Lichens, Liverworts, Mosses, and More: Apterra's other Photosynthesizers

Plant life on Apterra has never been as abundant and varied as it is right now. While localized extinctions have begun in many habitats, the rate of speciation worldwide is still more than enough to continue the trend towards diversification. The descendants of common buffalograss, the only vascular plant introduced during the planet's dawn, dominate the landscape, with some reaching dozens of meters high and all adapting to flourish in the budding ecosystems of their home. But while no other complex plants live here, photosynthetic life exists in other forms wherever one looks. From the open ocean to the barren arctic scrubland to the rich, humid temperate rain-pseudoforests, simpler organisms soak in the sunlight. Overshadowed as they are by the proud trees and pseudotrees, non-vascular and single-celled photosynthesizers are easy to overlook at first glance, but without them this world would never have become livable for the towering Tracheophytes whose shadows they grow in.

The toxic algae were among the first to make their presence impossible to ignore. The dinoflagellates Gonyaulax and Karenia were spread widely across the planet before animal life was introduced. Their runaway success began almost immediately Post-Abandonment; Apterra's young ecosystem with few other life-forms to stabilize the food chain meant that organic debris was readily available, fueling algal blooms on a scale unseen on earth. The open sea was stained a reddish color, sometimes for millennia on end. Producing deadly toxins to ward off herbivores, the algae caused massive fish kills, especially among the early generations of Gambusia that first took to the sea and had yet to evolve resistance to their poison. After using up all the available nutrients and oxygen, much of the red tide would die, rendering the surrounding ocean even more lifeless. 

With time, some varieties evolved to stick together, forming dense mats that can still be spotted to this day around many coastlines. These first originated on sandy beaches as a result of Apterra's extreme tides, which carried floating cells far inland and left them stranded as the water ebbed. To avoid desiccation, some strains gained the ability to produce a sticky mucus-like coating when exposed to air, essentially glueing them together at low tide. These early mats would quickly break apart into individual cells again once the tide rose, but as algal populations surged, so did the scale and frequency at which these protective structures formed. As they grew larger, they would stick together longer, with the delay representing the time the seawater took to reach the deepest-buried cells. During this interval, the disintegrating mat would rise with the tide, buoyed by air bubbles trapped in its extracellular gel matrix. The uppermost layers would once again be exposed to air, fail to reabsorb their coatings, and maintain their attachments to one another. As long as water evaporated from the surface at the same rate it traveled upward through the mat by capillary action, this arrangement was stable. Thus, while the mat-like growth habit originated as a complete accident, some dinoflagellates were able to expand upon it, becoming specialized for this lifestyle. Different cell lines competed for space at the ideal height - neither buried under too many others nor too exposed to cell-killing ultraviolet light - and soon each individual mat (or, in the case of larger mats, each multi-square-meter "province" of the expanse) was a monoclonal colony, with cells outside the optimal zone serving supporting roles on behalf of genetically identical photosynthetic cells. This remains the strategy of modern mat-forming macroalgae: a single stratum, no more than a few hundred cells thick, is responsible for photosynthesizing and producing new sacrificial cells to form the lower and upper layers that provide, respectively, buoyancy and UV protection.

Whether colonial mats or unicellular pelagic drifters, dinoflagellates continue to overwhelm algivorous sea creatures, accumulating on the seabed and cooling Apterra's climate. While deadly red tides continue to disturb the sea with decreasing regularity, harmless green algae are gaining a foothold as Apterra's new dominant autotrophic plankton. Volvox is far and away the most successful of these; evolving from a freshwater ancestor, this multicellular genus is now also distributed across marine environments. Volvocine algae are the world's most numerous large phytoplankton, floating along with the current and becoming the preferred diet for filter-feeders everywhere. They rose to prominence due to Apterra's lack of diatoms, and with little competition, Apterran Volvox quickly outclassed their earthly counterparts in body size, with some now reaching up to 3 millimeters in diameter. While smaller forms still exist (as do relatives with lower cell counts, such as Chlamydomonas, Tetrabaena, and Pleodorina), the prevalence of such massive algae has kicked off an arms race with filter-feeding zooplankton. Volvox-eating species of rotifers, water fleas, and copepods are pushed towards exceptional sizes, with smaller individuals unable to fit the biggest Volvox into their mouths.

Cyanobacteria, commonly known as blue-green algae, are the simplest photosynthesizers. Several genera were introduced intentionally, especially Spirulina, Arthrospira, and Trichodesmium, to serve as the foundation of the planet's aquatic ecosystems. The latter, commonly known as sea sawdust, was instrumental to the survival of Apterra in the earliest days, serving as the ocean's primary nitrogen fixer. It and its descendants are also capable of forming diffuse, floating mats, though these are generally nontoxic and support hubs of nutrients and biodiversity in the otherwise barren open sea. In freshwater, Nostoc was more common; while often possessing a filamentous habit, it could also form thicker, solid biofilms attached to underwater surfaces, as well as moist areas on land. Its role was similar to Trichodesmium, attaching to the roots of certain grasses and providing a source of usable nitrogen. In modern Apterra, each of these genera has hundreds of descendants. While some have already diverged down evolutionary pathways unseen on earth, others still retain their ancestors' lifestyle, giving calories and nutrients to a world that couldn't survive without them. A few larger plants, like drunk-grasses and many woodlouse-grasses, have even begun to evolve nodules on their roots to support colonies of nitrogen-fixing cyanobacteria.

Moving firmly onto land, non-vascular bryophytes can be seen in nearly all environments. Mosses are most successful in the tropics, with many holdouts persisting in places like the Remnant Rain-Pseudoforest. An assortment of species can also be found in temperate regions, growing in thick clumps on rocks, rotting logs, or the trunks of pseudotrees - though palm-grasses are usually free of them, as arboreal isopods tend not to tolerate surface-covering epiphytes. Sphagnum grows in boggy areas like the barrier island south of Ailuropia, sequestering carbon in the form of peat and contributing slightly to ongoing global cooling. In its case, though, the carbon does not remain trapped forever, as the roots of mycads, sweetstalks, and other swamp plants occasionally reach deep enough to access it. Liverworts are also common, being some of the only plants that can thrive on the shady pseudoforest floor of Apterra's tropics. At the same time, light-loving mosses and liverworts both grow on the coldest tundras where no pseudotrees or turfgrasses can survive. 

A similarly wide range of habitats are host to lichens. Not a monophyletic clade or even truly plants, lichens are built from the symbiosis of multicellular fungi and algae. Cyanobacteria often join this partnership, as does yeast, creating a near-infinite array of possible forms. Exposed rocks are the preferred habitat of many Apterran lichens, though a few can be found attached to bladevines growing high in pseudoforest canopies. Arctic species are often the best available food source for rattaloxen, mountain kiwis, and other high-altitude or high-latitude herbivores, as they can live at temperatures lower than what even the bryophytes can withstand. As time goes on and increasing numbers of grasses slip into extinction, these "primitive" plants and plant-like organisms will be the saving grace for many more terrestrial and aquatic animals that would otherwise be doomed to the same fate.