Interactions of polyhalogenated aromatic hydrocarbons with thyroid hormone metabolism

This thesis deals with the possible interactions of polyhalogenated aromatic hydrocarbons and/or their metabolites with thyroid hormone metabolism. This chapter summarizes firstly the effects of thyroid hormone on the induction of biotransformation enzymes by PHAHs. Secondly, the results on the inhibition of thyroid hormone sulfation by hydroxylated metabolites of PHAH are summarized. Some conclusions and remarks on the overall implications of the results are given at the end of this chapter.The effects of thyroid hormone on the induction of biotransformation enzymes by polyhalogenated aromatic hydrocarbonsThe first part of this thesis focussed on the question whether or not the PHAH-induced decrease of plasma T4 is an adaptive endocrine response of the animal to cope with the onset of toxic effects by PHAHs. For this purpose, the possible regulatory effect of thyroid hormones on biotransformation enzymes was investigated, using rats differing in thyroid state which were exposed to TCDD or PCBs as model inducers of biotransformation enzymes.In Chapter 2 , the thyroid state of euthyroid (Eu), thyroidectomized (Tx) and Tx rats in which T3 or T4 levels are restored using osmotic minipumps were compared. The decreased circulatory levels of plasma T4 and T3, the increased pituitary feedback response (plasma TSH levels), as well as changed functional responses (decreased hepatic D1 and malic enzyme activities, and increased brain D2 activities) in Tx rats were largely restored to Eu levels in Tx+T4 rats and, except for plasma TT4 and brain D2 activity, in Tx+T3 rats. These results indicated that the thyroid hormone-replaced Tx rats were valid models to study peripheral effects of TCDD. Three days after exposure to 10 mg TCDD/kg body weight, plasma TT4 and FT4 levels were significantly reduced in Eu rats and in Tx+T4 rats, and plasma T3 was significantly reduced in Tx+T3 but not in Eu or Tx+T4 rats. Hepatic T4 UGT activity was induced by TCDD while T3 UGT activity was only slightly increased in the different exposed groups. These results strongly suggest that the thyroid hormone-decreasing effects of TCDD are predominantly extrathyroidal and mediated by the marked induction of hepatic T4 UGT activity.The effects of thyroid state modulation on the induction of detoxification enzymes by TCDD in experimental animals are described in Chapter 3 . In all rats, TCDD largely induced CYP1A1/1A2 activity (EROD), CYP1A1 protein content, and CYP1A1 mRNA levels. TCDD exposure also resulted in higher total hepatic cytochrome P450 content, hepatic p-nitrophenol UGT activity, and GST 1-1 protein levels, but had no effect on hepatic NADPH cytochrome P450 reductase activity, overall GST activity and GST 2-2, 3-3, and 4-4 protein levels and iodothyronine sulfotransferase activity. Thyroid state did not affect the total cytochrome P450, and GST activity and protein levels, but slightly decreased CYP1A1/2 activity, NADPH cytochrome P450 reductase activity, PNP UGT activity and iodothyronine sulfotransferase activity were demonstrated in Tx rats, as compared to Eu rats.In the second animal experiment, the interaction between thyroid state and PCBs in the regulation of CYP1A1 and CYP2B expression is described ( Chapter 4 ). Male Tx Sprague-Dawley rats, Eu rats, and rats made hyperthyroid by infusing T3 were treated with a single ip dose of the CYP2B inducer PCB 153 and the CYP1A inducer PCB 126. The thyroid states of the rats were confirmed by measurement of plasma T4, T3 and TSH and of functional parameters such as hepatic D1 activity, malic enzyme activity and a-glycerolphosphate dehydrogenase activity. Total hepatic cytochrome P450 content was increased by PCB treatment in all groups, but was not affected by thyroid state. NADPH cytochrome P450 reductase activity was decreased in Tx rats and increased in hyperthyroid rats, while PCB treatment had no effect. PCB 126 specifically induced T4 UGT activity, measured in the absence of detergent, and CYP1A activity, protein and mRNA levels, whereas PCB 153 induced T4 UGT activity, measured in the presence of the detergent Brij 56, and CYP2B activity, protein and mRNA levels. Thyroid state, neither hypo nor hyper, significantly affected T4 UGT activity or CYP1A and CYP2B activities, protein or mRNA levels.The almost complete lack of response of basal and PCB- or TCDD-induced activities of biotransformation enzymes to changes in thyroid state observed in our studies is in contrast to effects published by others (Kato and Takahashi et al. , 1968; Rumbaugh et al. , 1978; Leakey et al. , 1982; Müller et al. , 1983a/b; Skett, 1987; Yamazoe et al. , 1989; Arlotto and Parkinson, 1989; Murayama et al., 1991; Chowdhury et al. , 1983; Moscioni and Gartner, 1983; Pennington et al. , 1988; Goudonnet et al. , 1990; Williams et al. , 1986; Pimental et al. , 1993). This may be due to differences in strain and sex of the animals, the severity and duration of the hypo- and hyperthyroid states induced as well as the duration and dose of TCDD/PCB