The setting is Ronneby, a municipality of roughly 28,000 inhabitants in southern Sweden. For decades, aqueous film-forming foam, or AFFF, used in firefighting training at a nearby military airbase seeped into the groundwater that fed one of the town’s two water treatment plants. When the contamination was finally discovered in 2013, total PFAS concentrations in the water exceeded 10,000 nanograms per liter, dominated by perfluorooctane sulfonic acid (PFOS) and perfluorohexane sulfonic acid (PFHxS), with perfluorooctanoic acid (PFOA) also present. Retrospective analyses of archived newborn blood spots suggest the exposure began as early as 1985 and climbed steadily until around 2005 to 2007. The contaminated supply was shut off on December 16, 2013, but by then, some residents carried serum PFAS concentrations up to a hundred times higher than those seen in the general Swedish population.

That unusual exposure gradient is precisely what made Ronneby valuable to researchers. Most previous studies of PFAS and reproductive hormones have been conducted in populations with only background exposure, and their findings have been inconsistent. Regulatory bodies, including the European Food Safety Authority and the US National Academies of Sciences, have judged the evidence linking PFAS to reproductive hormone alterations insufficient. By studying children whose bodies span a range of exposures from background levels to extreme contamination, the Swedish team could probe exposure-response relationships that background-level studies simply cannot resolve.

The researchers drew on biobanked serum samples collected between 2014 and 2016 from participants in the Ronneby PFAS Biomarker Cohort, aged 5 to 20 years. The final study group comprised 87 girls and 84 boys aged 5 to 11, and 117 boys aged 12 to 20; girls aged 12 and older were excluded because menstrual cyclicity and contraceptive use would have introduced unmanageable variability. About 23 percent of participants came from nearby Karlshamn, where drinking water PFAS levels were below 5 nanograms per liter, providing a reference group. Serum concentrations of PFOS, PFHxS, and PFOA were measured by liquid chromatography-tandem mass spectrometry, and the sum of these three compounds served as the primary exposure measure. Hormones, including follicle-stimulating hormone (FSH), luteinizing hormone (LH), prolactin, sex hormone-binding globulin (SHBG), and total testosterone, were quantified with electrochemiluminescence immunoassays at an accredited clinical laboratory.

The exposure levels were striking. Among the younger children from Ronneby, median combined serum concentrations of the three PFAS exceeded 90 nanograms per milliliter, and among adolescent boys the median reached 173 nanograms per milliliter, with some individuals above 600. For context, the European Food Safety Authority considers a serum level of about 6 nanograms per milliliter to be the threshold below which the tolerable weekly intake of PFAS is assumed to be maintained. PFOS and PFHxS were strongly correlated with one another, reflecting their shared source in the contaminated water, and roughly one quarter of participants were classified as prenatally exposed based on residential records from the year of their birth.

The hormone findings split cleanly along developmental lines. Among children aged 5 to 11, higher PFAS concentrations were associated with elevated prolactin in both sexes, an association that held in the continuous models for girls and boys alike. Boys in this younger group also showed lower FSH as exposure increased, while boys in the medium exposure category had higher SHBG. When the researchers formally tested for sex differences, however, the interaction terms were not statistically significant, meaning they could not conclude that boys and girls responded differently. Among adolescent boys aged 12 to 20, the pattern shifted: higher PFAS levels were associated with lower LH, lower total testosterone, and a lower free androgen index, a calculated measure of bioavailable testosterone that combines testosterone and SHBG. In the categorical analysis, the reduction in LH was most evident in the medium-high exposure group, while testosterone and the androgen index declined across the higher exposure categories.

These results align closely with findings from other highly exposed populations. In the C8 Health Project in the Mid-Ohio Valley, where drinking water was contaminated by PFAS production facilities, higher PFOA and PFOS concentrations were linked to delayed puberty and lower testosterone in both boys and girls. More recently, adolescents living near a 3M chemical plant in Belgium showed similar patterns, with higher PFHxS and PFOA associated with lower LH and testosterone and higher SHBG in teenage boys. The convergence of evidence across three independent contamination hotspots on two continents strengthens the case that the associations are not statistical artifacts, though the observational design still cannot prove causation.

