Red Weed

On Earth, a view of plant life may envelop a person in warm, serene feelings. Plants are organisms we take for granted as helpful, passive creatures, sometimes even viewing them as if they exist solely for our own purposes, more tool than lifeform.

On Mars, a view of the local flora may cause, if it is capable of feeling, a sense of dread in the local wildlife. Perhaps in the distant past, the vegetables of this planet used to be just as passive and generous as our own, but millions of years of environmental deterioration have made even the flora savage. In the last savannah can be found giant, sponge-like growths that digest microorganisms from the air in cavernous guts while in the sparse shrublands there are bushes that send out their offspring to feed on other plants.

Arephyta, the Martian taxon most similar in morphology to Earth’s Plantae, once used to make up the majority of the planet’s flora, as the fossil record attests. But unlike our planet’s algae and plants, arephytes never evolved oxygenic photosynthesis. They are stuck with a much more archaic metabolism, where sunlight is used to turn hydrogen sulphide and carbon dioxide into sugar, the waste-product being elemental sulphur instead of oxygen. When Mars was young, this dependence on H2S was no problem. Volcanoes regularly nourished the atmosphere with sulphuric gases while the waterways and wet soils were likely filled with microscopic sulphate-reducers that created H2S as a waste product. But as the planet has aged and dried, so have the conditions which have allowed these organisms to flourish. Many arephytes have gone extinct eons ago, the last survivors of the most basal types now desperately cling to the hangs of volcanoes and hot springs, where conditions still provide a faint echo of the elder days. The thrones of the plant kingdom have been usurped by former “planimals”, the spongisporians and fractarians, which have bet on the right horse and engage in symbiosis with oxygen-producing chloroplasts.

However, one group of arephytes has managed to adapt to this changing world. Arthrophyta is a clade characterized by bilateral symmetry and segmentation, traits we usually associate with animals. But the major characteristic that differentiates them from their archaic forebearers is that they have gained an additional type of endosymbiont: sulphur-reducers. These are anaerobic areont-cells that the plant houses in tightly-sealed bulbs along its stem. The deal is simple: The plant provides anoxic conditions inside its body and waste-sulphur, the symbionts reduce this sulphur back into hydrogen sulphide. An ingenious cycle. With this, the arthrophytes have been able to maintain a wider distribution than their cousins, and in the past grew into their own forests in a world already dominated by fractarian scale-trees. Still, they were and are limited compared to their competitors. Being able to recycle one’s own sulphur is an excellent adaptation, but it is not truly the same as producing H2S. It is a closed cycle and so the arthrophytes are still dependent on any additional hydrogen sulphide they may receive from the soil.

One peculiar group of arthrophytes has found a solution to this problem: Carnivory. The ancestors of this group likely started out not unlike Earth’s sundew, growing in nutrient-poor soils and trapping smaller animals as an extra-source of nitrogen. As they did, putrefying microorganisms must have fermented the sulphurous molecules of the prey’s body and created waste H2S, which immediately came in handy. Over millions of years a close endosymbiosis was forged, which eventually resulted in diets far more sophisticated and terrifying than that of any venus flytrap on Earth.

When an animal steps into the tentacle-like leaves of the red weeds, it immediately becomes ensnared, each attempt at escape triggering more proto-nerve reactions in the plant to hold onto the prey firmer, eventually tiring and choking it to death. In an ordinary carnivorous plant from Earth, this is where digestion would already begin, with enzymes secreted from the plant surface decomposing the food. But these plants digest their prey with microorganisms inside their body, in fact in the anaerobic chambers that formerly housed the sulphur-reducers, now transformed into something that could be called a true stomach. And it is by the method that these plants get food to their stomachs that they have earned their sinister reputation… and colour. These are vampire plants. Each leaf is adorned by tiny needles, much like the hairs of Earth’s stinging nettle, but instead of injection, these needles are adapted towards suction. Through osmotic processes, a pressure difference is created between the stomachs and the veins leading up to the needles. Once a prey animal breaks away the needles’ seal and gets stung, they work like syringes and draw out the blood and other fluids from the organism, sucking it dry over time. It is a gruesome process, sometimes an audible slurping sound has been recorded by observers. Some plants even inject their prey with an acid that aids in decomposition of body tissues into a digestible sludge, much like a spider. The nutrients from the victim’s fluids are digested in the stomachs, leading to the production of H2S, nitrogen and other helpful resources. The excess water now courses through the predator’s xylem, an excellent boon when living in the dry wastes of Mars.

