Module 16: Climate Biochemistry

Heat-shock proteins (HSPs) are the universal molecular chaperones that refold thermally denatured proteins and escort terminally damaged ones to the proteasome. The canonical HSP70 ATP cycle (Bukau & Horwich, 1998):

Thermally inducible HSP70 genes carry heat-shock elements (HSEs) bound by the trimerised transcription factor HSF1 when cytosolic unfolded-protein load exceeds threshold. Tomanek (2010) measured species-specific induction thresholds in congeneric Mytilus mussels: the cool-temperate M. trossulus induces HSP70 at\(\sim 23\;{}^{\circ}\mathrm{C}\), whereas the warm-adapted invasive M. galloprovincialis does not engage HSF1 until\(\sim 28\;{}^{\circ}\mathrm{C}\). The difference predicts the northward expansion of M. galloprovincialis observed along the California coast since the 1990s.

HSP90 and the Capacitor Hypothesis

HSP90 chaperones metastable client proteins including ~200 kinases and transcription factors. Rutherford & Lindquist (1998) proposed that HSP90 buffers cryptic genetic variation — when heat stress saturates HSP90, previously hidden mutations become phenotypically visible and subject to selection. This is a molecular mechanism for “evolutionary capacitance” and may accelerate adaptation to climate change.

Fitness Trade-offs

Sørensen et al. (2003) review the cost of HSP expression: each HSP molecule required 8 ATP for synthesis plus ongoing ATP for client refolding. Constitutive HSP overexpression slows Drosophila development by ~15% and reduces fecundity, giving rise to a classic protection–performance trade-off. The optimal induction threshold is therefore environment-specific, and rapid warming can leave populations stuck at an obsolete HSF1 set-point.

At the opposite thermal extreme, organisms survive by preventing, tolerating or controlling ice nucleation. Three classes of cryoprotectants dominate:

• Colligative: glycerol, glucose, sorbitol, trehalose. Depress freezing point via Raoult’s law (\(\Delta T = K_f\,m\), \(K_f = 1.86\;\mathrm{K\cdot kg/mol}\)). Wood frog Rana sylvatica accumulates up to 0.3 M glycerol+glucose and survives with 65% of body water frozen (Storey & Storey, 1988).
• Non-colligative antifreeze proteins (AFPs): four types (AFP I-IV) plus antifreeze glycoproteins (AFGPs) adsorb to ice primary prism faces and create a non-equilibrium gap between the melting and freezing points — the thermal hysteresis. DeVries (1969) discovered AFGPs in the notothenioid Dissostichus mawsoni, which maintain serum at\(-2.1\;{}^{\circ}\mathrm{C}\) in\(-1.9\;{}^{\circ}\mathrm{C}\) seawater.
• Vitrification by trehalose: tardigrades, artemia and anhydrobiotic nematodes replace cellular water with trehalose glass (Crowe et al., 1984). The glass prevents membrane fusion and stabilises proteins via the water-replacement hypothesis.

AFP Adsorption-Inhibition Theory

The Kelvin effect predicts that a spherical ice nucleus of radius \(r\)melts at \(T_m - 2\sigma T_m/(r\rho L)\). AFPs adsorb irreversibly to the ice surface between discrete pinning sites separated by \(d\), so the ice can only grow as microscopic curved bumps between pins. This raises the local effective curvature and depresses the freezing point by:

Evolutionary Origin

Chen et al. (1997) traced notothenioid AFGPs to a duplication of the trypsinogen gene\(\sim 5\text{-}14\) Mya, coincident with final cooling of the Antarctic Circumpolar Current and the glaciation of Antarctica. The AFGP nine-amino-acid repeat (Thr-Ala/Pro-Ala)\(_n\) arose from an in-frame insertion in the trypsinogen signal peptide — one of the cleanest “neofunctionalisation” stories in molecular evolution.

UV-B radiation (280–315 nm) damages DNA (thymine dimers) and photosystem II (D1 protein). Plants, especially in alpine and tropical systems, synthesise a shielding armoury of flavonoids— including chalcones, flavones, flavonols and anthocyanins—through the phenylpropanoid pathway (Winkel-Shirley, 2001). The rate-limiting enzyme chalcone synthase (CHS) condenses one \(p\)-coumaroyl-CoA with three malonyl-CoA to give naringenin chalcone.

Anthocyanins absorb strongly in the UV and green (500–550 nm), giving autumn maples and high-altitude saxifrages their red colour. Chalker-Scott (1999) reviewed the multiple roles of anthocyanins: UV sunscreen, osmoregulation during cold acclimation, scavenging of reactive oxygen species, and herbivore deterrent. Close & Beadle (2003) documented that the ratio of anthocyanin to chlorophyll increases sharply in juvenile Eucalyptus leaves above treeline, consistent with photoprotective function.

