The microfilm kingdom: sea grape

Microfilm organisms differ from brothic species mainly on the grounds that microfilm cells tend to be larger, and the internal organisation of the cell tends to be more defined, with different processes gathered into membranous pseudo-organelles. Most microfilm species also demonstrate cell adaption strategies, where the cell will take on different forms dependant on the local environment and the presence or absence of other microfilm species in the local environment.

Sea grapes are named for the fruiting bodies - known as the composite stage of the life cycle - which three particular species of micrfilm organisms produce as the sea temperature falls during autum and early winter. The organisms described can be found throughout the temperate and subtropical oceans of the northern hemisphere.

The species found in the composite body are:

• The basal component species, which can be found living year round in thin films covering rocks at a depth of 0-3m along rocky coastal areas across the northern hemisphere. The range of this species extends from latitudes 25°N up to 65°N, though its preferred range is much narrower at 5° north and south of 45°N. The organism is fairly simple, with an average diameter of 25μm.
• The photosynthetic component species, which is also spherical in shape with an average diameter of 30μm. It occurs in greatest concentrations in the top 0.5m of water between 25°N and 45°N, though it can survive at latitudes of 50°N and has been reported at the equator. It is one of the major components of the surface-level oceanic microflora.
• The chemosynthetic component, which is a much larger species whose rod-like cells can reach over 75μm in length. Its preferred habitat is sandy littorals where it will form thin crusts feeding off detritus and the sand itself - it has no photosynthetic capability. It can also be found interspersed within basal component colonies on sub-littoral rocks where it works in symbiosis with its partner: the one leaching essential minerals from the rock while the other uses the minerals to effect more efficient photosynthesis to produce enough nutrient for both.

Life cycles

During the spring and early summer months each of the component species lives independently of each other in their preferred niches. The photosynthetic component organisms are found only as free-living single cells, while the other two species have a preference for forming microfilm colonies across rocks and sands - this is known as the uncoupled stage of the life cycle.

Towards midsummer, chemosynthetic organisms will often undergo a population explosion with an associated encystment of cells which move into the water column and are dispersed to other areas. Some of these cysts will settle on basal organism colonies where they will germinate: multi-species colonies are more capable of out-competing basal-only colonies as coastal nutrient levels, and oxygen levels, start to fall following the summer solstice. This marks the beginning of the commensal stage of the life cycle.

The photosynthetic component also has a population explosion as surface water temperatures peak in the lead-up to the autumn equinox. However, these organisms do not start to invade basal-chemosynthetic colonies until water temperatures fall in the weeks following equinox. The invasion is systematic: individual photosynthetic component cells lodge themselves within the colony and displace basal cells, which in turn migrate beneath the chemosynthetic layer of cells where they clump together to start forming small, hollow balls of cells known as a basal megacyst.

Over the course of the next 10-15 days these megacysts grow rapidly, forming prominent lumps (between 1-2mm in diameter) beneath the photosynthetic-chemosynthetic microfilm. The basal cells then excrete carbon dioxide into the megacyst, making it rise through the microfilm until it eventually breaks away, taking a portion of the microfilm with it - this is the base material from which the embryonic composite stage of the life cycle will form.

Once the composite organism breaks free of the microfilm the cells that form it organise themselves into distinctive layers, with the photosynthetic cells on the outside covering the chemosynthetic cells, which in turn surround the basal cells of the megacyst. The outermost layer of photosynthetic cells (also known as protective cells) undergo a radical metamorphosis, losing their ability to photosynthesise and instead becoming transparent and extruding a gel which hardens in contact with salt water. The inner layer also begins to organise itself into a series of ridges around which the chemosynthetic cells cluster. By the time of the winter solstice all the cells become inactive as the organism enters a short period of hibernation.

Rising sea temperatures triggers the end of hibernation and the start of maturation. Within the organism the basal megacyst breaks down as the basal cells form a few dozen mature cysts (also known as storage cells); the rest of the basal cells die. In response, the photosynthetic and chemosynthetic cells use the nutrients released by the basal cells to rapidly multiply, breaking out of the hardened gel shell and expanding the organism to a maximum diameter of 25mm - this is the adult composite stage, and the form in which the sea grape is most useful to humans.

Within the adult organism, a line of photosynthetic cells at the edge of each ridge undertakes a sporulation, possibly involving the swapping of genetic material between cells. Similarly, clumps of chemosynthetic cells along the sides of the ridges form fruiting bodies - though there is little change in the physical appearance of the cells involved. As the spring solstice passes and the water temperature continues to warm the sea grape now becomes highly bouyant - vast numbers of them are washed up along the shores of the continent, in particular the eastern and northern shorelines of Ewlah.

A final gift to the organism from the dying basal cells is the production of toxins: the other cells within the organism are (mostly) immune to these poisons, but they are effective against many other microfilm species which may have - accidentally or intentionally - hitched a lift within the body of the sea grape. These toxins also make the sea grape unpalatable to larger predators which may harvest the crop as it washes ashore.

However, the toxins are only effective against Type One organisms, making the sea grape a useful early crop for human settlements living near to the coastlines. The grape flesh is not pleasant, but the aqueous fluid within it is a rich emulsion of sugars and light oils.

The final act of the life cycle is the breakdown of the composite organism, releasing the mature basal cysts, photosynthetic spores and chemosynthetic fruiting bodies into the water where they can germinate and settle into their respective uncoupled stage habitats. Just before the grape ruptures, the other remaining cells will each ingest a droplet of the aqueous fluid - enough to (hopefully) see them survive until they can find a hospitable niche for further growth and replication.