treatment. Overall, it can be concluded that hepatic NADPH cytochrome P450 reductase activity is dependent on thyroid state, whereas total cytochrome P450 as well as CYP1A1 and CYP2B together with UGT, GST and sulfotransferase activities show little or no thyroid hormone dependence. These slight effects are unlikely to represent an endocrine adaptation to a chemical stressor (TCDD). Therefore, the PHAH-induced decreased T4 levels , as well as other aspects of PHAH-induced alterations in thyroid hormone metabolism, are most likely a direct reflection of the developing toxicological response of the animals toward PHAH exposure.Inhibition of thyroid hormone sulfation by hydroxylated metabolites of polyhalogenated aromatic hydrocarbonsThe second part of this thesis focussed on the question whether or not hydroxylated metabolites of PHAHs (PHAH-OHs) are able to inhibit thyroid hormone sulfation in vitro as well as in vivo .Chapter 5 presents the investigations concerning the possible inhibitory effects of PHAH-OHs on iodothyronine sulfotransferase (SULT) activity. Rat liver cytosol was used as a source of sulfotransferase in an in vitro assay with 125I-labelled T2 as a model substrate. Hydroxylated metabolites of PCBs, PCDDs and PCDFs were found to be potent inhibitors of T2 SULT activity in vitro with IC50 values in the low micromolar range (0.2-3.8 mM). The most potent inhibitor of T2 SULT activity within our studies was the PCB metabolite 3-hydroxy-2,3',4,4',5-pentachlorobiphenyl with an IC50 value of 0.2 mM. A hydroxyl group in the para or meta position appeared to be an important structural requirement for T2 SULT inhibition by PCB metabolites. Ortho hydroxy PCBs were much less potent and none of the parent PHAHs were capable of inhibiting T2 SULT activity. In addition, the formation of T2 SULT-inhibiting metabolites from individual brominated diphenyl ethers and nitrofen as well as from some commercial PHAH mixtures (e.g. Bromkal, Clophen A50 and Aroclor 1254) by CYP450 catalyzed hydroxylation was also demonstrated.Consequently, the inhibition of thyroid hormone sulfation by PHAH-OHs was studied in more detail, investigating isozyme specificity and inhibition kinetics ( Chapter 6 ). The difference in inhibition pattern demonstrated for SULT activity present in rat liver and brain cytosol, is probably caused by a difference in isozyme pattern. It was shown that PCB-OHs inhibited T2 sulfation by interacting with the rat isozyme SULT1C1 and an additional isozyme responsible for T2 sulfation in female liver cytosol, probably rat SULT1B1, but not SULT1A1. On the other hand, human phenol SULT1A1 was inhibited by PCB-OHs, but not the human isozyme SULT1A3. In conclusion, we suggested that at least human SULT1A1, and rat SULT1C1 and perhaps rat SUL1B1 are involved in the inhibition of T2 sulfation by PCB-OHs. However, more information is needed about the various isozymes involved in iodothyronine sulfation in humans as well as in rats, before definite conclusions can be drawn.Furthermore, it is shown that T2 is a good model substrate for the active hormone T3 when investigating the inhibition of thyroid hormone sulfation by hydroxylated metabolites of PHAHs. The inhibition kinetics strongly suggested that the nature of the T2 sulfation inhibition by PCB-OHs is competitive. To obtain more decisive information, tests with purified isozymes should be performed. It was also demonstrated that PCDD-OHs and PCB-OHs themselves are substrates -albeit poor- for SULT enzymes, which further supports the competitive inhibition of thyroid hormone sulfation by PHAH-OHs.To bridge the gap between in vitro experiments using cytosol and the in vivo situation, we investigated the inhibition of thyroid hormone sulfation in hepatoma cell lines ( Chapter 7 ). Two PCB-OHs, 4-hydroxy-2',3,3',4',5-pentachlorobiphenyl and 4-hydroxy-3,3',4',5-tetrachlorobiphenyl, together with the known sulfation inhibitor pentachlorophenol (PCP) were tested in the rat hepatoma cell line FaO and the human hepatoma cell line HepG2. PCP inhibited T2 sulfation in vitro in FaO and HepG2 cells, although it was 1000 times less potent in whole cells than in rat liver cytosol. Micromolar concentrations of the two tested PCB-OHs hardly affected T2 conjugation in FaO cells, but reduced T2 sulfate formation in HepG2 cells. Inhibition of T2 sulfation was more pronounced using medium without FCS than in medium with 5% FCS, due to a lower uptake of inhibitor by the cells in the presence of serum, as demonstrated using radiolabeled PCP.These in vitro results indicate that hydroxylated PHAHs are potent inhibitors of thyroid hormone sulfation. Since thyroid hormone sulfation may play an important role in regulating "free" hormone levels in the fetus, and hydroxylated PCB metabolites are known to accumulate in fetal tissues after maternal exposure to PCBs, these observations in vitro might have implications for fetal thyroid hormone homeostasis and development.The in vivo experiment in which was tested if PHAH-OHs are able to inhibit T2 sulfation, was described in Chapter 8 . Pregnant rats were exposed to 25 mg