The Noble Sea Sponge and its Role in Global Carbon Cycling

Source Article: Maldonado, M., López-Acosta, M., Sitjà, C. et al. Sponge skeletons as an important sink of silicon in the global oceans. Nat. Geosci. 12, 815–822 (2019) doi:10.1038/s41561-019-0430-7

If Silica ran the world

Global cycling of chemicals and nutrients like Nitrogen (N), Phosphorus (P), Iron (Fe) and Silicon (Si) drive some of the most important biological processes on our planet, primary production and photosynthesis. 

The carbon cycle is especially interconnected with primary production because photosynthesis is one of the natural ways that carbon dioxide (CO2) is removed from the atmosphere and transformed into carbon forms that do not contribute to the greenhouse effect. 

Phytoplankton, or photosynthetic algae, are responsible for more than half the primary production on earth. When CO2 is removed from the atmosphere by these organisms much of it is then stored in the ocean.

Many of these marine phytoplankton rely on dissolved Si to survive because it forms an integral part of their hard skeletons.

The Sili-who cycle?

Si is the second most abundant material in the earth’s crust, and is commonly found in rocks and marine sediments. It is also present in dissolved forms in the ocean. Silicon constantly cycles between its mineral and dissolved forms due to several biological processes and weathering reactions. 

Because phytoplankton growth, and therefore Si use, is related to CO2 consumption, we can measure some of the global carbon cycle by measuring Si uptake.

In the ocean there is one major input of Si, runoff from land, and one major output, uptake by plants and animals. To measure the balance of these we must quantify the amount of Si that is put into the ocean and then what is taken up by marine organisms and incorporated into their hard skeletons. 

After the organisms die, those hard parts survive and sink to the sea floor where they become part of marine sediments. Some of those Si-rich sediments eventually break down and re-release as dissolved Si that can be reused, but some persist long enough to get buried and are then stored under the sea floor for long time periods.

Finding dead plankton on the seafloor..?

We can study the Si cycle by observing materials deposited on the seafloor in sediment cores. Cores are an important record that tracks what sediment falls to the sea floor over time, and what happens to materials after they are deposited.

 

Using tools such as sediment cores, scientists continue to make discoveries about the Si cycle which refines the way we understand it. Although for many years they assumed there is an equal amount of Si that is put into the ocean and removed from the ocean, newly discovered Si sources from land have thrown those estimations for a loop.

Currently, the global Si budget has 30 million metric tons (equal to over 550,000 african elephants) of Si which enters the ocean from land and according to Si cycle calculations, are not either taken up by organisms or buried over time. So where does it go, and how does that affect our understanding of the carbon cycle?

Spongebob SilicaPants

Enter the sea sponge. Our oceans contain 8,550 species of this filter-feeding organism, and they are important structural components of reef systems.

 

Unlike squishy and porous Spongebob, most species of sea sponge have hard skeletons, sometimes made of silica. Although sea sponges take up dissolved silica from the water, unlike phytoplankton they do not perform photosynthesis so it is not connected to carbon uptake in the ocean. 

Most Si researchers rarely find pieces of sponge skeletons in marine sediments and as a result always assumed that dead sea sponge does not contribute to the global Si cycle compared to phytoplankton.  

Current studies are now revealing that we have discounted these organisms because we could not actually detect their silica in marine sediments with classic Si measurement methods. But, if sea sponges do play a big part in ocean Si and chemical cycling, we are missing a large piece of the marine carbon cycle puzzle.

Cameras tell the real silica story 

And so, Manuel Maldonaldo and colleagues decided that in order to come up with a true Si cycle estimate, and relate it to carbon dioxide uptake in the ocean, they needed to see how much sponges actually contribute to the Si cycle. 

Using a new technique, they take photos of marine sediments with microscope cameras and a computer identifies how much sediment material is actually broken pieces of sea sponge silica. 

The researchers analyzed 17 cores taken all over the world’s oceans, and they used a combination of older methods and this new microscope technique to estimate how much biologically produced Si was buried in the cores and identify if it was produced by either phytoplankton or sea sponge.

Not just “silli” sponges!

This study is one of the first records of sponges making up a substantial part of sediment Si, about 29-99% in half the cores. The findings also state there is difference in Si burial rate between silicon produced by different organisms.

About 7.5 times more sponge Si deposited on the surface is eventually buried in deeper parts of the sediment than phytoplankton produced Si. 

Sponges are a Si sink which we did not even know existed, and in certain conditions they can end up burying a significantly larger amount of Si over time than photosynthetic, carbon removing phytoplankton. 

With there new findings, Si researchers have a lot of new questions to answer about ocean wide chemical cycling, and how that can impact climate, especially as scientists continue to understand climate change and carbon cycling. 

It is essential to have a clear and accurate understanding of the ocean’s role in removing carbon from our atmosphere. Now we just need to figure out the sea sponges’ potentially large part!

 

Featured image: Wikimedia.