Why Mixotrophy Matters: A Competitive Edge

This is Jim Fiorendino, your host for On the Ocean.

Runners often have a big meal of pasta, or another carbohydrate-rich food before a marathon. Carbohydrates provide lots of energy when broken down, which runners need to compete. A runner who has eaten lots of carbohydrates may have a competitive advantage over a runner who has not. In the same way, mixotrophic marine phytoplankton may have a competitive advantage over strictly autotrophic phytoplankton.

Phytoplankton in the ocean were once thought to be strictly autotrophic, which means they make their own food and do not need to eat. In contrast, heterotrophs like zooplankton obtain their food by consuming other organisms. Recently, the importance of mixotrophy, or the ability to derive nutrients and food from both autotrophic and heterotrophic pathways, has become apparent within the marine microbial world.

Phytoplankton are a numerous and diverse group present in all the world’s oceans. Despite their diversity, phytoplankton compete for the same resources, specifically light, carbon, and other nutrients such as nitrogen, phosphorous, and iron. There are so many different species of phytoplankton that it seemed impossible so many could exist while competing for the same resources; surely some species would outcompete the others and drive the losers to extinction. This problem has been the subject of extensive research in the past, and has been described as the Paradox of the Plankton.

Scientists now know that, though the marine environment may seem uniform, it is quite variable. Many phytoplankton are specialized, exploiting subtle differences in their environment that other species cannot, and ensuring their survival. If a species is to succeed over some other species, it must outcompete other organisms and maintain a steady growth rate. Mixotrophy may have offered a competitive advantage to certain species of phytoplankton by supplying an additional source of food that other species did not have. This may have allowed certain species to continue to be successful when conditions were unfavorable for strict autotrophs relying solely on photosynthesis.

This has been On the Ocean, a program made possible by the Department of Oceanography and a production of KAMU-FM on the campus of Texas A&M University in College Station. For more information and links, please go to ocean.tamu.edu and click On the Ocean.

 

What is Mixotrophy?

This is Jim Fiorendino, your host for On the Ocean

What if you were hungry, and could have lunch by taking a walk in the sun, and later, for dinner, enjoy a hamburger? There are organisms in the ocean, called mixotrophs, that can do exactly that!

In the past, organisms have been described as autotrophs or heterotrophs based on how they meet their nutritional needs. A heterotroph needs to eat to survive, while an autotroph can make its own food, and does not need to eat. Humans are heterotrophs, while organisms like plants are autotrophs.

Phytoplankton are important autotrophs present in all the world’s oceans. These microscopic organisms rely on photosynthesis to produce the food they need to survive. During photosynthesis carbon dioxide, water, and light energy from the sun are converted to food for phytoplankton. Several marine phytoplankton, previously thought to be strictly autotrophs, have been found to utilize both autotrophy and heterotrophy. These organisms, known as mixotrophs, can obtain the food they need from both photosynthesis and predation on other marine organisms.

In a typical marine food chain, phytoplankton form the base, photosynthesizing and creating food for themselves. Heterotrophs like zooplankton graze on phytoplankton. Zooplankton are, in turn, eaten by higher predators like fish. Recently, scientists have realized that treating marine ecosystems as a chain of predators and prey is inaccurate, particularly when attempting to describe the interactions of microscopic organisms.

Instead of a simple, tiered food chain, with autotrophs at the bottom and major predators at the top, a more accurate depiction of marine predator and prey relationships would look like a web. Restructuring the marine food web to account for mixotrophy has many important implications for ocean ecosystems that scientists are now trying to understand.

This has been On the Ocean, a program made possible by the Department of Oceanography and a production of KAMU-FM on the campus of Texas A&M University in College Station. For more information and links, please go to ocean.tamu.edu and click On the Ocean.

Script author: James M. Fiorendino

 

Sampling the Sea Surface Microlayer

This is Jim Fiorendino, your host for On the Ocean.

Clouds form when water droplets or ice crystals grow on aerosols in the atmosphere. These aerosols may originate as organic material in the world’s oceans. Breaking waves and the bursting of bubbles propel droplets of water containing organic material into the atmosphere, which may become aerosolized and promote cloud formation. An important source of the organic material in droplets of seawater are marine phytoplankton, which leak exopolymers and organic matter into the surrounding water. Exopolymers are carried to the surface of the ocean by bubbles and become concentrated in the sea surface microlayer, which is a thin skin at the ocean’s surface roughly 50 micrometers thick.