The prolactin finding is particularly intriguing because it is comparatively rare in the PFAS literature. Positive relationships between fluorochemical exposure and prolactin were reported in occupational studies in the 1990s, but few modern studies have examined the question. Prolactin is best known for its role in lactation, but it also feeds back on the hypothalamic-pituitary-gonadal axis, and pathological hyperprolactinemia in adults can suppress gonadotropin release and reduce sex steroid production. The Ronneby team is careful to note that their data do not demonstrate a prolactin-mediated suppression of the hormonal axis, since prolactin was elevated in young children but not in adolescent boys, in whom LH and testosterone were instead reduced. The hormone-specific associations may reflect other mechanisms or differing susceptibility across developmental stages, and experimental studies suggest PFAS may impair testosterone production by disrupting steroidogenesis in the Leydig cells of the testes, though exact mechanisms remain uncertain.

The study has limitations worth weighing. Blood samples were drawn at varying times of day, and hormones such as testosterone and LH follow circadian rhythms that peak in the early morning, introducing variability that likely attenuated the observed effect estimates rather than creating spurious ones. Immunoassay-based sex steroid measurements are less sensitive than mass spectrometry, particularly at the low concentrations typical of children, and most estradiol values fell below the laboratory’s reporting limit and could not be analyzed. The researchers also could not disentangle prenatal from postnatal exposure, because the contamination was already pervasive by the mid-1980s and the two exposure routes are highly correlated in this cohort. Sensitivity analyses adjusting for body mass index produced similar estimates for most hormones.

The clinical significance of the hormonal differences remains uncertain. Testosterone concentrations in the exposed adolescents were still within age-appropriate reference ranges, and pediatric reference intervals for prolactin are lacking, making interpretation difficult. What the study does establish is that real-world, AFFF-driven PFAS contamination, the same scenario now unfolding around airports, military bases, and industrial sites worldwide, is associated with detectable alterations in the hormonal regulation of children at multiple levels of the endocrine system. With legacy PFAS persisting in the environment despite regulatory restrictions and being replaced by newer compounds whose health effects are poorly characterized, the researchers call for longitudinal follow-up of highly exposed populations, more sensitive hormone measurement methods, and standardized sampling to determine whether these early hormonal shifts translate into delayed puberty or long-term reproductive consequences.

Subject of Research: Associations between serum PFAS concentrations and reproductive hormone levels in highly exposed children and adolescents

Article Title: Serum PFAS Concentrations and Reproductive Hormones in Highly Exposed Children and Adolescents

Article References: Hammarstrand, S., Andersson, E., Colldén, H., Lindh, C., Nielsen, C., Jakobsson, K., & Andersson, E. M. (2026). Serum PFAS Concentrations and Reproductive Hormones in Highly Exposed Children and Adolescents. Environmental Advances, Article 100764. https://doi.org/10.1016/j.envadv.2026.100764

Image Credits: AI Generated

DOI: 10.1016/j.envadv.2026.100764

Keywords: PFAS, endocrine disruption, puberty, reproductive hormones, testosterone, prolactin, drinking water contamination, firefighting foam, Ronneby, children's health, HPG axis, environmental epidemiology

The central insight of the review is that fish reproduction is not an autonomous biological program that runs on schedule regardless of surroundings. Instead, it is orchestrated by the hypothalamic-pituitary-gonadal axis, or HPG axis, a hormonal cascade that translates environmental signals into gonadal growth, gamete production, and spawning. When water conditions drift outside tolerable ranges, this signaling chain falters. The hypothalamus releases gonadotropin-releasing hormone less effectively, the pituitary secretes fewer gonadotropins, and the gonads produce less testosterone and estradiol. The result is delayed maturation, reduced egg and sperm quality, and ultimately poorer seed production. Because hatchery managers frequently focus on feed and genetics while treating water quality as a background variable, the review suggests that suboptimal environmental conditions may be an underappreciated cause of disappointing breeding outcomes across the industry.

