Sponge and seagrass – evidence for a marine love story?

News • 18 April – 14 May 2022

Photos by Elisa Guarcini and Ulisse Cardini

Text by Johanna Berlinghof

Posidonia oceanica growing in association with Chondrilla nucula

We are fascinated by marine symbiotic relationships and in our research, we aim to unravel the secrets of interactions between plants, invertebrates, and their microbial partners. Therefore, it’s hardly surprising how excited we were when we heard about a possible symbiosis between the Neptune seagrass Posidonia oceanica and the sponge Chondrilla nucula. We wanted to find out what is going on, how these organisms interact, and how they could benefit from each other.

The idea

A good way to investigate symbiotic interactions is to have a closer look at the biological transformation of chemical elements. Nitrogen is an essential element for plant growth and marine nitrogen cycling is mainly carried out by microorganisms. Nitrogen-fixing microbes can turn atmospheric nitrogen, which is unusable by most lifeforms, into bioavailable molecules that can be taken up by the plant host. Other microorganisms can “recycle” nitrogen molecules and transform them back into atmospheric nitrogen. These key processes in the marine nitrogen cycle are also called nitrogen fixation, nitrification, and denitrification. While the plant host could benefit from nitrogen compounds produced by the microbial community of the sponge, the sponge in turn may benefit from carbon compounds released by the plant. To investigate the biogeochemical cycling of nitrogen, we planned to conduct incubation experiments, where we added different nitrogen compounds, depending on the process we wanted to look at.

The sampling site, called Schiacchetiello

Preparation of the incubation experiments

The fieldwork

We planned to do three experiments, one for each key process (nitrogen fixation, nitrification, and denitrification). In each experiment, we enclosed our organisms of interest in a container so that no water or air is exchanged with the environment. We will refer to this approach as a chamber incubation.

The incubations were carried out at Schiacchetiello beach in the Gulf of Naples. Here you can find a curious pattern in the growth form of the sponge. Commonly you will see them growing on rocky or calcareous surfaces, but here, they can be found growing like a ring surrounding the lower part of the leaves. Then, other larger sponges seem to either fuse together or grow from a shoot to the neighbors.

We collected P. oceanica and C. nucula growing alone and in association and incubated them in transparent incubation chambers fixed on crates. We incubated the sponge alone, the plant alone, both together, and both absent (which means using only seawater) to see how each of the key processes is affected by each organism. For the nitrogen fixation experiment, we added ¹⁵N₂ gas as isotopic tracer; for the nitrification experiment ¹⁵NH₄⁺ and for denitrification we added ¹⁵NO₃^–. These tracers are molecules that contain a slightly different version of nitrogen. It has one additional neutron in its atom and is in nature less abundant than the version you can see in the periodic table. Because we have the tools to detect this heavier version of the nitrogen, we can add a known quantity to our chambers and track where it is going.

One part of the incubation chambers was covered with opaque plastic bags for the dark incubation, while the other part remained in the light. When the crates were finally back in the water, floating around to ensure natural temperature, light level, and wave exposure, we had to wait for six hours until the incubations were stopped. During this time, we had the opportunity to take environmental samples, underwater pictures, or just enjoy the beautiful location. After the incubation, we took samples from each incubation chamber to analyze organic and inorganic nutrients, dissolved gases, and isotopic composition of the tissue. We are waiting excitedly until we have some first results!

The experience

After one year of working almost exclusively digital, this was my first opportunity to do some hands-on fieldwork for my PhD project. Even though everything was tightly scheduled, and we were dealing with some manpower shortage in between (thanks to Covid…), we were able to carry out every incubation experiment and take (almost) all samples that we aimed for. It was a great experience, and I learned a lot; for example, you can plan everything as well as possible beforehand, but you still have to be prepared for difficulties in the field, either with the material or the technology. But for every issue we found a solution and more than once my supervisor Ulisse Cardini found a creative way of fixing our problems. Working together with other scientists was an excellent opportunity to broaden my knowledge and I left Naples after four weeks full of impressions and new ideas. I definitely have to come back to see the Vesuvius!

The team

The project is supervised by Dr. Ulisse Cardini, and he also made sure that the fieldwork and all the logistics were running smoothly. I was also accompanied by Luis Montilla (PhD student at the SZN), Luigi Gallucci (Master student in Marine Biology and Ecology at the University of Naples Federico II), and Elisa Guarcini (Bachelor student in Natural Sciences at the University of Naples Federico II). Many thanks to this fantastic team for your support during my stay in Naples and for all the fun moments! Power and patience!