Kleptoplastidy: Some Phytoplankton are Thieves

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

Did you know that some phytoplankton are thieves? Phytoplankton are microscopic marine organisms that make their own food through photosynthesis. Photosynthesis is a chemical reaction that requires light, carbon dioxide, and water. Photosynthesis occurs in special structures within phytoplankton cells known as plastids. Without plastids, phytoplankton cannot photosynthesize.

There are some phytoplankton that lack plastids of their own, yet still depend on photosynthesis for survival. They accomplish this by eating other phytoplankton and stealing their plastids. Typically, these predatory phytoplankton will digest their prey, but preserve their prey’s plastids. In some cases, the entire prey cell may be maintained for a short time. This behavior is known as kleptoplastidy. Phytoplankton that engage in kleptoplastidy are mixotrophic, since they both photosynthesize and feed on other phytoplankton.

Once acquired, the plastids that mixotrophic phytoplankton steal from their prey continue photosynthesizing and producing food. The plastids will last for several days or weeks, but must eventually be replaced. Some species of phytoplankton can replicate captured plastids, extending their lifespan. Interestingly, plastids that have already been stolen can be passed to another mixotrophic plankter. The toxic bloom-forming alga Dinophysis ovum obtains its plastids from the marine ciliate Mesodinium rubrum. Mesodinium rubrum does not create its own plastids, instead stealing them from a tiny strictly photosynthetic phytoplankter.

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.

 

Mixotrophy and Harmful Algal Blooms

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

Mixotrophic organisms get their food from both autotrophic and heterotrophic pathways. For marine phytoplankton, this means both photosynthesizing and consuming other marine microbial organisms. Scientists have recently come to appreciate the importance of mixotrophy within marine microbial communities, as well as larger marine ecosystems. In addition to offering a potential competitive advantage over non-mixotrophic phytoplankton, mixotrophy may be an important factor in the development of harmful algal blooms, or HABS.

HABs occur when phytoplankton populations grow to densities capable of causing deleterious effects to humans or ecosystems. In areas where human activity is loading marine environments with excess nutrients, such as coastal cities, the risk of HABs is especially great. This input of excess nutrients is known as eutrophication, and can stimulate the growth of potentially harmful or toxic species of plankton.

Mixotrophs are unique because eutrophication can promote their growth both directly and indirectly. Eutrophication encourages the growth of phytoplankton directly by increasing the abundance of important nutrients needed for phytoplankton to grow. Mixotrophs may also benefit indirectly from eutrophication if organisms they like to eat become more abundant. Strictly photosynthetic phytoplankton do not share this additional benefit.

The mixotrophic phytoplankter Dinophysis ovum is known to form HABs in the Gulf of Mexico. This species produces toxins that accumulate in shellfish like oysters and cause diarrhetic shellfish poisoning. Scientists at the Texas A&M University Department of Oceanography are currently attempting to catch and grow Dinophysis ovum in a laboratory so it can be studied. Ultimately, through laboratory experiments, scientists hope to be able to predict the occurrence of blooms of this species and prevent human illness.

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. D20140408T032741_IFCB007_00069The above image shows a single Dinophysis ovum cell taken by the Imaging Flow Cytobot (IFCB) in Port Aransas, Tx during a bloom in early 2014. More images of marine plankton taken by the IFCB can be viewed here

 

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