Giant Phytoplankton: an interview with Dr. Tracy Villareal

Dr. Villareal is a Professor in the Department of Marine Science at the University of Texas at Austin and a Research Scientist at the University of Texas Marine Science Institute in Port Aransas, Texas. His research interests include understanding the processes and interactions that structure phytoplankton communities, with a focus on the autecology of the largest known species of phytoplankton and harmful algal blooms along the Texas coast. He is leading the Giant Phytoplankton segment of research for The Longest Swim.

Half of the world’s oxygen comes from primary producers living in the ocean. Elements like nitrogen, phosphorous, and iron are essential for photosynthesis, and their relative abundance significantly influences the productivity of microscopic marine plants, called phytoplankton. Because photosynthesis requires sunlight for energy it predominantly takes place in the ocean’s upper layer. However, if nutrients at the surface are depleted, photosynthesis (and oxygen production!) could slow or stop. Some of the giants in the phytoplankton world may avoid this problem by migrating to deeper water to harvest nitrogen, then rising back up for photosynthesis. Ben and the crew will record any sightings of this phytoplankton they make during the swim, as well as sample large aggregations for later analysis. Learning how this species acquires nutrients can help scientists understand large-scale nutrient cycling in our oceans, as well as the future impacts of global warming.

What kind of research will you be doing with Ben and the crew?
Ben will be looking for Rhizosolenia mats while he is in the water swimming. These are large, visible aggregations of diatoms. We are hoping he can help us extend information on their range in the Pacific Ocean. If they can do collections, we are hoping that Dr. Brett Baker here at UT can examine them for unique bacterial assemblages using the powerful genetic tools that he uses.

What makes working with The Longest Swim such a unique opportunity?
Few oceanographers actually get in the water to look at large, rare biologicals like this. We’ve been fortunate to have some research time for limited surveys in the eastern part of the Pacific. However, having data all the way across the Pacific Ocean is something we did not think we would be able to get. It’s very expensive for a research vessel to do this, so it’s a rare opportunity for us.

Can you explain a little about nutrient cycling in the ocean?
We are mostly concerned with nitrogen. In these waters, most of the nitrogen comes from internal recycling as animals eat the plants and then excrete the nitrogen. The plants take it up and the cycle continues. There are other sources like nitrogen fixation where certain microbes can use nitrogen gas. In addition, nitrate can be mixed up from deeper in the ocean either by winter mixing in some areas, local eddies, or by biological transport. Ultimately, the nitrogen coming into the system has to balance the nitrogen leaving the system as material sinks or is swept into the deep sea, or the whole system runs down. Only nitrogen-fixation and nitrate input can supply this, so it is an important part of the process to look at.

And how does this giant phytoplankton affect our understanding of those processes?
Biological transport occurs as phytoplankton move between the dark nutrient–rich areas and the well-lit, but nutrient poor surface waters . That’s what the data says Rhizosolenia mats are doing. It’s a process we call vertical migration. Rhizosolenia mats are big and relatively easy to study. We are using them as a model system to understand their role in nutrient cycling. That’s why the distribution information is so important. The bacteria project will tell us a great deal about what types of interactions are occurring on the mats. Recent advances in how bacterial genomes are processed have really transformed our understanding of the tree of life and the role bacteria are currently playing in the ocean. These mats have never been examined this way. They could be very novel and rich in unique assemblages.

Could this research have an impact on life on land?
Most certainly. Phytoplankton make oxygen, a lot of oxygen. As the oceans warm and change, the patterns of primary production and oxygen production are changing as it becomes harder and harder to mix nitrate-rich water up. We call this increased stratification. Giant phytoplankton thrive in these more stratified seas and could provide an important source of nutrients for oxygen production.

Pictured: Dr. Tracy Villareal (top) and Dr. Ma’moon Al-Rshaidat (second from top).

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