Plant Life of Amthalassa

Photosynthesis and chemistry

Plants on Amthalassa use methane as their primary carbon source, using energy from the suns to convert this methane into larger hydrocarbon chains and diatomic hydrogen. This hydrogen is released into the atmosphere, providing the animal life with an essential gas for their respiration.

Just like life on Earth, life on Amthalassa not only depends on carbon, but also on nitrogen, hydrogen, and oxygen. These are the basic building blocks of life on both planets. Nitrogen is much more readily available to Amthalassan plants in the form of the planet's ubiquitous ammonia, which allows these plants to grow far more efficiently than those of Earth. Ammonia is essential for the construction of proteins on Amthalassa, and just as the plants release hydrogen from methane, so too do they release hydrogen from ammonia. 

Although nitrogen replaces oxygen in many of the hydrocarbons used by life on the planet, oxygen is still an important element. It is obtained both from water ice in the soil, and from nitrous oxide in the atmosphere, as well as the less abundant carbon dioxide and carbon monoxide to an extent. Still, the element is less important to Amthalassan life than it is to Earth life, so plants don't need to obtain nearly as much to grow and function. 

Most plants use a purple photosynthetic pigment, similar to that used by purple photosynthetic bacteria on Earth. 

Plants are able to store energy by creating hydrocarbons with carbon-carbon triple bonds. By reacting it with hydrogen, two smaller hydrocarbons are produced and energy is released. Because of this, plants often grow thick tubers underground containing alkynes. There are often also alkynes present in seeds and fruit to encourage animals to eat them, as well as leaves to an extent. This method of energy storage was particularly useful during periods where the planet had a thick tholin layer blocking out light, allowing plants to survive the dimmer winters. 

Structure and variety

A typical Amthalassan kelp tree leaf

Most large land plants have broad and flat leaves to optimise surface area for gas exchange, as well as increasing the area exposed to the suns. At the base of many specialised leaves are gas sacks containing pure hydrogen; this hydrogen is produced as a byproduct photosynthesis. This evolutionary adaptation exists to give the plants greater lift so they don't need to be as sturdy to support a larger size; although hydrogen is present in the planets atmosphere, air on Amthalassa is far denser than pure hydrogen at the same pressure. Because of this, many trees lack strong branches and are instead attached to the ground by thin, flexible stems. They can often look somewhat similar to kelp on Earth. 

Plants retaining older features look far different from these purple kelp trees. Many plants lacking these hydrogen sacks have thinner, branching needle-like leaves, and most have sturdy branches and trunks, especially the larger ones. This is closer to what the majority of trees looked like over 200 million years ago. Hydrogen bubbles evolved relatively late in the planet's history, probably as a response to a period lower light levels. While many of these thin leaved plants' branches are made of a wood-like substance, the majority of species use exoskeletons composed primarily of calcium carbonate for support. They vary in colour far more than kelp trees, with some being shades of red or orange rather than purple. 

The ground is covered not in grass, but in a purple moss-like plant. Many species have adapted to become far more resistant to dryness than Earth moss, so moss is often found in arid areas where light-blocking trees are unable to grow.

There is a certain group of plants that has developed the ability to move. They most closely resemble thin leaved plants in appearance, and are usually fairly small since the planet's animal life dominates most mobile niches. They are extremely slow moving, since they don't have muscles. Instead, they move through a combination of changing the elasticity of sections of their branches and altering ammonia pressure.

There are many species of microscopic photosynthetic plankton in the planet's oceans. They are extremely common; so common in fact that the oceans of the planet are tinged a slight purplish blue rather than just blue. Their success is linked to the greater usefulness ammonia has to Amthalassan life than water does to life on Earth, since it can be used to construct proteins. The high methane levels also play a role since, as will be established later, it allows plant life to be far more common on Amthalassa. 

Aeroplankton is also common on the planet, although not enough to colour the sky in most cases. The presence of complex hydrocarbons in the upper atmosphere produced by the interaction of methane with sunlight is attractive to these aeroplankton. In fact, at various points throughout planet's history they have become common enough to entirely consume the tholin layer in the upper atmosphere, only to have a massive drop in population after it runs out. Currently, there is no thick tholin layer, but there have been many dimmer and colder points in history when this blocked most of the suns' light. 

Biomes 

The the planet is very densely forested, owing to the high levels of methane, as well as the greater availability of usable nitrogen than on Earth. The planet is also very wet and humid, which leads to much of the solid surface being covered in tropical and temperate rainforests and swamps. 

Close to the equator is wet rainforest, with no dry season and fairly even temperatures year round. Extremely large trees grow here, and there is a great deal of variety. Both regular kelp trees and thin leaved trees are common, but some of the tallest trees are kelp trees with far thicker, wooden trunks. The trunks have a honeycomb structure and are filled with hydrogen, so they can be surprisingly light and strong. Thin leaved trees are also far larger in this biome than they are elsewhere, forced to grow tall to compete for light. 

There are seasonal rainforests a bit further from the equator, with far smaller trees. These are dominated primarily by regular kelp trees as well as a few thin leaved trees. The tree density is far higher than in the rainforests closer to the equator, since more light is able to reach the ground. In the wetter parts of the temperate zones are similar forests, albeit with plants more adapted to a temperate climate. Most of the drier parts of the temperate zones are still heavily forested with kelp trees, but not as densely. 

There are a few arid parts of the planet, and even deserts, although since the planet is so humid these only really exist in the rain shadows of mountains. To the west of mountains in the tropics, and to the east of mountains in the temperate zones, there can be found moss dominated arid steppes. In some cases, especially near the horse latitudes, as one moves closer to the mountains the steppes eventually make way for even drier barren deserts. Most deserts are covered in red iron oxide rock and sand, but this varies and many deserts even have white sand made of water ice. 

In the colder regions, thin leaved trees are actually far more common than kelp trees. The conditions past the arctic circle are still warm enough for dense boreal forests to grow - in fact forests exist surprisingly close to the poles thanks to the planet's high (for ammonia life) and fairly even temperature - but since it gets very dark in the winter most plants in these parts of the planet seasonally lose their leaves and subsist off of stored alkynes in their tubers. In the snowier regions very close to the poles plants have a more triangular shape, much like the pine trees of Earth, so as to allow ammonia snow to slide off.