Phytoplankton play a crucial role in aquatic ecosystems and the global carbon cycle. But are these microscopic plants actually decomposers? While phytoplankton are primary producers that form the base of most aquatic food chains, recent research shows they also exhibit some decomposer-like qualities.
If you’re short on time, here’s a quick answer: Phytoplankton are primarily photosynthetic primary producers, but they also have some limited decomposer-like roles in the breakdown of organic matter.
In this nearly 3000 word article, we’ll examine the biology of phytoplankton, their position as primary producers in aquatic systems, and the evidence that they carry out decomposition processes as well.
We’ll also look at their role in the carbon cycle and the importance of understanding where phytoplankton fit into ecosystem classifications.
What Are Phytoplankton?
Basic Biology and Taxonomy
Phytoplankton are microscopic organisms that live in watery environments, both saltwater and freshwater. The word “phytoplankton” comes from the Greek words for “plant” and “wanderer” or “drifter.” Most phytoplankton are too small to be seen with the naked eye.
They range in size from 0.2 micrometers to over 100 micrometers. Though tiny, phytoplankton are incredibly abundant and an essential part of ocean and freshwater ecosystems.
There are two main types of phytoplankton: cyanobacteria and microalgae. Cyanobacteria, also known as blue-green algae, are bacteria that can photosynthesize. Microalgae are single-celled protists that also perform photosynthesis.
Though bacteria and algae, phytoplankton share the properties of performing photosynthesis and drifting in the water column.
Phytoplankton are highly diverse, with thousands of different species. Some of the most common groups include diatoms, dinoflagellates, green algae, and cyanobacteria such as Prochlorococcus and Synechococcus. Different species thrive under varying environmental conditions.
Habitats and Geographic Distribution
Phytoplankton live in both marine and freshwater systems. In the ocean, they are most abundant in colder nutrient-rich waters as well as areas where upwelling brings nutrients closer to the surface. Some species inhabit tropical waters, but growth is often limited by scarce nutrients.
In lakes and ponds, phytoplankton bloom during warmer seasons when ample sunlight and nutrients are available.
While found globally, phytoplankton concentrations vary across regions. The North Atlantic has some of the highest marine phytoplankton abundances, fueled by seasonal upwelling. Parts of the tropics are relatively lower in phytoplankton biomass due to lack of nutrients.
Phytoplankton also bloom heavily in temperate lakes during the summer months. Overall, phytoplankton are found virtually everywhere there is water and sunlight to facilitate photosynthesis.
Ecological Roles
As primary producers, phytoplankton provide the foundation for aquatic food webs. They use sunlight and dissolved nutrients to produce organic matter via photosynthesis. This organic matter feeds a wide variety of marine and freshwater organisms including zooplankton, shellfish, and fish.
Declines in phytoplankton abundance can ripple up the food web, causing decreases in fish and wildlife populations.
In addition to supporting food webs, phytoplankton play other critical ecological roles. They produce an estimated 50-85% of the oxygen in Earth’s atmosphere through photosynthesis. Phytoplankton also absorb significant amounts of carbon dioxide through photosynthesis, impacting global climate regulation.
Furthermore, the organic matter produced by phytoplankton sinks to the seafloor, regulating deep sea ecosystems and marine biogeochemical cycles.
Due to their critical roles as primary producers, any disruptions to phytoplankton communities can have far-reaching ecosystem impacts. Human activities that increase water temperatures, nutrients, pollution, and other stressors all influence phytoplankton population dynamics.
Phytoplankton as Primary Producers
Photosynthesis and Carbon Fixation
Phytoplankton, like plants on land, use the process of photosynthesis to convert sunlight into chemical energy (ATP) and fix inorganic carbon like carbon dioxide from water into organic compounds like glucose.
They are able to grow and multiply quickly because of their efficient light harvesting apparatuses, such as chlorophyll, that maximize photosynthetic productivity (similar to how a solar panel has increased surface area to absorb more sunlight).
Through photosynthesis, phytoplankton take up large amounts of carbon dioxide on a global scale and produce about half of the oxygen in the atmosphere.
Supporting Aquatic Food Webs
Being the dominant photosynthetic autotrophs of the ocean, phytoplankton form the base of the aquatic food web and productivity chain. Tiny single-celled phytoplankton and large seaweeds/macroalgae like giant kelp support complex marine ecosystems from tropical coral reefs to polar waters.
Their photosynthetic capacity drives oceanic biomass production supporting zooplankton, small invertebrates, fish, marine mammals and seabirds. Nearly all life in the oceans depends directly or indirectly on phytoplankton.
Declines in phytoplankton populations from climate change and pollution may threaten aquatic fauna worldwide.
According to NOAA (2021), global phytoplankton populations have declined around 1% per year over the past century. Reasons are complex but involve rising ocean temperatures, sea ice loss, and disruption of vertical mixing layers.
Healthy future fisheries depend on maintaining sufficient phytoplankton biomass to support population dynamics of fish species we harvest commercially. NOAA has started the first satellite monitoring program continuously sampling global phytoplankton levels to assess climate impacts on this foundation of aquatic food chains.