Light is perhaps the most powerful lever. Photoperiod is widely regarded as the primary environmental cue triggering puberty in fish, and it acts through a neuroendocrine network involving melatonin, dopamine, serotonin, kisspeptins, and gonadotropin-inhibitory hormone. In chub mackerel, extending day length and shifting light wavelength raised levels of testosterone and 17β-estradiol, while in rainbow trout longer photoperiods promoted ovulation and spermatogenesis in tandem with elevated circulating sex steroids. But the relationship is not simply more light, more eggs. In some species, excessively long photoperiods caused gonadal regression, and altered light cycles disrupted the synchronization of clock genes in zebrafish ovaries, a change associated in that species with ovarian tumor formation. Melatonin, secreted in darkness, acts as a negative regulator of the HPG axis, so photomanipulation that ignores species-specific rhythms can suppress rather than stimulate reproduction.

Temperature exerts equally profound effects, and the review documents them across cold-water and warm-water species alike. Elevated temperatures inhibited ovulation and reduced fertility and egg survival in Atlantic salmon and Arctic charr, decreased fertility and larval survival in rainbow trout and red seabream, and cut fertility and offspring survival in wolffish. In sea bass, high temperatures even induced male sex reversal, reshaping the sex ratio of an entire cohort. The molecular mechanisms are increasingly clear: warmer water suppresses expression of the aromatase gene Cyp19, the enzyme that converts testosterone to estradiol, undermining vitellogenesis and egg development, while colder water boosts dopamine secretion, which inhibits gonadotropin release. Elevated temperature also raises melatonin secretion, compounding the endocrine interference. For hatcheries, the implication is that thermal regimes must be tuned not just for growth but for the precise hormonal choreography of final egg maturation and spawning.

Acidity and buffering capacity form another quiet battleground. Fish generally thrive between pH 6.0 and 9.0, and deviations ripple through nearly every reproductive stage. In acidic water, migratory fish showed near-total hatching failure, the characin Astyanax lacustris produced eggs with reduced yolk and deformed chorions, and salmonids abandoned digging and upstream spawning behavior. Rainbow trout sperm fails to activate below pH 7 or 7.8, a fact with obvious consequences for captive breeding. Remarkably, pH can even bend sex ratios: in the West African cichlid Pelvicachromis pulcher, acidic conditions skewed offspring toward males, apparently by dampening aromatase expression during early development. Acidification delays spawning and induces ovarian atresia in salmonids by blocking vitellogenesis, and acid stress in rainbow trout appears to impair oocyte maturation, possibly through chromosomal damage during meiosis. Alkalinity compounds the problem, since fluctuations destabilize pH buffering and dissolved oxygen while stressing fish hormonally; medaka exposed to high alkalinity suffered reduced fecundity, impaired ovarian development, and downregulation of reproductive genes including those for gonadotropin and GnRH.

Water hardness, the concentration of calcium and magnesium salts, plays a subtler but species-dependent role. Ideal calcium hardness falls between 75 and 200 milligrams per liter as calcium carbonate, and deviations affect egg membrane permeability, sperm motility, and embryonic development. In guppies, harder water increased reproductive output, yet in Siamese fighting fish it impaired hatchability and nest building, a striking demonstration that one size fits no one. Hardness above 120 parts per million severely affected gonadal development in angelfish and suppressed maturation in rosy and tiger barbs, while in channel catfish, low-calcium water of 4.7 milligrams per liter reduced hatching rates. Excess calcium can even crystallize on the surface of Betta eggs, blocking water absorption into the perivitelline space and causing the eggs to dehydrate and shrink.