Aroclor 1254/kg body weight or to the well-known phenol sulfation inhibitor PCP (25 mg/kg body weight) from day 10 till day 18 of gestation. Fetuses and dams were sacrificed on gestation day 20 (GD20). PCP and PCB metabolite levels in fetal serum and tissues were high. Aroclor 1254, but not PCP exposure resulted in an induction of hepatic EROD and T4 UGT activity in dams.PHAHs are known for their disrupting effects on thyroid hormone metabolism, as shown in Figure 9.1. In this animal experiment, Aroclor 1254 exposure caused an increase in T4 UGT activity, resulting in decreased TT4 levels. Treatment with PCP also resulted in decreased serum TT4 levels, but increased FT4 levels, in dams and fetuses. The ratio FT4/TT4 was increased indicating a reduced plasma TTR binding capacity in fetuses and dams following both treatments. D1 activity in liver decreased in dams and fetuses after treatment with Aroclor 1254 and PCP. This decrease is probably caused indirectly by the lowered T4 levels. D2 activity in brain decreased by exposure to PCP in dams but no effect was found in fetuses, and increased by exposure to Aroclor 1254 in fetuses, with no effect in dams. The increasing D2 activity is a response of the brain to low T4 levels, to maintain the T3 homeostasis.The positive control PCP was shown to increase the T2 SULT activity measured in maternal liver and brain cytosol. Studies using varying T2 concentrations and different protein concentrations suggested competitive inhibition of PCP carried over in the in vitro assay as well as true induction of T2 SULT activity. This effect of PCP on thyroid hormone sulfation in vivo apparently did not result in lower levels of the product T4S, since fetal and maternal serum levels of T4S were not changed after treatment with PCP. This negative answer may be explained by an increased availability of substrate (FT4; maternal) together with a reduced D1 activity by PCP treatment, resulting in a reduced enzymatic breakdown of T4S.Exposure to Aroclor 1254, which resulted in the formation of hydroxylated metabolites, did not significantly change the T2 SULT activity in maternal or fetal brain or liver cytosol, nor the serum levels of T4S.Remarkably, the T3S and T4S levels were very low in fetal rat serum in this study, especially when compared with the reported high iodothyronine sulfate levels in fetal human and sheep serum. This can not be explained by low SULT activity levels or high D1 activity levels in rat fetuses on day 20.Overall implications of the observed PHAH effects on thyroid hormone metabolismPHAHs induce a wide spectrum of toxic effects in rats. Some effects have been suggested to be linked to a hypothyroid situation, such as the "wasting syndrome", decreased feed intake, and increased cholesterol concentrations. Indeed, reduced serum T4 concentrations have been observed following exposure to PHAHs (Bastomsky et al. , 1977; Gorski and Rozman, 1987; Hermansky et al. , 1988; Brouwer, 1989; Beetstra et al. , 1991), and it is tempting to speculate about a relationship between the hypothyroxinemia and the observed toxic responses. However, induction of a hypothyroid situation or a hypothyroxinemia by PHAHs could also be regarded as an adaptive endocrine response to diminish the PHAH-induced toxicity. One argument in support of this interpretation is the observed protective effect of thyroidectomy on TCDD-induced lethality and immune toxicity (Rozman et al. , 1985).In this study, it is proposed that the T4 decrease could well have a regulatory role in the induction of hepatic biotransformation enzymes, as was reported before (see Chapter 1 ). The present investigations suggest that the lowering effects of PHAHs on T4 levels are only a toxic effect of PHAHs and not an adaptive response to regulate the induction of biotransformation enzymes. The differences with other reports on modulating effects of thyroid hormone state on biotransformation enzymes may be explained by differences in the time and dose of inducers as well as by a difference in hypo- or hyperthyroid state. Nevertheless, the T4 decreases in the hypothyroid animals in our study are similar to the PHAH-induced T4 decreases. Therefore, the model was good enough to investigate our hypothesis.The second part of this thesis demonstrated that the sulfotransferase enzyme is another thyroid hormone-binding protein, besides D1 and TTR, which can be competitively inhibited by hydroxylated metabolites of PHAHs. In a relatively narrow range of low micromolar concentrations, PHAH-OHs were able to competitively inhibit T2 SULT acttivity in vitro , in a SULT isozyme and tissue specific manner.Studies using a perinatal exposure setup were performed to test inhibition of T2 sulfation in vivo . It was demonstrated that the well-known sulfation inhibitor PCP was able to indeed competively inhibit T2 SULT activity, but also was able to upregulate the sulfotransferase protein amounts. Aroclor 1254 exposure resulted in a slight inhibition of T2 SULT activity, probably caused by hydroxylated metabolites formed. This inhibition, together with lower substrate (FT4) levels found after Aroclor treatment did not result in decreased serum T4S levels, which is probably caused by a concomitantly decreased inactivation route, i.e. a decreased D1 activity, together with a higher availibility of substrate (FT4) after PCP exposure.Remarkably, the serum T4S levels in fetal rat are low compared to the levels in sheep and human fetal serum samples (Wu et al. , 1992a/b; 1993a/b; Santini et al. , 1993). This could not be explained by already higher D1 activities or a relatively low sulfation activity in the control fetus around GD20. For this reason, we concluded that the fetal rat probably is not a very good model for humans in terms of investigating the impact of toxic compounds on fetal thyroid hormone sulfation. However, it should be mentioned that, although PHAHs and their metabolites interfere at many sites with thyroid hormone transport and metabolism, the fetus apparently is able to cope with those changes and can keep its homeostasis in T3.Another interesting point deduced from this study, is that PCP, which could be a model for PCB-OHs, itself showed effects on thyroid hormone levels and metabolism, indicating the importance of phenolic organohalogens compounds for disrupting effects on the thyroid hormone system. This also indicates that the disrupting effects of PCBs on the thyroid hormone system are for a large part caused by the hydroxylated metabolites formed. The own toxicity of PCB-OHs and related phenolic organohalogens inducing a separate set of effects together with the recently observed high fetal accumulation of hydroxy-PHAHs, give reason to further investigate the potential toxicity of these compounds on thyroid hormone metabolism and transport (see also Figure 9.1). It is worth mentioning that besides the "old" organohalogen pollutants that have been phased out since the 1980's, there is a wide range of new products on the market, such as brominated diphenylethers (PBDEs), chlorinated benzenes, bisphenol A and so on. PBDEs, which are nowadays used as flame retardants, have been demonstrated at increasing levels in our environment (De Boer et al. , 1989; Sellstrom et al. , 1996), and are probably able to cause similar effects as PHAHs. Serum T4 decreases have already been reported in rats after exposure to PBDEs (Darnerud et al. , 1996) or PCDEs (Rosiak et al. , 1997). Also, hydroxylated metabolites of PBDEs have been found to competitively inhibit the T4 binding to TTR in vitro (Meerts et al. , 1998).The human diet contains a diverse spectrum of naturally occuring and xeno-compounds that affect thyroid hormone metabolism. These include the organohalogens and related contaminants, and in addition, a large number of food components. Flavones and flavonoids have been reported to interfere with thyroid hormone binding proteins such as D1 (Auf'mkolk et al. , 1986; Cody et al. , 1989) and TTR (Lueprasitsakul et al. , 1990; Köhrle et al. , 1986). Flavonoids such as quercetin were similarly found to be able to inhibit phenol sulfotransferase activity in vitro (Walle et al. , 1995; Eaton et al. , 1996), and also other food additives were potent inhibitors of phenol sulfation (Bamforth et al. , 1993). The potential adverse human health impact of these compounds depends on a number of factors, including dietary intake, metabolism and pharmacokinetics, compound potency, serum concentrations, relative binding to serum proteins, and interactions or cross-talk with other endocrine pathways. In a risk evaluation, it should be taken into account that humans are exposed to a mixture of compounds with effects on thyroid hormone metabolism. If the mechanism of interference is similar for all these classes of compounds, the effects might very well be additive, or interactive. Additionally, the very persistent PHAHs are probably of more importance from a risk assessment point of view than the natural food components having a higher degradation rate.The effects of PHAHs on the thyroid hormone system in this study have been obtained in rats, are the results relevant for the human situation. Occupational or accidental exposure to high levels of PCBs or PBBs results in changes in serum T4 levels as was found by Bahn et al. (1980), Kreiss et al. (1982), Murai et al. (1987), and Emmet et al. (1988). Moreover, in pregnant women exposed to background levels of PHAHs mainly through diet, a significant negative correlation was observed between human milk levels of PHAHs and plasma T4 and T3 levels (Koopman-Esseboom et al. , 1994). In addition, increases in plasma TSH and both increases and decreases in plasma T4 levels were found in newborn babies following exposure to increasing PHAH levels through in utero and lactational transfer (Pluim et al. , 1993; Koopman-Esseboom et al. , 1994). Besides, prenatal exposure to PCBs is related to disorders in neurological development of children, found in some in epidemiologic studies (Rogan et al. , 1986; Jacobson et al. , 1990). It still is however not clear if these effects of PHAHs on thyroid hormone levels and metabolism may have possible effects on (brain) development.</p

