تنجو إسفنجيات أعماق البحار في الظلام الدامس بطرق لم نكن نعرفها من قبل

When we think of marine life, we usually picture colourful coral reefs or dense seaweed forests filled with fish and other critters. The ocean that comes to mind is the one touched by sunlight.

However, most of the ocean is not like that. By volume, roughly 95% of the ocean consists of the permanently dark, cold deep sea. Despite such hostile conditions though, there is life in the ocean’s abyss.

Deep-sea marine sponges are among the organisms that live in these mysterious dark waters. They form “gardens” that are among the largest ecosystems on the planet, some spanning thousands of square kilometres on the ocean floor. They act as ecosystem engineers, providing habitats to many other organisms living on the seafloor.

Individual sponges can also pump and filter thousands of litres of water every day through their bodies. The nutrients they release support other organisms. Yet we know remarkably little about how sponges survive, let alone thrive, in the inhospitable environment of the deep-sea.

Symbiosis with microbes is an important part of how marine sponges live. We’ve been studying deep-sea sponges to better understand life in the ocean’s depths. So far, we’ve found some sponges are packed with microorganisms that use energy from chemical reactions.

This is called chemosynthesis and is commonly found in other deep-sea organisms, such as mussels and tubeworms living in hydrothermal vents – deep-sea “hot springs”.

Our new study, published today in the journal Microbiome, shows sponges and their microbial partners also use a second strategy to make a living in the deep sea.

Two strategies, one sponge

All living organisms produce waste. Just like humans produce urine, many sponges produce ammonia as one of their waste products.

In this study, we analysed the Calyx species of deep-sea sponges from a depth of 830 metres.

About 16% of their microbial partners use the familiar chemosynthesis process. With ammonia as the energy source, they use carbon dioxide dissolved in the water to build biomass – it’s a bit like plants growing through photosynthesis from sunlight, but in the dark.

In well-lit shallow waters, many sponges and corals have photosynthetic microbes that help them build biomass from carbon dioxide. Our findings show that in the dark depths of the ocean, sponges have microbial partners that use ammonia instead of light for the same process.

The remaining 84% of microbial partners are where it gets really interesting. Instead of chemosynthesis these microbes use heterotrophy, which means consuming organic matter to generate energy and biomass (like the vast majority of animals, humans are also heterotrophs) .

The problem here is that there’s little organic matter in the deep sea. Whatever falls down from the surface waters, such as dead plankton and algae, gets stripped by bacteria and small crustaceans of anything easily digestible as it sinks through the water column.

So, the little amount of organic matter that reaches the seafloor is generally poor food for the sponge itself. But, as we discovered, not necessarily for its microbial partners.

It turns out the heterotrophic microbes in Calyx sponges have lots of enzymes specialised in breaking down complex compounds, such as xylan and pectin, which make up the hard-to-digest cell walls of algae.

Feeding on these algal skeletons would allow the microbes to thrive and to transform organic molecules into nutrients their sponge host can use.

Protecting what we don’t yet understand

Our study shows that sponges and their microbial partners are complex, biogeochemical reactors. They use and recycle ammonia “urine”, carbon dioxide and hard-to-digest organics to generate biomass.

The biomass can then support the growth of other organisms, such as brittle stars and fish, in turn supporting the broader community of animals living on the dark seafloor.

Unfortunately, these ecosystems are under pressure from human activities. Deep-sea trawling physically destroys sponge gardens. Deep-sea mining, now being actively pursued for rare metals used in batteries and electronics, threatens to disrupt the deep-sea habitat in ways that might take centuries to recover.

The United Nations has recognised deep-sea sponge gardens as vulnerable marine ecosystems, a formal acknowledgement of both their ecological importance and their fragility. But recognition alone is not enough.

If we destroy these habitats before we fully understand their role in carbon transformation, then we may lose a critical piece of Earth’s carbon cycle before fully realising it was there.

Torsten Thomas receives funding from the Betty and Gordon Moore Foundation USA and the Australian Government.

Alessandro N. Garritano does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.