The Mighty Mixotroph and How It Rewrites Our Understanding of Plankton
Zabdiel Roldan Ayala aims to understand how climate change will affect this third category of plankton.
Diving into mixotrophy
The interior of the lab shared by Zabdiel Roldan Ayala and others studying under Dr. Nicole Millette is covered in stickers. They depict characters from popular culture like Gravity Falls and Pokémon, all manner of sea creatures, and some very corny science puns. The large fridge in the back left corner of the lab houses the mixotrophic plankton he uses for his research. It contains dozens of cultures, each housed in a large flask. It looks like something straight out of Jurassic Park.
Removing one of these flasks, you’ll quickly observe the millions of plankton congregating near the surface, forming an oblong shape as they flood towards the uneven light creeping through the window on the opposite side of the room. Looking through the lens of the microscope invites you into an almost alien world, hundreds of plankton swirling around. This particular species, Alexandrium monilatum, has a shape and color closely resembling a tennis ball, each one constantly pirouetting as it dances across the swirling water in its container. Those settled to the bottom can be seen building chains and slowly inching across the glass surface like microscopic caterpillars.
For as long as I had been taught about plankton, there were two main categories: phytoplankton, which gain their energy from photosynthesis, and zooplankton, which gain their energy from the consumption of other microorganisms. Mixotrophs are capable of both behaviors.
"A very common example of a mixotroph outside of the plankton community is the Venus Flytrap. It’s a plant that is able to photosynthesize but also consumes insects to acquire the nutrients that it's lacking,” said Roldan Ayala. Through his research, Roldan Ayala hopes to gain a better view of how changing temperatures will affect the microscopic world inhabited by mixotrophic plankton.
Roldan Ayala is a ninth-year researcher. His journey with plankton started at the University of Puerto Rico at Humacao, where, as an undergraduate, he studied phytoplankton community composition. After receiving his bachelor's in coastal marine biology, he hopped on a plane and started work on his master's the following year at Queens College at the City University of New York, where he earned a degree in earth and environmental science. He is now a Ph.D. candidate under Dr. Nicole Millette at the Virginia Institute of Marine Science, having recently passed his qualification exam.
I have been a volunteer in Roldan Ayala’s lab since the fall of 2024. Located in Chesapeake Bay Hall at the VIMS Gloucester Point campus, about a 30-minute drive from the College of William and Mary, it’s situated right where the York River meets the Chesapeake Bay, with a fresh sea breeze and gentle waves encircling its edges.
Sitting by a towering window adorned with a translucent mural of the Chesapeake Bay — and despite working together for almost an entire academic year, this was one of the first times we’d had an in-depth discussion about what ignited our shared passion. My interest in mixotrophs was primarily based on how they fundamentally altered my understanding of plankton. Roldan Ayala voices a similar motivation.
“They flipped my understanding of plankton upside down,” he said. “I was always interested in phytoplankton since undergrad. When I was looking for Ph.D. opportunities, I saw this lab, which focused on mixotrophy in phytoplankton, and when I started reading it, it blew my mind. A large portion of the phytoplankton community can be mixotrophic.”
In fact, it is now thought that in some marine ecosystems, more than half of the plankton community can be mixotrophic. I found this shocking, particularly in light of the impact plankton have on climate change.
“It seems like it’s the perfect organism, so you begin to ask the question: why aren’t all cells mixotrophs?” said Roldan Ayala. This is a question that still puzzles scientists. Scientists have known of the existence of mixotrophs for some time, but studying them hasn’t become viewed as an independent discipline until fairly recently. There are still many unanswered questions about them. I asked Roldan Ayala what he sees as the biggest knowledge gaps in the field.
“A lot of the research that goes into mixotrophy is in the laboratory. Our methods were designed to target phytoplankton specifically or zooplankton specifically. One of the areas the mixotroph community is trying to advance in the upcoming years is to develop methodologies to be able to study them in their natural habitat,” said Roldan Ayala. In the ocean, plankton’s every move is controlled by the whims of the crashing waves — a far cry from the stable conditions inside a refrigerated flask. This is why one of Roldan Ayala’s experiments aims to help bring the field into, well … the field.
Why is studying mixotrophs important?
Roldan Ayala has four main test organisms: Procentrum micans, Heterocapsa triquetra, Alexandrium monilatum, and Karenia brevis. He explains that each of these species has an important but different role in the environment. Procentrum and Heterocapsa are both prominent members in local estuaries. They make up a sizable portion of the planktonic food web. He elected to use them both because Procentrum tends to bloom in the warmer months, while Heterocapsa is better adapted to colder weather. Alexandrium and Karenia are both toxic species that bloom along the eastern coast of the United States. Knowledge about these two species could help us better understand harmful algal blooms that plague their ecosystems.