Heat, cold, UV and desiccation all generate reactive oxygen species (ROS) — superoxide O\(_2^{\cdot-}\), hydrogen peroxide H\(_2\)O\(_2\)and hydroxyl radical OH\(^{\cdot}\) — primarily from mitochondrial complex III and chloroplast photosystem I. The enzymatic defence cascade (Halliwell, 2006):

Glutathione peroxidase uses the tripeptide GSH (gamma-Glu-Cys-Gly) to reduce H\(_2\)O\(_2\)and organic peroxides, oxidising itself to GSSG; glutathione reductase recycles GSSG to GSH using NADPH from the pentose phosphate pathway. When ROS production overwhelms this cycle, lipid peroxidation of membrane PUFAs produces malondialdehyde (MDA) and 4-hydroxynonenal (4-HNE) — both used as biomarkers of climate-induced oxidative damage in fish, corals and plants.

Coral Oxidative Bleaching

Heat stress in Symbiodinium dinoflagellates inhibits the D1 subunit of PSII, causing electron overflow to O\(_2\) and a ROS burst that triggers the coral host to expel its symbionts (Lesser, 2006; Yakovleva et al., 2009 documented catalase induction). Clade D Durusdinium has a more heat-stable PSII and a 10× higher catalase activity, giving holobionts that host it a \(\sim 1.5\;{}^{\circ}\mathrm{C}\)advantage in thermal tolerance (Stat & Gates, 2011).

Galloway et al. (2008) quantified the global doubling of reactive-nitrogen flux (N\(_r\)) over the 20th century — from\(\sim 15\) to \(\sim 190\;\text{Tg\,N/yr}\) — driven by industrial Haber–Bosch NH\(_3\)synthesis and combustion-derived NO\(_x\). Every N atom fixed cascades through soil, water and air, producing NO\(_3^-\) runoff, NH\(_3\)volatilisation, N\(_2\)O greenhouse forcing and tropospheric-ozone precursors, before being finally denitrified back to N\(_2\).

N\(_2\)O is a long-lived (~116 yr) greenhouse gas with a 100-yr GWP of 273. Bacterial denitrification via NirK/NirS (copper and cytochrome-cd\(_1\)nitrite reductases) drives most emissions:

In the ocean, diazotroph cyanobacteria (Trichodesmium, UCYN-A, Richelia) fix N\(_2\) to NH\(_4^+\) using the Fe-Mo cofactor of nitrogenase. Warming expands their range poleward but increased oceanic CO\(_2\) may lower pH below nitrogenase tolerance — a climate-nitrogen interaction still being resolved (Hutchins et al., 2007).

Photosystem II (PSII) is the single most photolabile complex in any chloroplast or cyanobacterial thylakoid. Its D1 subunit accumulates irreversible singlet-oxygen damage at a turnover rate of ~1 molecule every 30 min under full sun, and the D1 repair cycle (Aro et al., 1993) is a dedicated machinery to replace it: lateral migration of damaged PSII from the grana to stroma lamellae, proteolysis by FtsH/Deg, import of newly translated D1, reassembly of the oxygen-evolving cluster (Mn\(_4\)CaO\(_5\)), and return to the grana.

Heat stress simultaneously accelerates D1 damage (more ROS) and inhibits FtsH-mediated proteolysis (ATP demand competes with Rubisco activase), so the net D1 pool collapses. This is the molecular basis for both coral bleaching and mid-afternoon photosynthetic depression in C\(_3\) crops under heatwaves. The xanthophyll cycle (violaxanthin → zeaxanthin via VDE) dissipates excess excitation energy as heat via non-photochemical quenching (NPQ); rapid NPQ relaxation is a trait under active breeding selection to boost field crop yield under fluctuating light (Kümmel & Long, 2016).

To cope with salinity, pressure and temperature, cells accumulate compatible osmolytes: small organic molecules that raise osmotic pressure without disrupting protein function. Yancey (2005) classifies them as polyols (glycerol, sorbitol), amino acids (proline, taurine), methylamines (betaine, TMAO) and urea.

Deep-sea fish concentrate trimethylamine N-oxide (TMAO) linearly with depth — about \(60\;\mathrm{mmol/L/km}\) (Yancey et al., 2014). TMAO is a classic piezolyte that counteracts the denaturing effect of hydrostatic pressure on enzymes. Plants under drought accumulate proline and glycine betaine via the P5CS/BADH pathways; both stabilise Rubisco and cell membranes. Ectoine produced by halotolerant bacteria (Halomonas) is now commercialised as a cosmetic active — a microbial climate-adaptation strategy turned industrial.