Dissolved oxygen may be the most consequential parameter of all, because aquatic habitats offer far less oxygen than air and extracting it costs energy. The review recommends maintaining at least 5 milligrams per liter at all times, and the evidence for why is stark. Koi carp held at 0.8 to 1 milligram per liter for one to three months showed disrupted steroidogenesis; goldfish at 0.8 milligrams per liter for eight weeks had reduced vitellogenin, poor egg growth, and halted spermatogenesis; male carp exposed to 1 milligram per liter for twelve weeks lost sperm motility; and Atlantic croaker suffered testicular damage after four weeks at 1.7 milligrams per liter. In zebrafish, hypoxia downregulated genes across the HPG axis, and in tambaqui it induced oxidative stress that shortened sperm motility duration. The authors frame chronic hypoxia as a genuine endocrine disruptor, one that impairs steroid production, gamete quality, fertilization, and embryogenesis, with severity scaling to both the depth and duration of oxygen deprivation.

Turbidity and nitrogenous waste complete the picture of an environment that can silently erode breeding success. Suspended particles scatter light, alter its spectrum, and interfere with the visual communication and mate selection on which many fish depend; in giant gourami, turbidity above 200 nephelometric turbidity units halted spawning entirely, and even 5 NTU reduced hatching rates in spotted gar. Meanwhile, the metabolic byproducts of intensive culture are potent reproductive toxicants. Unionized ammonia, toxic at 0.1 to 0.6 milligrams per liter, suppressed follicle-stimulating and luteinizing hormones in Nile tilapia and cut fertilization and hatching rates; in goldfish, 50 milligrams per liter for 48 hours triggered ovarian apoptosis and collapsed expression of steroidogenic genes such as CYP11a1 and CYP19a2. Nitrite converts hemoglobin to methaemoglobin and, in male zebrafish, caused severe testicular damage while suppressing gnrh2 and luteinizing hormone beta through the hypothalamic-pituitary-gonadal-liver axis. Nitrate, usually considered benign, disrupts steroid synthesis through nitric oxide chemistry: it binds heme proteins and cytochrome P450 enzymes including aromatase, and promotes S-nitrosylation of CYP11A, CYP17, CYP19A1, and the steroidogenic factor SF-1, effectively dismantling the enzymatic machinery of sex hormone production.

The review also highlights stressors that hatchery routines themselves create. Handling and confinement spike cortisol, which suppresses gonadal steroids and gonadotropin secretion; in snapper, capture stress blunted the ovulatory response even to exogenous hormone treatments, and in brown trout, chronic confinement followed by acute handling significantly reduced 11-ketotestosterone and testosterone. The authors advise keeping broodstock undisturbed after hormonal injections so cortisol can return to baseline before induced breeding. Nutrition, too, is inseparable from reproduction: sexual maturation requires crossing a genetically defined energy threshold, dietary lipids of 12 to 18 percent improved fecundity in rabbitfish, n-3 HUFA levels up to 1.6 percent boosted fecundity in gilthead seabream, and tryptophan supplementation advanced spermiation in ayu. Beyond the farm, heavy metals, pesticides, metallic nanoparticles, and microplastics act as endocrine disruptors that accumulate in gonads and interfere with the HPG axis, meaning even well-managed facilities inherit contamination from their water sources.

The practical takeaway is a set of concrete targets that any hatchery can audit: species-appropriate photoperiod, temperature within optimal breeding ranges, pH between 6.0 and 9.0, alkalinity of 60 to 150 milligrams per liter as calcium carbonate, hardness of 75 to 200, dissolved oxygen at or above 5 milligrams per liter, turbidity between 30 and 80 centimeters of visibility, ammonia and nitrite each below 0.02 parts per million, and nitrate between 0 and 200 parts per million, all paired with minimal handling stress and balanced broodstock nutrition. What elevates this review beyond a checklist is its molecular framing: by tracing each parameter to specific genes, enzymes, and hormonal pathways, it gives researchers a way to separate environmental effects from those of feed and genetics in breeding experiments. As aquaculture expands to feed a growing population while wild stocks decline, the difference between a thriving hatchery and a failing one may come down to invisible shifts in water chemistry that fish sense long before their keepers do.

