THE OCEAN offers a home for many marine organisms including plankton. Although many fail to classify it as marine organisms, phytoplankton are the most prevalent and vital species in the ocean. As their name suggests, phyto meaning “plant” and plankton meaning “being made to wander or drift” in Greek, they are microscopic plant organisms living in watery environments. They form the foundation of marine food webs, ultimately being responsible for many organisms’ living. This is because they are an autotroph[1] that independently undergoes photosynthesis[2]. Many marine biologists previously forecasted that phytoplankton production would decrease following the rising sea temperatures, yet a recent study reveals the opposite.

An unexpected increase in phytoplankton

   Like land plants, phytoplankton independently source energy through photosynthesis. Consequently, their growth depends on the availability of carbon dioxide, sunlight, and nutrients such as nitrate and phosphate[3]. The temperature also influences the phytoplankton’s living. This is particularly because they are clustered on the ocean surface where they can easily capture the abundant sunlight. A catalyst that instigates an enzymatic reaction is susceptible to temperature changes, and a temperature beyond its optimum temperature alters the shape, which deactivates the catalyst. Thus, increasing temperature due to climate change inhibits some catalysts of enzymatic reactions involved in the carbon dioxide intake, limiting the availability of carbon dioxide.

   The warming temperature further affects the availability of nutrition by altering the ocean current. The nutrients required for phytoplankton growth are abundant in the deep ocean and they become available to phytoplankton as the ocean current flows. However, the warmer the ocean surface gets, the less mixing there is between those waters and the deeper, more nutrient-rich water; as the nutrients become scarce on the surface, where phytoplankton grows, productivity declines[3]. Such a phenomenon is described as stratification, where there is a distinctive difference between the low-density surface water and saltier deep water. As global warming continues, the mixing of water is suppressed, stratification worsens, and distracts the nutrient supply of phytoplankton. This hampers the ocean’s ability to sequester carbon dioxide, consequently exacerbating climate change[4].

   For these reasons, many marine biologists have forecasted that phytoplankton production would decrease in the future. Surprisingly, phytoplankton productivity has increased over the past few years, despite the low nutrition availability. The main nutritive salts are phosphate and nitrate dissolved in ocean water, which source phosphorus and nitrogen for phytoplankton. As insufficient nutrition threatened their survival, they switched their metabolism strategy. They adjusted to the changing environment by substituting phosphorus with sulfur, which is still abundant in warm water, via nutrient uptake plasticity. Furthermore, nitrogen fixation carried out by some phytoplankton species further supports the growth in areas of low concentrated nitrogen[5]. This refers to the process of obtaining nitrogen from the atmosphere; since phytoplankton live near the surface, close to the atmosphere, they can easily source nitrogen from the atmosphere. Such remarkable ability to flexibly control their nutrient uptake enables phytoplankton to persist in nitrogen or phosphorus-depleted environments[6].

Acting as green warriors

   Phytoplankton combat climate change by lowering carbon dioxide levels in both the atmosphere and ocean. Since they are plants that photosynthesize, they have the “green” ability to convert carbon dioxide into oxygen. This lowers the carbon dioxide concentration on the ocean surface and increases the level of oxygen. Furthermore, they sequester carbon dioxide to the ocean depths, away from the atmosphere, as they work as biological pumps. As phytoplankton die, some carbon they took in via photosynthesis sink to the ocean depths, transforming them into new forms of carbon[6]. Because the deep ocean below the thermocline[7] is isolated from the atmosphere, it takes carbon out of contact with the atmosphere for several thousand years or longer[8]. This is phytoplankton’s unique ability that other land plants are incapable of. Globally, this biological pump transfers 10 billion metric tons of carbon from the atmosphere to the ocean depths each year[4]. This demonstrates that phytoplankton absorb greater amounts of carbon than normal land plants, so the increase in their productivity can ameliorate climate change. 

   Phytoplankton support the base of aquatic food webs; they feed primary consumers like zooplankton and small fish, who are then consumed by secondary consumers, and so on, ultimately acting as the source for humans' diverse nutrition. Insufficient phytoplankton disrupts the survival of many other marine animals, as carbon dioxide is no longer converted into biomass. When an organism dies, decomposition takes place, which consumes oxygen and depletes other organisms of nutrition. Whether directly or indirectly, every organism ultimately depends on phytoplankton for living, so it is crucial to maintain its population level balanced.

    Taking this into consideration, we can infer that the instigation of phytoplankton blooms using engineered nanoparticles can help to reduce carbon dioxide. The nanoparticles can be designed to fertilize the growth of phytoplankton in various ways. Being coated with simple and environmentally friendly polymers may urge plankton to be buoyant within the phytoplankton zone, preventing them from sinking to the bottom of the sea. The particles could also be shaped to deliver key nutrients or prevent them from being eaten to help their growth[9]. It would be effective to utilize phytoplankton’s persistent “green” ability to reduce carbon dioxide, despite the rising temperature.

An uncertain outlook

   Although phytoplankton emerges as a practical solution to climate change, uncertainty remains. Warming temperatures may cause certain phytoplankton to change from carbon absorbers to carbon emitters. Most phytoplankton obtain energy through photosynthesis, yet some do so from both photosynthesis and by consuming other organisms. Photosynthesis takes carbon dioxide out of the atmosphere, but the consumption of other organisms results in emissions of carbon[4]. Additionally, an excessively rapid growth of phytoplankton may be harmful to marine ecosystems. This is because they produce toxins, and the constant accumulation of such toxins threatens the entire ecosystem. Algal blooms are an exponential growth of phytoplankton and occur when a suitable environment is provided: abundant light, warm temperature, and high nutrition availability. They are an international concern due to their lasting impacts on human life beyond marine ecosystems. Algal blooms directly influence the diet and nutrition sources of humans as the depletion of oxygen levels due to amassed toxins threatens the quality and safety of seafood. High levels of nitrogen or phosphorus along with the blooms also limit beach accessibility. Particularly, coastal communities dependent on income generated through fishing and tourism combat not only climate change, but also economic struggles[10]. Since fish and shellfish are often farmed in a confined area, it makes them difficult to escape the sudden and severe blooms. Algal blooms in Nordic fjords resulted in the death of about 7.5 million farmed Atlantic salmon, costing the industry over $90 million[11].

   Likewise, phytoplankton can be a double-edged sword; they may serve to prevent climate change by removing carbon away from the atmosphere, yet are also exposed to the high risk of becoming harmful algal blooms. While phytoplankton’s nutrient uptake plasticity seems to shape a hopeful future for a “greener” Earth, it is crucial to precisely control and anticipate their movement. This requires close observatory reference over the long term. In the future ahead, the research will aim to further analyze how phytoplankton would respond to complex changes such as nutrition depletion and ocean acidification[5].

[1] Autotroph: An organism that can produce its own food using abiotic components without having to depend on other sources

[2] Photosynthesis: The process by which plants produce energy in the form of glucose and oxygen using sunlight, water and carbon dioxide

[3] NASA Earth Observatory

[4] Columbia Climate School

[5] Institute for Basic Science

[6] Dynamic model of flexible phytoplankton nutrient uptake

[7] Thermocline: The transition layer where the water temperature drastically changes

[8] The Oceans and Marine Geochemistry

[9] “Using nanoparticles and phytoplankton to help combat climate change”

[10] National Oceanic and Atmospheric Administration

[11] LG Sonic