Due to the abundance of iron in the planet’s crust, many of the higher animals use haemoglobin, myoglobin and erythrocruorin to transport oxygen, much like many organisms on Earth do. This means their blood is likewise red and, now stolen, tints the natural colour of these arthrophytes from the inside. As many of their metabolic pathways do not require oxygen, it remains unknown what the plants use the globins coursing through their vessels for. Its colour, together with its aggressive, choking nature, is how the plant has earned its colloquial name: Red Weed.

By going on the offensive, these remnants of a forgotten flora have burned their way across the ecosystem, growing with an astonishing vigour and luxuriance. Whole areas of the sparse shrublands have become their own microbiome where the cactus-like red weeds form dense carpets and miniature carmine forests. Many different species and morphologies exist. Some have the typical broad, tentacular leaves like a sundew, others broadcast long, thin strings from their stems almost like tripwires. The latter’s tendrils creep like slimy, wet animals across the wasteland, covering field, ditch, shrubs and sleeping animals with living, scarlet feelers, crawling, crawling… A third type of red weed, the vampire-waterlilies, is only very rarely encountered, as it lies dormant for most of the year. But during the thawing season, wherever this extraordinary growth encounters a stream of water it straightaway becomes gigantic and of unparalleled fecundity, clinging and growing with frightening voraciousness. Its diaspores are simply poured down into the water to be deposited and covered into the sediment for the next season and its swiftly growing and titanic water fronds speedily choke the water, creating transitory pools in which amphibious creatures lay their spawn.

The greatest differences between the species usually lie in the “heads” of the plants, which are their reproductive organs. Something akin to the beautiful flowers or fruit has never evolved on Mars among any of the known flora. Instead, sickening sporangia grow from these sickle-shaped tops, dispersing wretched diaspores into the wind and soil. Some of the red weeds’ diaspores can be necroparasitic if they are ingested. Usually, they lie dormant inside the host’s tissue for its entire life, but when it finally dies of natural causes, putrefying chemicals will cause the diaspore to germinate and feed on its host’s corpse until bursting out of the decaying body. A very few do not wait for the host’s death.

Another advantage of the red weeds is that very few herbivores feed on them, usually due to the damage that their needles may cause on mouthparts but also because most herbivorous animals seem to find it unpleasant to bite into food filled with other animals’ blood. But there are a few that do feed on the red creepers, helping to keep their advance at bay. Bennus, rannus and other periostracans have solid scolecodont beaks formed out of former teeth and so are immune to the syringes’ stings. They simply bite into the leaves and rip them out, destroying any built-up osmotic pressure, and then chew with pleasure on the succulent snack. Perhaps they do not mind the taste due to their own omnivorous diet. Some onychognaths also have beaks, but the soft inner-linings of their mouths still make it difficult for them to bite into red weeds.

On a final note, it is interesting to discuss why such a type of carnivorous plant has not evolved on Earth, even though all the required mechanisms theoretically exist in our own flora. It is most likely a question of necessity, none of the oxygenic earth-plants needing to evolve such drastic measures like the metabolically impaired arephytes to stay in the evolutionary game of life. But it could also be a question of time. Carnivorous plants on Earth are only a recent phenomenon, most fossil and molecular evidence pointing towards an origin not older than the Late Cretaceous. Perhaps the humble sundew and the flytrap are still at a very early stage of their kind’s evolution. As Earth itself will inevitably sink in habitability and competition from the expanding C4 plants will drive such forms into harsher habitats in another 500 million years, perhaps strategies just as drastic if not more extreme than those of the red weeds will evolve here too. In a billion years, when the expansion of the sun has turned our home into another red desert, this will maybe also become a planet of vampire weeds. Perhaps they will even sprout legs and go by themselves on the hunt for the animalistic post-humans that will cling to the wasteland.