The study of aquatic microorganisms

Aquatic ecosystems are populated by many thousands, perhaps millions of species of microorganisms that interact in complex microbial food webs. Microbes comprise much of the base of pelagic food web in marine and freshwater environments, and many of these species are important predators of other microbes within food webs.  As such, they are producers of organic material, they contribute to the trophic transfer of this material to larger forms of life through food webs interactions, and they participate in decompositional processes that remove wastes and regenerate basic nutrients. A thorough understanding of the biology and ecology of microbial species is essential because of the fundamental roles played by these species in aquatic ecosystems.


What are protists?

Most of the research in our laboratory focuses on the general biology of Protists. The term ‘protist’ refers to organisms that can exist as single cells (although many can form simple colonies) that have a cellular organization that is more complex than bacteria. Like the multicellular plants, animals and fungi, the single-celled protists possess organelles such as nuclei, mitochondria and chloroplasts. The older terms microalgae and protozoa are often still used to refer to protistan species that possess ‘plant-like’ and ‘animal-like’ nutrition, respectively, while those that are able to conduct both processes in a single cell are referred to as mixotrophic species (literally, having ‘mixed’ nutrition).

Evolutionarily, animals, plants and fungi are presumed to have evolved from ancient protists through the derivation of multi-cellular body plans. Collectively, the protists are incredibly diverse, and the evolutionary diversity represented by these species still far outweighs that of the plants, animals and fungi within the eukaryotic tree of life, although this fact is often overlooked because of the diminutive sizes of most protists.

Most protistan species are microscopic, but they span a very wide range of sizes. Most range in size from approximately 1 micrometer (1 m ~ 0.00004 inch; similar in size to bacteria) to a few hundred micrometers (~0.008 inch), although a few species reach a few centimeters (~1 inch) in diameter. Many protists exist as free-living species, living and growing individually in natural ecosystems via photosynthesis or the consumption of other microbes. Many others live in close associations with other micro- and macroorganisms in mutualistic relationships (in which both partners benefit), commensal relationships (in which one partner benefits and the other is neither benefited nor harmed) or parasitic relationships (in which one partner benefits and the other is harmed). Protistan parasites constitute some of the most debilitating and deadly diseases known to humans (e.g. malaria, African sleeping sickness), while photosynthetic protists living within the tissues of corals comprise some of the most extensive mutualistic relationships on the planet. However, the vast majority of protists are free-living species occurring not only in marine and freshwater ecosystems but also in soils and sediments throughout the world.

Most protists can reproduce rapidly. The primary mode of reproduction in most protistan species is called binary fission), a process in which an individual cell increases in size as it incorporates new cellular material (via photosynthesis or the consumption of food), and eventually splits into two independent cells. The process can occur several times per day in small protists. Some microalgae form ‘blooms’ of very high abundance in aquatic ecosystems when nutrient and light conditions are conducive to growth. This ability to grow and reproduce rapidly under favorable conditions allows some protists to reach high abundances in a relatively short amount of time, and makes them important in food webs and the cycling of elements in nature.

What We Do:
Studies of diversity, biogeography, ecology & physiology

The central goal of the research in our lab is to define the roles of protists in the ecology and biogeochemistry of aquatic ecosystems. This work takes numerous forms and directions, but it is all generally focused on characterizing the diversity of communities of protists in various ecosystems around the world, establishing their basic physiologies (how they make a living, how fast they grow), and investigating how these many and varied species come together to form food webs; i.e. defining the trophic relationships among these species. In the process, our work helps to establish the contribution of these species in overall community ecology and the flow of energy and elements through aquatic communities.

Specific research projects conducted by our lab often entail extensive field work as well as laboratory-based experiments. Whether conducted on ships, from land-based research stations or in the lab, these investigations generally fall under two very large umbrellas:

  1. Studies of Protistan Diversity and Function
  2. The Ecology of Harmful Algal Blooms

Studies of Protistan Diversity and Function: These programs form the ‘core’ ecological research of the lab, and are focused on measurements of biodiversity (numbers of species and their relative abundances within ecosystems), biogeography (the distribution of species in time and space), life histories (strategies of species for survival) and physiology (rates of feeding, respiration, growth and excretion). Ultimately, our goal is to provide an accurate description of the roles that these populations play in the production and utilization of energy, and the cycling of elements in oceanic and freshwater environments.

The Ecology of Harmful Algal Blooms: The study of Harmful Algal Blooms (HABs) forms a second, more focused effort in our lab. This research is aimed at understanding massive accumulations of photosynthetic protists (microalgae) that sometimes occur in coastal waters. These ‘blooms’ are often highly dominated by one or a few species of protists, and can be associated with harmful ecological outcomes due to the production of toxic or noxious substances by certain species of algae. Our research in this area of study is focused on documenting when and where these algae (and their toxins) occur, and understanding the environmental factors leading to their occurrence. We view these harmful events as ‘natural experiments’ that can shed light on how planktonic food webs are structured, and how they respond to changes in environmental conditions.

  • Caron Laboratory
  • University of Southern California
  • 3616 Trousdale Parkway, AHF 301
  • Los Angeles, CA 90089-0371