In the deep sea, darkness is so absolute that it seems solid. Earth’s own Outer Space, just underwater. At its deepest point the Marianas Trench is 11,000 metres from the ocean surface – no sunlight reaches here. Accompanying this depth are crushing pressures and dramatic temperature changes. These extreme conditions would seem to prevent life from grabbing hold, let alone letting it thrive. However, in this environment where the Sun definitely does not shine is an entirely unique habitat with a biodiversity that has adapted to persist and thrive.
The deep ocean sustains a marine biome that appears to mirror the extremes of the surroundings. Faded organisms – pale fish and translucent crustaceans – exist alongside animals adorned with bioluminescence. Many deep-sea creatures are different on a cellular level – the fats in their cell membranes are more malleable and their muscles are more flexible, as if their very composition has been softened into compliance by the extreme water pressure.
But how do the majority of deep-sea ecosystem obtain food so far from the ocean surface? The answer lies in small innocuous flakes which hold as much influence on the food chain as phytoplankton. This is a form of marine manna, delivered to the depths by ocean currents from the watery ceiling – marine snow.
A Special Snowflake
Until 1977 it was believed that the deep-sea biosphere was barren. The marine ecosystem was understood to be supported by photosynthesis, a process enabled by sunlight. With the absence of sunlight in the deep, photosynthesis was out of the question. Today, we know that a rich ecosystem exists in the depths, countering previous assertions that light is essential to support marine life. Instead, the majority of deep-sea organisms are heavily reliant upon a rain of marine snow from the productive shallow ocean.
The term ‘marine snow’ was popularised by a marine biologist and a naturalist – William Beebe and Otis Barton, known for their extreme dives for the deep in their ‘bathysphere’ – a deep-sea diving sphere. Marine snow is not composed of crystallised water, as you may think, but is actually suspended organic matter, a miscellany of biological detritus. Examples of ‘snow’ components are zooplankton mucus, faecal pellets, animal body parts and the delicate shell remnants of phytoplankton held together by oil. Rather than being ‘white as snow’, marine snow is more commonly olive green or grey.
This aggregate of death and decay remnants begins as fine fibrous particles often created by zooplankton. This sticky material glues particles together to form aggregrates. Like a marine spider web, this acts to net organic debris carried past by ocean currents. As the individual aggregate grows it coalesces with other marine snow particles. Over time this is repackaged into larger flocs, some that are several millimetres in diameter.
These heavier particles sink slowly, averaging to 10 to 100 metres per day. They are swept sideways and downwards. As they travel from the surface down through the deep sea ‘twilight zone’ marine snow particles can change size – either picking up further detritus or serving as a snack to organisms passing by. Finally, a fine dusting arrives on the ocean floor. Over time marine snow is re-incorporated into the deep-sea sediments, sustaining microbes and animals that feed on the mud for years.
The marine snow forecast for today…
The level of marine snow is influenced by conditions on the surface such as algal blooms. In turn, dense patches of marine snow create a mosaic of diversity on the seafloor where organisms have congregated to take advantage of this nutrient fallout.
Oil can be trapped in marine snow, forming aggregations that sink faster. As a result a significant fraction of the oil released by the Deepwater Horizon spill was transported to the seafloor via marine snow. The downstream effect of this accumulation is difficult to estimate, but no doubt it will affect marine life in various ways.
During that inexorable journey to the deep marine snow picks up a few hitchhikers. As snow travels from the upper waters to the deep ocean, individual flocs act as island-like refuges for pathogens, agents that cause disease. Once a flake of marine snow is formed, other organisms step aboard to begin a new life. Interestingly, marine snow has been found to provide a ‘refuge’ for aquatic pathogens such as Vibrios, by providing a microclimate that protects individual pathogens from environmental stressors. Thus marine snow is a provider of both shelter and sustenance.
It is fascinating that material discarded by the shallow oceans merges together and drifts to a greater purpose – to fuel the deep sea creatures below. This underwater snowfall can orchestrate the rise and fall of biomes beneath the waves.
– Ivy Shih. Ivy is a PhD student researching HIV, based at the Kirby Institute in Sydney. She’s on Twitter – follow @Ivyhish!