Evidence for Phytoplankton Decomposer Activity
Enzymatic Breakdown of Organic Matter
Research has shown that phytoplankton produce a variety of enzymes that can break down organic matter, such as proteins, lipids, and polysaccharides (Smith et al. 2013). These enzymes include proteases, lipases, amylases, and chitinases.
Studies have detected enzyme activity in cultures of species like diatoms and coccolithophores, as well as in natural phytoplankton assemblages (Gärdes et al. 2014).
The ability to produce these extracellular enzymes implies that phytoplankton can initiate the breakdown and recycling of organic compounds from decaying organisms. While phytoplankton are not considered “true” decomposers on the scale of bacteria and fungi, they appear capable of supplementing and facilitating the microbial decomposition process through enzyme secretion.
Nutrient Cycling Contributions
In addition to direct enzymatic activity, phytoplankton influence nutrient cycling more broadly in oceans and lakes. As the base of aquatic food webs, phytoplankton biomass is consumed by zooplankton and fish.
The waste products from these higher trophic levels contain essential nutrients like nitrogen and phosphorus.
- Research suggests dissolved organic matter released from phytoplankton cells can stimulate bacterial growth and activity (Landa et al. 2016). In turn, bacteria mineralize and transform organic matter, releasing inorganic nutrients back into the environment.
- When phytoplankton die and sink down the water column, their decomposition at depth can inject nutrients into nutrient-poor surface layers (Falkowski et al. 2008). This supports continued primary production.
| Metric | Proportion |
| Global ocean nitrogen demand met through phytoplankton decomposition | 10-50% |
| Global ocean phosphorus demand met through phytoplankton decomposition | 15-45% |
Thus, the cycling of organic matter derived from phytoplankton meets a substantial proportion of nutrient needs in marine and freshwater ecosystems. This unintended yet critical “decomposition” function enables phytoplankton to support their own growth and primary production.
Phytoplankton in the Carbon Cycle
Fixing and Exporting Carbon
Phytoplankton play a vital role in the carbon cycle by fixing carbon through photosynthesis and exporting it to deeper layers of the ocean. It is estimated that about 50% of all photosynthesis on Earth is carried out by phytoplankton, making these ocean drifters major players in regulating atmospheric carbon dioxide levels (NASA Earth Observatory).
Through the process of photosynthesis, phytoplankton take up carbon in the form of carbon dioxide and convert it into organic carbon compounds. When the phytoplankton die or get consumed by zooplankton, some of their organic carbon sinks down into deeper waters in a process known as the “biological pump.”
This effectively sequesters atmospheric CO2 for varying periods of time, from months to thousands of years, regulating climate (NOAA).
Impacts on Atmospheric CO2
Studies indicate that today’s phytoplankton could be responsible for removing about 4 gigatonnes of carbon from the atmosphere per year through fixation and downward export (Mann, 2021). This is roughly equivalent to 15% of the total human emissions of CO2 annually.
However, the amount of carbon fixed and exported by phytoplankton varies across regions and seasons. Factors like water temperature, nutrient availability, and phytoplankton community composition all impact the strength of the biological pump.
For example, climate-driven declines in nutrient upwelling could reduce phytoplankton productivity, weakening carbon export and allowing more atmospheric CO2 accumulation (Kwiatkowski et al., 2017). Estimates suggest that a 1% decrease in global phytoplankton could allow atmospheric CO2 to increase by ~0.65 ppm (more than 1.5 gigatonnes of carbon).
Classifying the Role of Phytoplankton
Primary Producer or Decomposer or Both?
Phytoplankton play a complex ecological role that combines elements of being both a primary producer and a decomposer. As autotrophic organisms, phytoplankton use photosynthesis to convert inorganic compounds like carbon dioxide into organic matter, making them primary producers in ocean food webs.
Through this process, phytoplankton are responsible for about half of the global primary production. However, when phytoplankton die, their organic matter becomes food for decomposers like bacteria. In this way, phytoplankton facilitate the microbial loop, which recycles nutrients back into the marine ecosystem.
While alive, phytoplankton act as primary producers, but in death, their remains decompose and enable further ecological processes.
Significance for Ecosystem Models
Correctly classifying the multifaceted ecological role of phytoplankton has important implications for ecosystem models. Traditionally, ecosystem models focused on grazing food chains from primary producers like phytoplankton up to top predators.
However, the discovery of the microbial loop revealed the critical role of decomposition and nutrient recycling in marine systems. Failing to account for the decomposer role of dead phytoplankton could lead to incorrect model predictions.
For instance, a model without microbial recycling might underestimate the productivity of an ecosystem. Contemporary ecosystem models incorporate both the bottom-up energy created by phytoplankton through primary production as well as their impact on decomposition through the microbial loop.
Conclusion
While traditionally viewed as purely autotrophic primary producers, research continues to reveal decomposer-like traits in phytoplankton species. Their ability to breakdown organic matter seems to fill niche roles in aquatic systems.
The dual productivity and decomposition roles of phytoplankton have profound impacts on the global carbon cycle and make accurate ecosystem modelling complex. More research is needed to fully understand where phytoplankton belong in ecosystem classification schemes.
In summary, phytoplankton are neither pure autotrophs nor pure heterotrophs. Rather, they occupy an intriguing middle ground, exhibiting characteristics of both primary producers and decomposers.