Long-term CO₂ production following permafrost thaw

Thawing permafrost represents a poorly understood feedback mechanism of climate change in the Arctic, but with a potential impact owing to stored carbon being mobilized1–5. We have quantified the long-term loss of carbon (C) from thawing permafrost in Northeast Greenland from 1996 to 2008 by combining repeated sediment sampling to assess changes in C stock and&gt;12 years of CO2 production in incubated permafrost samples. Field observations show that the active-layer thickness has increased by&gt;1 cm yr−1 but thawing has not resulted in a detectable decline in C stocks. Laboratory mineralization rates at 5 ◦C resulted in a C loss between 9 and 75%, depending on drainage, highlighting the potential of fast mobilization of permafrost C under aerobic conditions, but also that C at near-saturated conditions may remain largely immobilized over decades. This is confirmed by a three-pool

Biomass offsets little or none of permafrost carbon release from soils, streams, and wildfire: an expert assessment

As the permafrost region warms, its large organic carbon pool will be increasingly vulnerable to decomposition, combustion, and hydrologic export. Models predict that some portion of this release will be offset by increased production of Arctic and boreal biomass; however, the lack of robust estimates of net carbon balance increases the risk of further overshooting international emissions targets. Precise empirical or model-based assessments of the critical factors driving carbon balance are unlikely in the near future, so to address this gap, we present estimates from 98 permafrost-region experts of the response of biomass, wildfire, and hydrologic carbon flux to climate change. Results suggest that contrary to model projections, total permafrost-region biomass could decrease due to water stress and disturbance, factors that are not adequately incorporated in current models. Assessments indicate that end-of-the-century organic carbon release from Arctic rivers and collapsing coastlines could increase by 75% while carbon loss via burning could increase four-fold. Experts identified water balance, shifts in vegetation community, and permafrost degradation as the key sources of uncertainty in predicting future system response. In combination with previous findings, results suggest the permafrost region will become a carbon source to the atmosphere by 2100 regardless of warming scenario but that 65%–85% of permafrost carbon release can still be avoided if human emissions are actively reduced

Vulnerability of high latitude soil organic carbon in North America to disturbance

