MIXOTROPHIC NUTRITION OF PHYTOPLANKTON

Venus Fly Traps of the microbial world

Phytoplankton are protists that photosynthesize using sunlight and CO2 to obtain energy and produce organic matter. But there are phytoplankton that ingest other microorganisms in addition to photosynthesizing. These organisms can capture and digest prey including bacteria, cyanobacteria and other algae. Because they have the ability to obtain energy and materials at two trophic levels (i.e., they "mix" plant and animal nutrition in the same organism), they are often called "mixotrophic" as opposed to phototrophic (plant) or heterotrophic (animal). While the existence of mixotrophs has been known for over 100 years, relatively little was understood about their abundance or ecological impact until much more recently.  This lack of information is being remedied as many scientists throughout the world have begun to reconsider this  phenomenon during the last 20 years.

NSF funding to myself and co-PI Rebecca Gast, of Woods Hole Ocenaographic Institution, will allow us to continue to investigate mixotrophy in the Antarctic environment.  Recently we documented wide-spread occurence of mixotrophy in the Ross Sea, Antarctica and also in the Arctic (Moorthi, et al. 2009;  Sanders and Gast, in prep).  We will also continue studying an Antarctic dinoflagellate that ingests phytoplankton and retains functional chloroplasts (kleptoplasty), a somewhat different alternative nutritional strategy.

In collaboration with co-workers, I previously explored the distribution, relative abundance, and feeding abilities of planktonic mixotrophs in lakes and the ocean. We also examined physiological attributes of isolated species in laboratory experiments.  Mixotrophy may afford algae with some advantages (relative to purely phototrophic or purely heterotrophic protists of similar size) in particular environmental situations. Specifically, mixotrophs' ability to exploit two different forms of nutrition (phototrophy and heterotrophy) may allow them to cope with energy or nutrient limitation in nature. For example, most algae rely on the uptake of dissolved nutrients such as ammonia and phosphate to obtain sufficient nitrogen and phosphorus for growth. Mixotrophic algae may be able to obtain these elements by consuming prey and using the nitrogen and phosphorus from the digested prey (see selected references below). Other specific nutritional needs (vitamins, lipids) may also be gained via ingested prey. 

As a part of our projects, we surveyed a number of freshwater and marine plankton communities to determine the abundances of mixotrophic algae in nature. We have looked in such disparate environments as George's Bank off the New England coast, the Sargasso Sea near Bermuda, the Ross Sea (Antarctica), the Beaufort Sea (Arctic), and in several lakes in Georgia and in the Pocono mountains of Pennsylvania. Mixotrophs are present in all of these environments, but their abundances vary tremendously over time and distance.

Protists - The term protist is used to group single-celled (mostly) eukaryotes from most of the groups formerly classified as algae, protozoa, and slime molds. It is a grouping of convenience since members of the group have polyphyletic origins. This means that they were derived from two or more ancestral groups.

REFERENCES. For more of our papers on mixotrophy, including links to abstracts and or PDF files, see "Publications."

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