Volume VII: Nature — Chapter 10: Symbiosis and Coevolution

"Nobody on Théxar lives alone. The lichen on the stone is two. The reef in the depths is hundreds. The ocean is billions. And we ourselves are also a union: a body of trillions, agreed to breathe together." — Shan-En Institute of Xenobiology, treatise "The Bonds," +3,340 Y.E.

Related sections: → see [Volume I, Ch.04, "Biochemistry"], → see [Volume I, Ch.20, "The Origin of Life"], → see [Volume I, Ch.21, "Ecosystems"], → see [Volume VII, Ch.03, "Marine Ecosystems"], → see [Volume VII, Ch.09, "The Plant Kingdom"], → see [Volume VII, Ch.11, "Evolution Patterns"], → see [Volume VII, Ch.13, "Biosphere Interactions"] Length: 15 pages

Introduction

Life on Théxar arose in an environment where chemical gradients were sharp and variable, and energy was abundant, but "food" was scarce. Under such conditions the strategy of "absorb and digest the competitor" almost always yielded to the strategy of "let inside the one who knows how to do what you cannot." The result is a biosphere in which symbiosis is not the exception but the rule.

The six-letter genetic code and the 24 proteinogenic amino acids (→ see [Volume I, Ch.04, "Biochemistry"]) gave the symbiosis of Théxar an additional dimension: genetic exchange between species occurs more often, and horizontal gene transfer covers not only prokaryotes but also eukaryote-like forms. Some researchers call the biosphere of Théxar a "superorganism with a divided genome" — cautiously, since the boundary between symbiosis and fusion is deliberately blurred here.

This chapter examines the forms of symbiosis on Théxar, the key symbiotic systems, and their coevolutionary dynamics.

10.1 Typology of Symbiosis

10.2 Endosymbiosis: The Cell as a Union

10.2.1 The Silicon Symbiont

The key difference between the cells of Théxar and terrestrial cells is the silicon-carbon membrane channels (→ see [Volume I, Ch.04, "Biochemistry"]). The modern theory of the origin of these channels is symbiotic: about 1.4 billion cycles ago a certain proto-eukaryote absorbed a silicon bacterium (Silobacteriae) capable of polymerizing SiO₂ into transmembrane pores. Instead of digesting it, the host "tamed" the bacterium — and received:

• channels resistant to high temperatures and pressure;
• selective transport of silicon ions;
• radiation resistance (the silicon pores shield ionic fluxes).

The descendants of this absorbed bacterium have survived as membrane symbionts in every cell of every eukaryote-like organism of Théxar — analogous to the mitochondria of terrestrial life, but with a different function.

KEY FACT

The membrane silicon symbiont retains a greatly reduced genome: 37 genes encode proteins for silicon-pore assembly. Most former functions have passed under the host nucleus’s control.

10.2.2 Sulfur Endosymbionts

In deep-water and cave communities (→ see [Volume VII, Ch.05, "Cave Ecosystems"]) sulfoendosymbionts are widespread — chemoautotrophic bacteria living inside the tissue cells of the host:

Sulfoendosymbionts are transmitted vertically (through eggs and larvae) — one of the few known exceptions to the rule "chemosynthetic symbionts are acquired from the environment."

10.3 The Reef Union: Double Symbiosis

The reefs of Théxar are a classic example of a symbiotic system that arose as a response to the specifics of the orange spectrum (→ see [Volume VII, Ch.03, "Marine Ecosystems"]).

10.3.1 The Participants

10.3.2 The Deep Line of Reefs

The transition of reef communities to a depth of 90–190 sha (the Ven-Khal reef) became possible thanks to a shift in the balance between photo- and chemosymbionts:

10.3.3 Bleaching and Recovery

When the temperature rises 2.2–2.8°X above the norm, the polyps expel the chemosymbionts first (they are more temperature-sensitive), but retain the photosymbionts. This is the reverse of terrestrial "bleaching" (where photosymbionts are lost). Recovery takes 8–14 months; the surviving photosymbionts continue to feed the polyp during regeneration.

10.4 Terrestrial Symbiosis

10.4.1 Bryoforma Lichens

The lichen forms of Théxar are a union of a Mycophyta fungus and a phototroph (an alga or a cyanobacteria-like form). About ~11,000 species have been described. Their uniqueness is a triple composition: in addition to the fungus and the phototroph, every lichen contains a silicon bacterium that reinforces the crust.

Lichens are the pioneers of colonization of lava fields and cliffs; their breakdown of the substrate initiates soil formation (→ see [Volume VII, Ch.13, "Biosphere Interactions"]).

