Worden, A., Follows, M., Giovannoni, S., Wilken, S., Zimmerman, A., & Keeling, P. (2015). Rethinking the marine carbon cycle: Factoring in the multifarious lifestyles of microbes Science, 347 (6223), 1257594-1257594 DOI: 10.1126/science.1257594
Background and significance:
On the surface, a handful of seawater does not seem overly exciting or extraordinary. However, a closer look reveals a microscopic world beyond imagination (Ocean life under a microscope!). Each drop of seawater in the ocean can contain thousands, or even millions, of microorganisms that are undetectable with the naked eye! On its own, a single drop may not seem too impressive, but this adds up when you consider 75% of our planet is covered by ocean (Is your brain hurting yet?). Based on this unfathomable abundance, you may assume that marine microbes can have large-scale implications for our planet… and you would be correct. These tiny organisms produce oxygen, form the base of marine fisheries, regulate global climate, and cycle carbon and nutrients throughout the ocean (Figure 1).
The largest and most diverse group of microbes in the ocean are “the protists”. Marine protists are single-celled eukaryotes and have been present in the ocean for nearly 2 billion years. They span an incredible range of sizes from 10 cm to as small as 1 µm (less than half the width of a human hair!). Protists are bundled into two main categories: photoautotrophs (use sunlight for energy) and heterotrophs (predatory feeders). They can also be associated with symbiotic or parasitic relationships. Protists dominate the microbial community that exists within a drop of seawater. In this post, I hope to convey the importance of marine protists and highlight their amazing diversity.
Our tour of the protistan world begins with the photoautotrophs. Marine algae (or phytoplankton) are responsible for roughly half of global oxygen production, which rivals terrestrial land plants. Without phytoplankton production, we could not breathe on this planet! Phytoplankton live in the upper layer (euphotic zone) of the oceans where sunlight and nutrients are abundant. Through photosynthesis , they convert atmospheric CO2 into an organic carbon product that sustains the entire aquatic food web. The oxygen they produce allows respiration to occur in the ocean… and helps us breathe on land.
Phytoplankton come in all shapes and sizes. Some have geometrically-stunning outer coatings made of silica (diatoms), whereas others are covered in ornate calcium carbonate shells (coccolithophores). Another large phytoplankton group are the dinoflagellates, which can possess “finger-like” extensions that enable them to “soak up” more sunlight (Figure 2). Different phytoplankton groups are equipped to survive in different habitats (cold vs. warm water). Diatoms prefer the chilly, nutrient-rich waters near the poles, while coccolithophores and dinoflagellates love the tropics. If all three groups are present in the same environment, a fierce competition for resources (sunlight and nutrients) will surely ensue.
Heterotrophs and Mixotrophs:
Next, we turn our focus to the predatory protists. Heterotrophs consume phytoplankton in the upper euphotic zone and are in turn consumed by larger predators such as copepods and fish. This link in the food chain is essential in driving carbon transport through the food web. Protistan predators can consume prey that is less than, equal to, or greater than their body size. Moreover, heterotrophic protists have incredibly diverse feeding mechanisms. Most will engulf their prey whole (Figure 3), but some can generate currents to actively filter prey in the water column. Unlike many phytoplankton, heterotrophs have the power of movement and can swim using their cilia and flagella.
Feeding in the microbial world is not always black and white and there are some organisms that utilize multiple feeding strategies. Mixotrophs are organisms that are autotrophic, but can also feed when there is not enough light. Different species within the dinoflagellate group (mentioned earlier) can be either heterotrophic, mixotrophic, or autotrophic! Dinoflagellates are one of the few groups in the sea where all three feeding behaviors are found.
Symbiotic and Parasitic Relationships:
In addition to having multiple feeding modes, protists are found to be involved in symbiotic associations. These relationships are mutualistic to both host and symbiont and can contribute to the dynamics of the microbial world. Many of these involve nutrient exchange where the protist is the symbiont within an animal (an example is algae inside coral). Heterotrophic protists can also play the role of host and contain bacterial or other eukaryotic organisms. One example is nitrogen-fixing bacteria that reside in the cells or on the silica spines of diatoms and provide nitrogen to the host. Diatoms require nitrogen, so this would come in handy for survival in nutrient-poor regions of the oceans.
Protists can also be subject to parasitic invaders! Parasites that infect marine protists can alter the host’s ability to swim, feed, and reproduce and thus can lead to high mortality (Figure 4). Protists are also known to be parasites themselves. A certain species of dinoflagellate in particular loves to infect copepods (see post).
Conclusion and Significance:
Marine protists are the most dominate and functionally diverse group of organisms in the ocean! They have variable cell structure and size, a variety of nutritional modes, and are involved in symbiotic and parasitic relationships. Measuring the abundance and distribution of protists is challenging because the ocean is vast and can be very patchy (areas more concentrated than others). In addition, patterns of primary production (autotrophy) and grazing (heterotrophy) will change with time of day, season, or latitudinal position. Large-scale ocean models have been used to resolve some of these issues, but it is still hard to capture the full scope of protistan diversity. A combination of models, molecular techniques, and in-situ (on site) observations are needed to shed more light on the protist community. Protists are central to the marine food web and help to modulate global climate and elemental cycles. I hope this introduction has given you a peek at the complex microbial world that exists within a seemingly simple drop of seawater.
If I have caught your attention with this post and you want to learn more about these amazing critters, check this website out! (Plankton Chronicles)