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1. Implications of chronic hypoxia during development in red drum

   https://doi.org/10.1242/jeb.247618  

   Negrete, Benjamin; Ackerly, Kerri Lynn; Esbaugh, Andrew J (August 2024, Journal of Experimental Biology)

   ABSTRACT Respiratory plasticity is a beneficial response to chronic hypoxia in fish. Red drum, a teleost that commonly experiences hypoxia in the Gulf of Mexico, have shown respiratory plasticity following sublethal hypoxia exposure as juveniles, but implications of hypoxia exposure during development are unknown. We exposed red drum embryos to hypoxia (40% air saturation) or normoxia (100% air saturation) for 3 days post fertilization (dpf). This time frame encompasses hatch and exogenous feeding. At 3 dpf, there was no difference in survival or changes in size. After the 3-day hypoxia exposure, all larvae were moved and reared in common normoxic conditions. Fish were reared for ∼3 months and effects of the developmental hypoxia exposure on swim performance and whole-animal aerobic metabolism were measured. We used a cross design wherein fish from normoxia (N=24) were exercised in swim tunnels in both hypoxia (40%, n=12) and normoxia (100%, n=12) conditions, and likewise for hypoxia-exposed fish (n=10 in each group). Oxygen consumption, critical swim speed (Ucrit), critical oxygen threshold (Pcrit) and mitochondrial respiration were measured. Hypoxia-exposed fish had higher aerobic scope, maximum metabolic rate, and higher liver mitochondrial efficiency relative to control fish in normoxia. Interestingly, hypoxia-exposed fish showed increased hypoxia sensitivity (higher Pcrit) and recruited burst swimming at lower swim speeds relative to control fish. These data provide evidence that early hypoxia exposure leads to a complex response in later life. 

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2. Gulf toadfish (Opsanus beta) gill neuroepithelial cells in response to hypoxia exposure

   https://doi.org/10.1007/s00360-024-01547-3  

   Duh, Orianna A; McDonald, M Danielle (April 2024, Journal of Comparative Physiology B)

   Neuroepithelial cells (NECs) within the fish gill contain the monoamine neurochemical serotonin (5-HT), sense changes in the partial pressure of oxygen (PO2) in the surrounding water and blood, and initiate the cardiovascular and ventilatory responses to hypoxia. The distribution of neuroepithelial cells (NECs) within the gill is known for some fish species but not for the Gulf toadfish, Opsanus beta, a fish that has always been considered hypoxia tolerant. Furthermore, whether NEC size, number, or distribution changes after chronic exposure to hypoxia, has never been tested. We hypothesize that toadfish NECs will respond to hypoxia with an increase in NEC size, number, and a change in distribution. Juvenile toadfish (N = 24) were exposed to either normoxia (21.4 ± 0.0 kPa), mild hypoxia (10.2 ± 0.3 kPa), or severe hypoxia (3.1 ± 0.2 kPa) for 7 days and NEC size, number, and distribution for each O2 regime were measured. Under normoxic conditions, juvenile toadfish have similar NEC size, number, and distribution as other fish species with NECs along their filaments but not throughout the lamellae. The distribution of NECs did not change with hypoxia exposure. Mild hypoxia exposure had no effect on NEC size or number, but fish exposed to severe hypoxia had a higher NEC density (# per mm filament) compared to mild hypoxia-exposed fish. Fish exposed to severe hypoxia also had longer gill filament lengths that could not be explained by body weight. These results point to signs of phenotypic plasticity in these juvenile, lab-bred fish with no previous exposure to hypoxia and a strategy to deal with hypoxia exposure that differs in toadfish compared to other fish. 

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3. Hypoxia is common in temperate headwaters and driven by hydrological extremes

   https://doi.org/10.1016/j.ecolind.2023.109987  

   Diamond, Jacob S.; Moatar, Florentina; Recoura-Massaquant, Rémi; Chaumot, Arnaud; Zarnetske, Jay; Valette, Laurent; Pinay, Gilles (February 2023, Ecological indicators)