10.4.2 Mycorrhiza and Silicon Fungi

90% of the terrestrial plants of Théxar form mycorrhiza with Mycophyta fungi. The special feature is silicon hyphae: hyphae reinforced with SiO₂ allow the fungus to penetrate the silicon walls of host plants, creating a sturdy transport junction.

Silicon mycorrhiza has no terrestrial analogues: the fungus dissolves soil silica, delivers Si to the plant for its walls, and in return receives organic matter. This is the most important accelerator of the silicon cycle on land.

10.4.3 Epiphytic Cushions

The division Thexophyta forms "cushion cities" on Tor-Khar trunks: up to 140 species of epiphytes, mosses, and ferns on a single tree. Inside the cushion a closed micro-cycle forms:

1. Xha-Ven filters H₂S from the air;
2. Mosses accumulate water and nutrients;
3. Ant-analogues (Xhel-kreb) fertilize the substrate;
4. The roots of the epiphytes process everything into humus, which is reused by the crown.

A single Tor-Khar tree supports an epiphyte biomass of up to 4.2 ru — an ecosystem the size of a small forest on a patch with no ground contact.

10.5 Predator-Prey Coevolution

10.5.1 The Arms Race in the Savannas

The classic pair is the Ghal-thass (plain tiger) and the herd-dwelling Xa-ruunidae (kharru) (→ see [Volume I, Ch.18, "Fauna"]).

The thermo-chromatic masking of the kharru is a direct consequence of the fact that their chief predator sees the thermal trace. Skin spots capable of cooling 3–5°X relative to the body break up the IR silhouette of the animal beyond a distance of 40 sha. This trait spread in ~900,000 cycles — one of the fastest coevolutionary adaptations on the planet (→ see [Volume VII, Ch.02, "Thermochromadae"]).

10.5.2 Pollination

The pollinators of Théxar are diverse: six-winged insect-analogues, leaf-nosed bats, wind, and — uniquely — the gethers ("walking" grasses) of the Silophyta.

Getherophily is a unique case of active pollination: the gethers not only carry pollen, but also deliberately "walk" from one partner plant to another, following the gradient of the attractants they emit. The movement of the gethers and the reproduction of the silophytes form a coevolutionary pair ~28 million cycles old.

10.5.3 The Light Language in the Depths

In the aphotic zone, coevolution took a luminous form: the bioluminescent "tongues" of fish-analogues and predators exchange signals recognized only by a species-species pair (→ see [Volume VII, Ch.07, "The Biology of the Moryn"]).

10.6 Multisymbiotic Complexes

10.6.1 The Hydrothermal Field Community

About two thirds of the species of the molluscan tier carry at least one endosymbiont; every fourth species carries three or more. A hydrothermal field functions as a single "metabolic cooperative": the waste of one species is the substrate for the symbiont of another.

10.6.2 The "Sulfur Gardens" of the Moryn

The Moryn domesticated not individual species but symbiotic complexes (→ see [Volume VII, Ch.07, "The Biology of the Moryn"]):

The coevolution of the Moryn with their "agriculture" dates back at least 40,000 cycles — comparable to the age of Xha'Ruun farming.

10.7 Parasitism and Its Coevolutionary Consequences

10.7.1 Forms of Parasitism

10.7.2 The Hygienic Coevolutionary Cycle

The parasites of Théxar play a paradoxically positive role in the long-term dynamics of the biosphere: by maintaining the genetic diversity of hosts (frequency-dependent selection), they accelerate the adaptation of populations to environmental change. The "crisis" episode in the Crisis Era (−2,100 – −750) was connected with the weakening of this regulator (→ see [Volume VII, Ch.12, "Extinction Events"]).

10.8 Symbiosis and the Xha'Ruun Civilization

Chapter Summary

1. Symbiosis on Théxar is the rule, not the exception: the cells of all eukaryote-like organisms carry a silicon membrane symbiont absorbed ~1.4 billion cycles ago.
2. Reef systems run on double symbiosis (photo + chemo), which allowed reefs to colonize depths down to 190 sha; under thermal stress the polyps sacrifice the chemosymbionts, not the photosymbionts.
3. Terrestrial symbiosis includes triple lichens, silicon mycorrhiza (90% of plants), and epiphytic "cushion cities" of up to 4.2 ru of biomass.
4. Predator-prey coevolution gave rise to the thermo-chromatic masking of the kharru against the tiger's IR sensors — the planet's fastest adaptation (~900,000 cycles).
5. The Moryn domesticated symbiotic complexes as a whole — the planet's oldest "agriculture" (40,000+ cycles), built on chemosynthesis.