Rubisco is the slowest and most abundant enzyme on Earth (\(k_{cat}\approx 3\;\mathrm{s^{-1}}\),\(\sim 10^{15}\;\mathrm{g}\) global mass). Its kinetics are captured by the Farquhar-von Caemmerer-Berry (1980) model:

C\(_4\) plants (grasses, maize, sugarcane) shuttle CO\(_2\) from mesophyll to bundle-sheath cells via PEP carboxylase + malate, suppressing photorespiration and extending the thermal niche to\(40\;{}^{\circ}\mathrm{C}\). CAM plants (agaves, cacti, epiphytic orchids) fix CO\(_2\) at night as malate and decarboxylate during day to feed Rubisco behind closed stomata — the ultimate water-saving strategy. Climate-change projections expect C\(_4\) grasses to advantage further under warming but lose ground under elevated CO\(_2\) (because C\(_3\) plants benefit more from CO\(_2\) fertilisation; Ehleringer & Cerling, 2002).

On the respiration side, the temperature sensitivity\(Q_{10}\approx 2\text{-}3\) means that a\(4\;{}^{\circ}\mathrm{C}\) warming doubles soil and autotrophic respiration. Anderson (1973) showed that the photosynthetic quotient PQ = O\(_2\) evolved per CO\(_2\)fixed is ~1.1–1.3, and the respiratory quotient RQ is ~0.8. Ecosystems shift from net sink to net source when respiration outpaces photosynthesis — the essence of the temperate and boreal carbon-climate feedback (Schuur et al., 2015 for permafrost).

Plants sensing cold activate the CBF/DREB1transcription factors, which bind the CRT/DRE element upstream of ~100 cold-regulated (COR) genes including antifreeze proteins, dehydrins and enzymes of compatible solute biosynthesis (Thomashow, 1999). Overexpression of CBF3 in Arabidopsisconfers freezing tolerance from \(-5\) to \(-10\;{}^{\circ}\mathrm{C}\).

In parallel, membrane lipid composition is remodelled by fatty acid desaturases (\(\Delta 9\), \(\Delta 12\),\(\Delta 15\)), which introduce double bonds to maintain membrane fluidity at low temperature. Engineered expression of FAD7 in tobacco raises the chilling tolerance threshold by \(3\;{}^{\circ}\mathrm{C}\) (Murata et al., 1992). The same principle applies to fish (Carassius carp upregulate\(\Delta 9\)-desaturase with cold) and marine plankton.

Mammalian hibernators (ground squirrels, bears, bats) lower their body temperature to within a few degrees of ambient during torpor, cutting metabolic rate by\(\sim 100\)×. At the molecular level this requires a coordinated shutdown of transcription, SUMOylation-driven proteostasis, and reversible phosphorylation of metabolic enzymes (Carey et al., 2003). Climate warming that shortens the hibernation season may increase apparent reproductive output but simultaneously raises winter metabolic costs — a mechanism implicated in the decline of yellow-bellied marmots and in the demographic collapse of hibernating bats to white-nose syndrome (Pseudogymnoascus destructans, Lorch et al., 2011), because pathogens proliferate at the elevated winter temperatures.

In insects, facultative diapause is controlled by juvenile-hormone signalling and FoxO-mediated stress responses. Phenological mismatch from warming frequently traps insects in the active state past their food season; conversely, chronic mild warmth prevents the cold induction needed to break diapause. Both failure modes are observed in the pine processionary moth (Thaumetopoea pityocampa) range expansion across Europe over the past 30 years.

Microbial biochemistry underlies every major climate-carbon feedback. In thawing permafrost peat, methanogenic archaea (Methanosarcina, Methanocella) degrade acetate and H\(_2\)/CO\(_2\) to CH\(_4\)using the Ni-tetrapyrrole coenzyme F430; aerobic methanotrophs (Methylococcus, Methylocystis) oxidise up to 80% of the flux before it escapes. Schuur et al. (2015) estimate that warming could release\(30\text{-}300\;\mathrm{Pg\,C}\) from permafrost by 2100. The CH\(_4\):CO\(_2\) emission ratio depends on soil water content and redox state — a single percent shift toward anaerobic decomposition doubles the instantaneous warming contribution.

