of proline synthesis during osmotic stress in T. californicus is likely mediated by some form of post-transcriptional regulation of either P5CS or P5CR. Understanding the regulation this pathway may elucidate the mechanisms limiting the salinity ranges of marine taxa.

Ontogeny of intracellular isosmotic regulation in the european lobster Homarus gammarus (L.)

Intracellular free amino acids were measured in the abdominal muscle of the three larval instars, postlarvae, and juveniles of the lobster Homarus gammarus, acclimated to seawater (35 per thousand) and to a dilute medium (22 per thousand), to study intracellular isosmotic regulation throughout the development of this species. Transfer to low salinity was followed by a highly significant drop of free amino acids level in all developmental stages. The main regulated amino acids were glycine, proline, and alanine. The level of regulation of total free amino acids changed at metamorphosis: the decrease in total free amino acids at low salinity was 46% in the three larval instars, but it was only 29% in postlarvae and 20% in juveniles. These results suggest that free amino acids, mainly glycine, proline, and alanine, are involved in intracellular isosmotic regulation in the lobster, with different levels of involvement in pre- and postmetamorphic stages. The ontogenetic changes in intracellular isosmotic regulation are discussed in relation to the changes in extracellular regulation (osmoregulation) in the lobster.

Low-molecular-weight sulfonates, a major substrate for sulfate reducers in marine microbial mats

Several low-molecular-weight sulfonates were added to microbial mat slurries to investigate their effects on sulfate reduction. Instantaneous production of sulfide occurred after taurine and cysteate were added to all of the microbial mats tested. The rates of production in the presence of taurine and cysteate were 35 and 24 microM HS(-) h(-1) in a stromatolite mat, 38 and 36 microM HS(-) h(-1) in a salt pond mat, and 27 and 18 microM HS(-) h(-1) in a salt marsh mat, respectively. The traditionally used substrates lactate and acetate stimulated the rate of sulfide production 3 to 10 times more than taurine and cysteate stimulated the rate of sulfide production in all mats, but when ethanol, glycolate, and glutamate were added to stromatolite mat slurries, the resulting increases were similar to the increases observed with taurine and cysteate. Isethionate, sulfosuccinate, and sulfobenzoate were tested only with the stromatolite mat slurry, and these compounds had much smaller effects on sulfide production. Addition of molybdate resulted in a greater inhibitory effect on acetate and lactate utilization than on sulfonate use, suggesting that different metabolic pathways were involved. In all of the mats tested taurine and cysteate were present in the pore water at nanomolar to micromolar concentrations. An enrichment culture from the stromatolite mat was obtained on cysteate in a medium lacking sulfate and incubated anaerobically. The rate of cysteate consumption by this enrichment culture was 1.6 pmol cell(-1) h(-1). Compared to the results of slurry studies, this rate suggests that organisms with properties similar to the properties of this enrichment culture are a major constituent of the sulfidogenic population. In addition, taurine was consumed at some of highest dilutions obtained from most-probable-number enrichment cultures obtained from stromatolite samples. Based on our comparison of the sulfide production rates found in various mats, low-molecular-weight sulfonates are important sources of C and S in these ecosystems.

Aminotransferase and the production of alanine during hyperosmotic stress in Paramecium calkinsi

When Paramecium calkinsi encounter hyperosmotic stress, intracellular free alanine increases. In vivo assays indicate that the reaction catalyzed by alanine aminotransferase contributes to the build up of alanine in response to hyperosmotic shock. 14C-pyruvate is converted to 14C-alanine in cells grown axenically at 200 mosm. When shifted to 600 mosm, the rate of conversion of pyruvate to alanine increases, and conversion at either 200 or 600 mosm is blocked by 1 mM aminooxyacetic acid (AOA), an inhibitor of aminotransferase. Intracellular free alanine increase is partially inhibited by AOA, and AOA prevents cells living in fresh water from acclimating to higher salinities, an indication that the increase in intracellular alanine is physiologically significant.

Developmental rate and viability of rainbow trout with a null allele at a lactate dehydrogenase locus

We show that a previously described isozyme polymorphism in rainbow trout (Salmo gairdneri) is the result of an enzymatically inactive (i.e., null) allele (n). Ldh3 null homozygotes (n/n) and heterozygotes (100/n) have reductions of about 20 and 12% in total lactate dehydrogenase (LDH) activity at hatching, respectively. As juveniles, (100/n) fish have reductions in LDH activity of 15, 37, and 21% in brain, heart, and white muscle, respectively. Embryos with different Ldh3 phenotypes from 11 families do not differ significantly in either survival or hatching time. However, a second measure of developmental rate, the amount of malate dehydrogenase (MDH) and phosphoglucomutase (PGM) activity in 33-day-old embryos, suggests that (100/n) embryos develop more slowly than (100/100) embryos. In three of four families examined, (100/n) embryos have significantly lower amounts of total MDH activity (8-10%). In one of these, (100/n) embryos also have significantly lower total PGM activity (15%). These data suggest that the reduction in total LDH activity is associated with small but detectable delays in developmental rate but nondetectable differences in survival to hatching.

Properties of allelic variants of phosphoglucomutase from the sea anemone Metridium senile

The phosphoglucomutase (Pgm) locus from populations of the sea anemone Metridium senile has three alleles in natural populations from the northeastern coast of North America. Two of the alleles exhibit clinal variation north of Cape Cod, suggesting a possible association of allele frequency with environmental temperature. This clinal pattern is reproducible and stable over at least brief periods of time. The allozymes encoded by each of the six Pgm genotypes have been partially purified and characterized. The symmetrical pH optimum for Vmax is pH 7.5; the apparent Km (Kmapp) of glucose-1-phosphate declines monotonically as the pH increases from 6.5 to 8.5. There are no pronounced differences in heat stabilities of PGM produced by various genotypes, nor are there significant differences in specific activities. There are no differences in the sensitivity of Vmax to temperature. Kmapp values are very low for all genotypes, ranging from about 2 to 12 microM, depending upon the temperature. Kmapp of glucose-1-phosphate declines as the temperature is raised for all genotypes, whether the pH is held constant or allowed to vary with the temperature. Under certain conditions, there are small significant differences among genotypes in Kappm values, but there is no systematic pattern to these differences. The present data provide no biochemical explanation for the maintenance of the Pgm cline by selection for functional differences under different thermal regimes.

Physiological effects of an allozyme polymorphism: glutamate-pyruvate transaminase and response to hyperosmotic stress in the copepod Tigriopus californicus

In order to regulate cell volume during hyperosmotic stress, the intertidal copepod Tigriopus californicus, like other aquatic crustaceans, rapidly accumulates high levels of intracellular alanine, proline, and glycine. Glutamate-pyruvate transaminase (GPT; EC 2.6.1.2), which catalyzes the final step of alanine synthesis, is genetically polymorphic in T. californicus populations at Santa Cruz, California. Spectrophotometric studies of homogenates derived from a homozygous isofemale line of each of the two common GPT alleles indicated that the GPTF allozyme has a significantly higher specific activity than the GPTS allozyme. Under conditions of hyperosmotic stress, individual adult copepods of GPTF and GPTF/S genotypes accumulated alanine, but not glycine or proline, more rapidly than GPTS homozygotes. When young larvae were subjected to the same hyperosmotic conditions, GPTS larvae suffered a significantly higher mortality than GPTF or GPTF/S larvae. These results suggest that the biochemical differences among GPT allozymes result in specific physiological variation among GPT genotypes and that this physiological variation is manifested in differential genotypic survivorships under some naturally occurring environmental conditions.