This synthesis addresses the vulnerability of the North American high-latitude soil organic carbon (SOC) pool to climate change. Disturbances caused by climate warming in arctic, subarctic, and boreal environments can result in significant redistribution of C among major reservoirs with potential global impacts. We divide the current northern high-latitude SOC pools into (1) near-surface soils where SOC is affected by seasonal freeze-thaw processes and changes in moisture status, and (2) deeper permafrost and peatland strata down to several tens of meters depth where SOC is usually not affected by short-term changes. We address key factors (permafrost, vegetation, hydrology, paleoenvironmental history) and processes (C input, storage, decomposition, and output) responsible for the formation of the large high-latitude SOC pool in North America and highlight how climate-related disturbances could alter this pool\u27s character and size. Press disturbances of relatively slow but persistent nature such as top-down thawing of permafrost, and changes in hydrology, microbiological communities, pedological processes, and vegetation types, as well as pulse disturbances of relatively rapid and local nature such as wildfires and thermokarst, could substantially impact SOC stocks. Ongoing climate warming in the North American high-latitude region could result in crossing environmental thresholds, thereby accelerating press disturbances and increasingly triggering pulse disturbances and eventually affecting the C source/sink net character of northern high-latitude soils. Finally, we assess postdisturbance feedbacks, models, and predictions for the northern high-latitude SOC pool, and discuss data and research gaps to be addressed by future research

Soil resources and element stocks in drylands to face global issues

Drylands (hyperarid, arid, semiarid, and dry subhumid ecosystems) cover almost half of Earth’s land surface and are highly vulnerable to environmental pressures. Here we provide an inventory of soil properties including carbon (C), nitrogen (N), and phosphorus (P) stocks within the current boundaries of drylands, aimed at serving as a benchmark in the face of future challenges including increased population, food security, desertification, and climate change. Aridity limits plant production and results in poorly developed soils, with coarse texture, low C:N and C:P, scarce organic matter, and high vulnerability to erosion. Dryland soils store 646 Pg of organic C to 2 m, the equivalent of 32% of the global soil organic C pool. The magnitude of the historic loss of C from dryland soils due to human land use and cover change and their typically low C:N and C:P suggest high potential to build up soil organic matter, but coarse soil textures may limit protection and stabilization processes. Restoring, preserving, and increasing soil organic matter in drylands may help slow down rising levels of atmospheric carbon dioxide by sequestering C, and is strongly needed to enhance food security and reduce the risk of land degradation and desertification

Only halving emissions by 2030 can minimize risks of crossing cryosphere thresholds

Peer reviewe

Permafrost and Climate Change: Carbon Cycle Feedbacks From the Warming Arctic

Rapid Arctic environmental change affects the entire Earth system as thawing permafrost ecosystems release greenhouse gases to the atmosphere. Understanding how much permafrost carbon will be released, over what time frame, and what the relative emissions of carbon dioxide and methane will be is key for understanding the impact on global climate. In addition, the response of vegetation in a warming climate has the potential to offset at least some of the accelerating feedback to the climate from permafrost carbon. Temperature, organic carbon, and ground ice are key regulators for determining the impact of permafrost ecosystems on the global carbon cycle. Together, these encompass services of permafrost relevant to global society as well as to the people living in the region and help to determine the landscape-level response of this region to a changing climate

Experimental warming differentially affects vegetative and reproductive phenology of tundra plants

Rapid climate warming is altering Arctic and alpine tundra ecosystem structure and function, including shifts in plant phenology. While the advancement of green up and flowering are well-documented, it remains unclear whether all phenophases, particularly those later in the season, will shift in unison or respond divergently to warming. Here, we present the largest synthesis to our knowledge of experimental warming effects on tundra plant phenology from the International Tundra Experiment. We examine the effect of warming on a suite of season-wide plant phenophases. Results challenge the expectation that all phenophases will advance in unison to warming. Instead, we find that experimental warming caused: (1) larger phenological shifts in reproductive versus vegetative phenophases and (2) advanced reproductive phenophases and green up but delayed leaf senescence which translated to a lengthening of the growing season by approximately 3%. Patterns were consistent across sites, plant species and over time. The advancement of reproductive seasons and lengthening of growing seasons may have significant consequences for trophic interactions and ecosystem function across the tundra.publishedVersio