SSM

Figure 1: Exopolymer concentration results from sea surface microlayer sampling.                   Thornton DCO, Brooks SD and Chen J (2016) Protein and carbohydrate exopolymer particles in the sea surface microlayer (SML). Frontiers in Marine Science 3:135.

Studying the sea surface microlayer is important in understanding the transfer of organic material and other particles to the atmosphere. The sea surface microlayer contains higher concentrations of organic material and exopolymers than the water beneath it, and is home to a unique microbial community capable of coping with high amounts of ultra-violet radiation from the sun. Sampling this thin layer of the ocean is difficult, and cannot be done from a research ship. Instead, scientists operate from rigid-hulled inflatable boats to avoid contamination from larger vessels. Sampling is conducted by dipping a large sheet of glass into the ocean. When the glass is pulled up, water from the sea surface microlayer clings to the glass and can be collected and stored for analysis. Results of sea surface microlayer sampling are shown in figure 1. Understanding the relationship between phytoplankton biology and cloud formation requires interdisciplinary research; Dr. Daniel Thornton of the Texas A&M University Department of Oceanography is currently collaborating with Dr. Sarah Brooks of the Department of Atmospheric Sciences on research regarding phytoplankton biology and cloud formation. Ultimately, through careful laboratory experiments and sampling in the field, Dr. Thornton and Dr. Brooks hope to link cloud formation processes to the biology of marine phytoplankton. The research being conducted by Dr. Thornton and Dr. Brooks is funded by the National Science Foundation.

This has been On the Ocean, a program made possible by the Department of Oceanography and a production of KAMU-FM on the campus of Texas A&M University in College Station. For more information and links, please go to ocean.tamu.edu and click On the Ocean.

Contributing Professor: Dr. Daniel Thornton

Growing Phytoplankton in a Marine Aerosol Reference Tank (MART)

This is Jim Fiorendino, your host for On the Ocean.

Phytoplankton are microscopic photosynthetic organisms present in all the world’s oceans. They are an abundant and diverse group known to produce organic compounds that may be an important part of cloud formation processes. Waves breaking and bubbles bursting at the surface of the ocean transfer organic material from the ocean to the atmosphere in droplets of water. Dr. Daniel Thornton of the Texas A&M University Department of Oceanography is currently researching how phytoplankton biology influences the chemical composition and production of organic material in the ocean. Certain processes, such as cell death, may be particularly important, as they result in the release of large amounts of organic material into the surrounding water.

Growing phytoplankton within a Marine Aerosol Reference Tank or MART (Figure 1) allows Dr. Thornton to study phytoplankton growth under varying conditions as well as the production of organic material by phytoplankton. The MART is sealed; inside, cultures of phytoplankton are grown in 63 liters of water, with a headspace of atmosphere roughly twice the volume of the water in the MART. Currently, Dr. Thornton is studying the diatom Thalassiosira weisflogii, chosen because it is a cosmopolitan species that grows well in a laboratory setting.

Air and water samples are taken from the tank and analyzed using various instruments to determine the size and chemical composition of both aerosols and organic matter. One of these instruments, known as a cloud condensation counter, determines what proportion of aerosols can form clouds at specific temperatures and relative humidity. Several other analyses are performed as the phytoplankton grow and die, allowing any changes in organic matter and aerosols to be documented. In the future, Dr. Thornton plans to grow additional species from other major groups of phytoplankton to gain a more complete understanding of how the marine phytoplankton community contributes to organic matter production and, consequently, cloud formation. Images of organic material within a MART are shown in figures 2 and 3.

This has been On the Ocean, a program made possible by the Department of Oceanography and a production of KAMU-FM on the campus of Texas A&M University in College Station. For more information and links, please go to ocean.tamu.edu and click On the Ocean.

MART

Figure 1: Marine Aerosol Reference Tank (MART) during an experiment.                              Photo credit: Andrew Whitesell

Thornton_stained_particles

Figure 2: Coomassie stainable particles (CSP) in the Marine Aerosol Reference Tank (MART). These are expolymer particles that contain protein. They have been stained with Coomassie Brillant blue. Image by Andrew Whitesell.

Thornton_Exopolymers

Figure 3: Transparent exopolymer particles (TEP) in the Marine Aerosol Reference Tank (MART). These are expolymer particles that contain polysaccharides. They have been stained with Alcian blue. Image by Andrew Whitesell.

Contributing Professor – Dr. Daniel Thornton