Karenia specifically poses all sorts of issues along the coast of Florida. It is part of a large group of plankton named dinoflagellates, dino meaning two, and flagella referring to the tail-like structures on the end of the organism. It is rare in that, unlike most other dinoflagellates, it doesn’t have a shell or casing. This means that it is more susceptible to rupturing and subsequently releasing its toxin into the water and air. This toxin harms both humans and the environment. It can cause the deaths of ocean inhabitants like manatees and dolphins, it has a nasty smell, and it releases a respiratory irritant for humans. This phenomenon is known as a Red Tide for the crimson tinge it stains the waves with. It looks as if hundreds of gallons of red paint or blood were emptied into the ocean, stretching across the shoreline. NOAA estimates that these types of blooms account for $82 million in revenue loss annually within the United States.
If plankton is allowed to grow uninhibited, it can also deplete oxygen from the water and create what is known as a dead zone. When large mats of plankton die, microorganisms like bacteria feast and consume large amounts of oxygen. That leaves little available for other organisms, making it difficult for them to breathe. It is not unusual to see dozens of floating dead fish in these areas.
But it’s not all bad news. By way of photosynthesis, plankton serve as the main mechanism through which carbon is brought from the atmosphere into the ocean, the world’s largest carbon sink. That means these plankton are directly responsible for combating climate change. They consume carbon dioxide through photosynthesis and are also capable of trapping carbon on the ocean floor. Particles of organic matter conglomerate, creating a shimmering white array that descends through the water column in a phenomenon known as “marine snow.” Lots of this is actually dead plankton. Once these husks reach the bottom of the ocean, it becomes incredibly difficult for the carbon they contain to reach the atmosphere again.
Roldan Ayala states that some mixotrophs can be larger than other photosynthetic plankton, making them more effective as a carbon sink. They contain higher amounts of carbon, holding space for the parts that make ingestion and photosynthesis possible. On top of this, they serve as the foundation of the ocean’s food web, transferring energy and nutrients to the organisms that consume them.
Mixotrophs in our changing climate
The central question of Roldan Ayala’s research is how climate change, specifically a temperature change, affects mixotroph energy acquisition. Stepping out from the lab into the hallway, you are met with a row of large environmental chambers capable of monitoring temperature and humidity. This helps Roldan Ayala replicate the conditions of climate change for his test subjects. Their steel doors closely resemble walk-in freezers, and their walls are lined with bright lights.
I have frequently helped Roldan Ayala using a tool called a flow cytometer, which emits a laser to count how much bacteria — the prey in his experiment — is in a sample of water. It looks similar to a printer with a small needle hanging from its front end like a stalactite. He plans to use the output from this machine to gauge how much ingestion is happening.
“I am using the prey removal approach, which basically means that we are going to have two treatments. One treatment is going to have the mixotroph with its prey, and the other is going to have the prey by itself,” said Roldan Ayala. We take samples after a certain amount of time, and by looking at the difference in [bacteria population] growth rate when it’s alone and together with the mixotroph, we can calculate how much prey the mixotroph ingested.”
For photosynthesis, he is measuring the amount of chlorophyll as a sort of proxy. It can’t be used as a direct indicator of how much photosynthesis is actually happening, but it does give you a strong idea of the amount of time and energy the cell is dedicating to that pathway.
It is also important to measure both rates simultaneously to determine if they are transitioning from one pathway to another, or maintaining the same photosynthetic rate while simply increasing ingestion. A shift from photosynthesis to ingestion would mean that less carbon is being removed from the atmosphere, which could have very negative implications for climate change. For this study, Roldan Ayala will be using special labeled carbon isotopes.
“I am going to be labeling bacteria with C13, a stable isotope of carbon, and I’m going to be labeling CO2 and bacteria. By labeling CO2, I will be able to learn how much carbon they are acquiring from photosynthesis, and how much they are acquiring from consumption of prey,” said Roldan Ayala. Labeling in this case refers to using isotopes of carbon, molecules that have a different number of neutrons than usual. A normal carbon atom has an atomic mass of 12, while carbon 13 has an atomic mass of 13 because of its extra neutron. Experiments like this allow you to understand where carbon came from by the time it ends up in the mixotroph. This isn’t measuring these processes directly, but it is gauging the amount of carbon they get from photosynthesis compared to how much they get from ingestion.
Roldan Ayala also plans on combining existing methodologies to help study mixotrophs in the field. He and his collaborators will be the first to combine molecular and natural isotope techniques to study the effects of temperature on mixotrophs in the field. They are usually studied in the lab; field research is rare because of the harsh, rapidly changing conditions of the ocean. He expressed both concern about the potential challenge, but also spoke about his upcoming project with both excitement in his voice and determination in his eyes. He aims to develop an approach that can eventually be adapted by others to study mixotrophs in their environment. The goal is to directly observe how these species respond to climate factors, helping to close this knowledge gap and advance this budding field.
Things in the lab don’t always go the way a researcher hopes. This is no different for Roldan Ayala. He recently had a setback with one of his cultures, which contained a large amount of fungi he believes may have interfered with the results. Nevertheless, he perseveres. The sleepless nights and countless hours in the lab will all be worth it if he can understand this alien world a little better. The insights from his research have the potential to change our comprehension of how the ocean interacts with atmospheric carbon and serves as a carbon sink. I implore you to keep thinking about the small but mighty mixotroph. It could have huge implications for our climate.
Source List
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