Subject of Research: Effects of shifting abiotic water quality factors on fish breeding and reproduction in aquaculture

Article Title: Impact of shifting abiotic factors in aquaculture on fish breeding and reproduction: a review

Article References: Bhat, I. A., Rather, M. A., Ahmad, I., Ahmad, I., Mir, I. N., & Hussna (2025). Impact of shifting abiotic factors in aquaculture on fish breeding and reproduction: a review. Blue Biotechnology, 2(1), Article 3. https://doi.org/10.1186/s44315-025-00027-9

Image Credits: AI Generated

DOI: 10.1186/s44315-025-00027-9

Keywords: aquaculture, fish reproduction, HPG axis, water quality, photoperiod, temperature, dissolved oxygen, pH, ammonia toxicity, nitrate, broodstock management, endocrine disruption

Heat stress is a well-established enemy of sperm. Decades of work in livestock, laboratory animals, and humans have shown that even modest elevations in scrotal temperature can reduce sperm motility and concentration, increase abnormal sperm morphology, and inflict histological damage on the seminiferous tubules where sperm are produced. The mechanisms are familiar: oxidative stress surges, inflammatory signaling ramps up, and germ cells succumb to apoptosis. What has received far less attention, the authors argue, is the role of the central nervous system. The hypothalamic-pituitary-gonadal axis, the hormonal hierarchy that runs from GnRH neurons in the hypothalamus down through pituitary LH and FSH to the gonads, is itself vulnerable to thermal and inflammatory insults, and the study set out to ask whether protecting that axis might be part of what makes a protective compound effective.

To test this, the team used male C57BL/6J mice divided into three groups: a control group, a heat stress group receiving vehicle, and a heat stress group receiving taurine. The heat exposure protocol was deliberately rigorous, subjecting animals to 40 ± 1 °C for one hour daily over fourteen consecutive days, a regimen designed to mimic the kind of repeated, subacute thermal challenge that accumulates during hot seasons or occupational heat exposure rather than a single dramatic event. Taurine was administered intraperitoneally before each heat exposure, ensuring the amino acid was present in the circulation at the moment the thermal insult began. This pretreatment design is important because it models a preventive intervention, something that could plausibly be deployed before anticipated heat exposure rather than after damage has already accumulated.

The results in the heat-stressed vehicle group were sobering and consistent with the established literature. Sperm motility and concentration fell, the proportion of abnormally shaped sperm rose, and testicular histopathology revealed marked injury to the seminiferous architecture. Beneath these visible outcomes, the molecular picture was equally troubled: the testes showed elevated inflammatory markers, heightened oxidative stress, and increased apoptotic activity, the classic triad of heat-induced germ cell damage. Crucially, the disruption was not confined to the gonads. The hypothalamic expression of Gnrh was altered, pituitary expression of the gonadotropin subunits Lhb and Fshb shifted, and circulating reproductive hormone levels drifted away from their normal balance, evidence that the entire HPG axis had been destabilized by the repeated thermal challenge.

Perhaps the most novel element of the study lies in what the researchers observed in the hypothalamus itself. Repeated heat stress was associated with activation of microglia, the resident immune cells of the brain, accompanied by an amplified inflammatory response within hypothalamic tissue. Notably, the team documented increased overlap between microglia and GnRH neurons, a spatial association suggesting that these immune cells were physically interacting with, or clustering around, the very neurons that initiate the reproductive hormonal cascade. Hypothalamic microglia are increasingly recognized as sensitive sentinels of metabolic and systemic stress, capable of modulating neuronal function through inflammatory signaling, and prior work has implicated glial-neuronal interactions in the control of GnRH secretion. The new data place thermal stress squarely within this emerging framework of neuroimmune regulation of fertility.

Against this backdrop, taurine pretreatment produced strikingly broad protection. Mice that received the amino acid before each heat exposure retained significantly better sperm motility and concentration, showed fewer abnormal sperm, and displayed markedly less testicular histopathological injury than their vehicle-treated counterparts. At the molecular level, taurine dampened the heat-induced inflammation, reduced oxidative stress, and curtailed apoptosis in testicular tissue. The compound also partially restored endocrine homeostasis, normalizing the disturbed patterns of hypothalamic Gnrh expression, pituitary gonadotropin subunit expression, and circulating reproductive hormones. And in the brain, taurine reduced the hypothalamic microglial response, tempering both the inflammatory activation and the increased microglia-GnRH overlap observed in heat-stressed animals.

