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Seafloor Ecology Expedition 2019 – Log 4

A FireSting oxygen sensor, adapted by MBARI engineers for use in the deep sea, measures the amount of oxygen that sponges remove from the surrounding water.

Seafloor Ecology Expedition 2019 – Log 4

Postdoctoral Fellow Amanda Kahn

Studying the connection deep-sea sponges create between microbes in the water column and seafloor communities.

Deep-sea animals have so many different adaptations to find food because food is scarce in deep water.  Deep-sea sponges are common at Sur Ridge and grow to over a meter (3.3 feet) tall!  Sponges are filter feeders that specialize in eating bacteria, which are too small for most other animals to eat. By eating them, sponges turn some of the bacteria into sponge tissue and excrete the rest as clumps that make them large enough for other animals to eat. In a way, sponges create oases in the deep ocean.

To figure out how large an effect sponges have, we deployed a suite of instruments to study the water around sponges. Combined, the three sampling methods described below will teach us the role of sponges as consumers and transformers of microbes at Sur Ridge. These kinds of careful physiology measurements will supplement the mapping and observational work that the Deep Coral and Sponge Observatory at Sur Ridge will aim to target.

When we found a sponge to study, we first used the DeepPIV, developed by Principal Engineer Kakani Katija in MBARI’s Bioinspiration Lab, to shine a sheet of laser light across the sponge. The laser sheet illuminates particles moving in the water, which can be analyzed on a computer to assess water motion and speed. The DeepPIV allows us to measure the rate that sponges are moving water through their filter-feeding system, which allows us to calculate the volume of water processed by deep-sea sponges—it can be substantial! A shallower species of sponge was estimated to pump about 900 times its own body volume each day.

After we finished measuring the pumping rate using the DeepPIV, we used a FireSting oxygen sensor to measure the amount of oxygen each sponge is removing from the water. Measuring the respiration rate shows how hard the sponges are working to filter feed. The FireSting came as a piece of electronics with a sensor that couldn’t get wet. Postdoctoral Fellow Amanda Kahn and Senior Research Technician Chris Lovera coordinated with several MBARI staff including David French, Dale Graves, and several ROV pilots, to make the instrument waterproof, to allow its electronics to interface with the ROV, and to design it in a way that it could be handled by the manipulator arms of the ROV.

Finally, for the first time on this trip, we used a new instrument designed by MBARI Mechanical Engineer Larry Bird to collect samples of the water after it passes through the sponge’s filter system. The water can be compared with surrounding water to determine how much bacteria the sponge ate from the water, and any other changes that have been made to the water chemistry.

As a bonus to doing this work, some of the measurements require us to wait several minutes. During that time, we can zoom in on the vibrant, diverse community of animals that live on and around sponges.