Marine methylotrophic bacteria (OM43 clade, SAR324) consume DMS\(_p\) produced by phytoplankton under oxidative stress; the remaining DMS escapes to the atmosphere and nucleates cloud condensation nuclei — the Charlson-Lovelock-Andreae-Warren mechanism. Climate warming that stratifies the surface ocean weakens this negative feedback.

Noyes et al. (2009) review how warming amplifies the toxicity of persistent organic pollutants (POPs), pesticides and heavy metals. Mechanisms include: higher uptake rates (metabolism Q\(_{10}\)), lower detoxification capacity when HSP/GSH cycles are committed to thermal repair, and volatilisation + long-range transport to cold condensation traps. Neonicotinoid pesticides combined with heat stress produce >10× synergistic mortality in bumblebees (Baron et al., 2017), an important feedback for pollinator networks.

Plants respond to elevated CO\(_2\) and warming by shifting secondary metabolite profiles. Peñuelas et al. (2013) document increased C:N stoichiometry in leaves (the “dilution” effect) and rising tannin content under free-air CO\(_2\) enrichment (FACE) — a plant-defence upregulation that makes foliage less nutritious for herbivores. The jasmonic-acid signalling network (JA -> JAZ degradation -> MYC2 activation) integrates wound, heat and herbivore cues into a unified transcriptional response.

Volatile isoprene (C\(_5\)H\(_8\)) emission by oaks and poplars is a thermal protectant for thylakoid membranes; it also forms secondary organic aerosol (SOA) in the atmosphere and contributes to cloud formation over tropical forests (Guenther et al., 2012). Warmer temperatures raise isoprene emission exponentially (Q\(_{10}\sim 3\)), creating a feedback that may partially offset forest warming but also enhance tropospheric ozone.

Angilletta (2009) synthesises thermal performance curves (TPCs): skewed unimodal functions with optimum \(T_{opt}\) and critical thermal maximum\(CT_{\max}\). Evolution can shift\(T_{opt}\) but \(CT_{\max}\) is phylogenetically constrained: Kellermann et al. (2012) sampled 95 Drosophila species and found that\(CT_{\max}\) variation across species is <10% of latitudinal temperature variation. Tropical ectotherms therefore live closest to their thermal ceiling and have the least evolutionary room for adaptation.

The “fundamental thermal niche breadth rule” (Deutsch et al., 2008) states that tropical species have narrower thermal tolerance than temperate/polar species, so even small climate shifts push them past CT\(_{\max}\). Field evidence: Calanus copepod populations in the North Sea have shifted 1000 km since 1960 (Beaugrand et al., 2002), whereas tropical coral holobionts reshuffle Symbiodinium genotypes but cannot track temperature. Climate-adaptation policy must therefore weigh the ability to move (temperate) against the ability to switch physiology (tropical).

Assisted evolution in corals — artificially selecting heat-tolerant Durusdinium symbionts and conducting cross-breeding between populations (van Oppen et al., 2015) — is now being trialled on the Great Barrier Reef as a stopgap. The ethical and ecological risks (introgression, local maladaptation) mirror those of assisted migration in terrestrial conservation.

Throughout this course we have tracked climate-change impacts up the ecological hierarchy: radiative forcing → ocean/atmosphere circulation → ecosystem structure → species range shifts → extinction risk. This final module closes the loop by showing that every one of those processes bottoms out in a small set of molecular machines: the Rubisco active site, the HSF1 trimerisation interface, the AFP ice-binding face, the membrane desaturase enzyme, the nitrogenase Fe-Mo cofactor, the SOD-catalase-GSH cascade.

These machines have evolutionary half-lives on the order of millions of years, while climate is being reorganised on the order of a century. The central question of 21st century biology is therefore: which organisms can acclimate (regulate existing pathways), which can adapt (evolve new set-points), and which will go extinct because their molecular toolkit is fixed at what is, by 2100, the wrong temperature?

The three simulations across this module (HSP70 induction, Farquhar-VCB photosynthesis, AFP thermal hysteresis) illustrate the quantitative form of those trade-offs, but the parameters are species-specific. A useful research programme for the next decade is to measure CT\(_{\max}\), CBF/DREB set-points and HSF1 thresholds across the tree of life and feed them back into the climate-velocity framework of Module 15. Doing so transforms “climate change biology” from a phenomenological enterprise into a mechanistic one — grounded in chemistry, scaleable to the biosphere, and directly informative for conservation policy.

With that, our tour of climate change and biodiversity is complete. The physics and chemistry introduced here underpin every later decision about ecosystem services, protected-area design, fisheries management, assisted migration and geoengineering. Understanding them is the prerequisite for any credible response.