   Hypoxia, or dissolved oxygen (DO) at low enough levels to impair organisms, is a particularly useful indicator of the health of freshwater ecosystems. However, due to limited sampling in headwater networks, the degree, distribution, and timing of hypoxia events are not known across the vast majority of most river networks. We thus sought to clarify the extent of hypoxia in headwater networks through three years of instrumentation of 78 sites across eight temperate, agricultural watersheds. We observed broadly distributed hypoxia, occurring 4 % of the time across 51 of the 78 sites over 20 months. The hypoxia was driven by three mechanisms: storm events, drying, and rewetting, with drying as the most common driver of hypoxia (55 % of all hypoxic event types). Drying induced hypoxia was most severe in smaller streams (Strahler orders ≤ 3), whereas storm events pref- erentially induced hypoxia in the larger streams (Strahler orders 3–5). A large diversity in DO trajectories to- wards hypoxia depended on hydrologic event type, with subsequent expected differences in mortality profiles of a sensitive species. Predictive models showed the most vulnerable sites to hypoxia were small streams with low slope, particularly during hot, low discharge periods. Despite variation among hypoxic events, there was remarkable similarity in the rate of DO drawdown during hypoxia events (ca. 1 mg O2 L− 1 d− 1). This drawdown similarity may be a useful rule-of-thumb for managers, and we hypothesize that it is either a signal of increasing lateral inflow of low DO water or a signal of increasing downstream oxygen demand. Overall, we posit that hypoxia is likely a common feature of most headwater networks that often goes undetected. Headwater hypoxia may become more common under increasingly dry conditions associated with climate and water resource management changes, with important implications for biological communities and biogeochemical processes. 

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4. A new microphysiological system shows hypoxia primes human ISCs for interleukin-dependent rescue of stem cell activity

   https://doi.org/10.1101/2023.01.31.524747  

   Rivera, Kristina R; Bliton, R Jarrett; Burclaff, Joseph; Czerwinski, Michael J; Liu, Jintong; Trueblood, Jessica M; Hinesley, Caroline M; Breau, Keith A; Joshi, Shlok; Pozdin, Vladimir A; et al (February 2023, bioRxiv)

   ABSTRACT Background & AimsHypoxia in the intestinal epithelium can be caused by acute ischemic events or conditions like Inflammatory Bowel Disease (IBD) where immune cell infiltration produces ‘inflammatory hypoxia’, a chronic condition that starves the mucosa of oxygen. Epithelial regeneration after ischemia and IBD suggests intestinal stem cells (ISCs) are highly tolerant to acute and chronic hypoxia; however, the impact of acute and chronic hypoxia on human ISC (hISC) properties have not been reported. Here we present a new microphysiological system (MPS) to investigate how hypoxia affects hISCs isolated from healthy human tissues. We then test the hypothesis that some inflammation-associated interleukins protect hISCs during prolonged hypoxia. MethodshISCs were exposed to <1.0% oxygen in the MPS for 6-, 24-, 48- & 72hrs. Viability, HIF1α response, transcriptomics, cell cycle dynamics, and hISC response to cytokines were evaluated. ResultsThe novel MPS enables precise, real-time control and monitoring of oxygen levels at the cell surface. Under hypoxia, hISCs remain viable until 72hrs and exhibit peak HIF1α at 24hrs. hISCs lose stem cell activity at 24hrs that recovers at 48hrs of hypoxia. Hypoxia increases the proportion of hISCs in G1 and regulates hISC capacity to respond to multiple inflammatory signals. Hypoxia induces hISCs to upregulate many interleukin receptors and hISCs demonstrate hypoxia-dependent cell cycle regulation and increased organoid forming efficiency when treated with specific interleukins ConclusionsHypoxia primes hISCs to respond differently to interleukins than hISCs in normoxia through a transcriptional response. hISCs slow cell cycle progression and increase hISC activity when treated with hypoxia and specific interleukins. These findings have important implications for epithelial regeneration in the gut during inflammatory events. 

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5. Short- and long-term impacts of variable hypoxia exposures on kelp forest sea urchins

   https://doi.org/10.1038/s41598-020-59483-5  

   Low, Natalie H. N.; Micheli, Fiorenza (February 2020, Scientific Reports)

   Abstract Climate change is altering the intensity and variability of environmental stress that organisms and ecosystems experience, but effects of changing stress regimes are not well understood. We examined impacts of constant and variable sublethal hypoxia exposures on multiple biological processes in the sea urchinStrongylocentrotus purpuratus, a key grazer in California Current kelp forests, which experience high variability in physical conditions. We quantified metabolic rates, grazing, growth, calcification, spine regeneration, and gonad production under constant, 3-hour variable, and 6-hour variable exposures to sublethal hypoxia, and compared responses for each hypoxia regime to normoxic conditions. Sea urchins in constant hypoxia maintained baseline metabolic rates, but had lower grazing, gonad development, and calcification rates than those in ambient conditions. The sublethal impacts of variable hypoxia differed among biological processes. Spine regrowth was reduced under all hypoxia treatments, calcification rates under variable hypoxia were intermediate between normoxia and constant hypoxia, and gonad production correlated negatively with continuous time under hypoxia. Therefore, exposure variability can differentially modulate the impacts of sublethal hypoxia, and may impact sea urchin populations and ecosystems via reduced feeding and reproduction. Addressing realistic, multifaceted stressor exposures and multiple biological responses is crucial for understanding climate change impacts on species and ecosystems. 

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