Taurine is an intriguing candidate for this role. It is one of the most abundant amino acids in the body, abundant in immune cells, the brain, and the male reproductive tract, and it carries well-documented antioxidant and anti-inflammatory credentials. Previous studies have shown that taurine enhances spermatogenic function and antioxidant defenses in hypertensive rats, and the same research group has previously reported that taurine protects against heat stress-induced cognitive impairment in mice through hypothalamic mechanisms. The new work extends this protective portfolio into reproductive physiology and, importantly, links the peripheral and central benefits in a single experimental design. The authors propose that the coordinated improvement across testicular injury, endocrine imbalance, and hypothalamic inflammation suggests taurine acts on multiple nodes of the reproductive axis simultaneously, rather than as a simple testicular antioxidant.

The implications extend beyond the laboratory. As global temperatures climb and heat waves grow longer and more frequent, concerns about heat-related declines in male fertility have moved from the veterinary literature, where heat stress and bull fertility are long-standing economic concerns, into mainstream human health discussions. Epidemiological and experimental evidence already links hot seasons, occupational heat exposure, and scrotal heating to reduced sperm quality in men. If the neuroendocrine mechanisms described in this study translate to humans, they would suggest that the fertility cost of chronic heat exposure may be compounded by a central hormonal component, one that purely local interventions such as scrotal cooling would not address. A safe, widely available compound that buffers both the gonadal and the neuroendocrine consequences of heat would represent an attractive preventive strategy for livestock management, occupational health, and potentially human fertility preservation in hot climates.

The authors are careful, however, to draw a clear line around what their data can and cannot claim. The hypothalamic findings are associative: microglial activation, inflammatory signaling, and increased microglia-GnRH overlap were observed alongside reproductive dysfunction and its attenuation by taurine, but the study does not establish that microglial changes cause the reproductive impairment or that suppressing microglia is the mechanism by which taurine acts. Disentangling correlation from causation in the hypothalamus will require interventional studies, for example selective manipulation of microglial activity during heat stress, and dose-response and translational work will be needed before any recommendations for human supplementation could be contemplated. The intraperitoneal dosing used in mice also differs fundamentally from oral intake, which is how humans would encounter taurine in practice.

Even with those caveats, the study marks a meaningful conceptual shift. It reframes heat-induced male reproductive dysfunction as a whole-axis phenomenon, a coordinated failure spanning brain, pituitary, and testis, rather than a testis-centered injury with hormonal bystanders. And it identifies a plausible, accessible intervention with demonstrated efficacy across all three levels of that axis in a demanding repeated-exposure model. As researchers continue to probe the neuroimmune control of GnRH neurons and the growing burden of thermal stress on reproduction, taurine’s performance in this study offers both a mechanistic lead and a practical starting point. For a field racing to keep pace with a warming planet, a humble amino acid that quiets angry microglia while safeguarding sperm may prove to be one of the more quietly important findings of the season.

Subject of Research: Taurine attenuation of heat stress-induced male reproductive dysfunction involving HPG axis homeostasis and hypothalamic microglial changes in mice

Article Title: Taurine Attenuates Repeated Heat Stress-Induced Male Reproductive Dysfunction in Mice: Associations with HPG Axis Homeostasis and Hypothalamic Microglial Changes

Article References: Li, B., Ming, R., Liu, Y., & Guo, H. (2026). Taurine Attenuates Repeated Heat Stress-Induced Male Reproductive Dysfunction in Mice: Associations with HPG Axis Homeostasis and Hypothalamic Microglial Changes. Reproductive Sciences. https://doi.org/10.1007/s43032-026-02209-1

Image Credits: AI Generated

DOI: 10.1007/s43032-026-02209-1

Keywords: taurine, heat stress, male fertility, spermatogenesis, HPG axis, GnRH, hypothalamic microglia, oxidative stress, inflammation, apoptosis, neuroendocrine regulation, Attenuates