Silicon is one of the most abundant chemical elements in the universe and, after oxygen, the second one on Earth. In the ocean, it is part of sediments, minerals and rocks and, more importantly, it occurs dissolved in the seawater. This dissolved silicon plays a pivotal role in the ecological functioning of the global oceans. Among other major functions, dissolved silicon is required for the growth of diatoms, a sort of microalgae that build a siliceous case. Diatoms are essential food for many other marine organisms. The abundance of dissolved silicon in the seawater typically enhances the abundance of diatoms, which, in turn, foster ocean primary productivity through their photosynthesis and favor the development of robust food webs, leading to important fisheries in those areas. Diatom photosynthesis also consumes huge amounts of CO2.
Consequently, through the mediation of diatoms, the availability of dissolved silicon in seawater modulates the capability of the ocean to sequester atmospheric CO2 and reduce the greenhouse effect and the global warming of our planet. Because of the importance of these silicon-linked processes, scientists have endeavored to decipher which is the availability of dissolved silicon in the ocean, who is using it and how it happens. As for all other fundamental biogeochemical cycles of the ocean (carbon, nitrogen, phosphorous, etc.), the marine silicon cycle is thought to be in internal equilibrium. This means that the amount of dissolved silicon entering the ocean every year should be equivalent to the one going out. The output of silicon from the ocean is largely due to the burying of siliceous skeletons of dead diatoms in the marine sediments. If the equilibrium between silicon inputs and outputs in the global oceans broke, a major alteration of the processes of ocean primary productivity and sequestration of atmospheric CO2 would be unchained.
Interestingly, the recent discovery of important silicon inputs in the ocean through ground water and the melting of glaciers and polar ice caps has raised the budget of the inputs relative to the outputs, suggesting that the internal equilibrium could already be broken. As glaciers and polar caps continue to thaw, it is necessary to find out whether other important biological sinks of dissolved silicon occur in the ocean, in addition to the burying of diatom skeletons. This knowledge is critical to understand not only the current stage of equilibrium in the global budget of the marine silicon cycle, but also to assess how this balance may evolve in future predicted scenarios of global change.
During the past five years, Spanish researchers from the Sponge Ecobiology and Biotechnology Group led by Dr. Manuel Maldonado in the Center for Advanced Studies of Blanes (CEAB-CSIC) have investigated marine sediments from very different environments in different oceans, including shallow bays, coral reefs, continental slopes, and seamounts, and abyssal plains, among others. Many of the sediments were collected, preserved, and selected for the study by Dr. Gemma Ercilla, a co-author from the Institute of Marine Sciences (ICM-CSIC), others were requested from international repositories, and even one retrieved using a Remotely Operated Vehicle (Figure A). The study was supported by two consecutive grants of the Spanish Government and a H2020 grant (SponGES) to Dr. Maldonado’s team. Over the years, the team examined more than 160,000 skeletons of siliceous organisms in the sediments, uncovering that organisms other than diatoms are also playing an important role in the burying of silicon in the ocean. More specifically, it was discovered that sponges are responsible for burying approximately 48 million tons of silicon every year through the microscopic pieces that make their siliceous skeletons (Figure B).
This finding increases about 28% the size of the biological silicon sink in the ocean, which had previously been calculated considering only the skeletons of diatoms. Also importantly, the scientists discovered that the siliceous skeletons of sponges are, for unclear reasons, far more resistant to dissolution than those of diatoms. Such a property favored that the silicon content of the sponge skeletons cannot be detected by alkaline digestions of marine sediments, which is the most widely used method to quantify biological silicon. For the past 40 years, the now discovered methodological problem has led to a biased understanding of the abundance of biological silicon in marine sediments, particularly in ocean areas where sponges abound (Figure C), such as continental margins and seamounts. The resistance of sponge skeletons to dissolution facilitates that around 46% of the silicon in the form of sponge skeletons gets buried every year, while only about 8% of the silicon in the form of diatom skeletons does it. Dr. Aude Leynaert, from the University of Brest (France), whose team has a long tradition in the study of silicon fluxes through diatoms, helped to establish relevant comparisons between diatom and sponge siliceous skeletons.
The occurrence of impressive aggregations of highly-silicified sponges in many zones of the deep sea (Figure C) suggests that such communities will be crucial to improve our understanding of the use and burying of biological silicon in future studies. The silicon processed through those sponges, unlike that processed by diatoms, is disconnected from the photosynthesis. In this regard, the study by Dr. Maldonado and co-authors also rises the novel concept: the “dark silica”. It means siliceous skeletons produced in disconnection from the photosynthesis and the consumption of CO2, often in marine environments lacking sunlight where diatoms cannot even survive. Therefore, the quantification of the dark silica provided in this study does not only support that the silicon inputs and outputs of the marine silicon cycle are in equilibrium, but it also introduces the idea for further investigations that the functional connections between the carbon and silicon cycles in the ocean are not as straightforward as previously thought.