Phytoplankton are adapted as freely floating, microscopic organisms that drift with water currents. They do the mighty job of photosynthesis and are the most prominent producers of an aquatic food web. To do photosynthesis, they need to be afloat in the euphotic i.e. light penetrating zone of ponds or rivers. But what are the secret of phytoplankton adaptation in this zone? And how do they ensure their position in the topmost layer of aqua sites?
Well, the unique Size and shapes of phytoplankton help them to remain floating in the water. There are many other factors and adaptive features, that combinedly ensure the floatation of phytoplankton in the uppermost layer of water.
In this article, we have discussed the causes of phytoplankton adaptations, including their special morphological features to adapt to the water body.
Causes of Adaptation of Phytoplankton in Water
To know the causes and process of phytoplankton adaptation in water, it is important to be clear about their position in the water ecosystem.
Position of Phytoplankton in Water Body
Phytoplankton hold a delicate position within the water body, balancing their needs for both light and protection. To get proper light, they need to float on the surface of the water. Otherwise, they might not be able to get food sources.
Again, phytoplankton must not expose themselves to the harsh elements beyond the water’s surface. The direct heat of sunlight and the arid embrace of the open air can be harmful, even deadly, to these tiny organisms. To protect themselves, they remain discreetly submerged beneath the euphotic zone, where they can access the life-giving light without risking desiccation or overheating.
But the real problem is, that phytoplankton are denser and heavier than water. This is because of their cellular components. The cell walls of phytoplankton are composed of materials like cellulose or silica, which have a density greater than that of water. This makes them inherently denser, causing them to naturally sink when not prevented by specific mechanisms. Plankton possess a bunch of adaptive features. Those features are discussed here.
Classification of Adaptation of Phytoplankton in Water
The adaptation of phytoplankton depends on a variety of factors. Density is one of them. The comparison of density between phytoplankton and water decides the suitable adaptation.
Adaptation of Phytoplankton Based on Density
Based on density and locomotion capability, there exist three different kinds of adaptation measures in phytoplankton.
1. Negatively Buoyant Phytoplankton
If the density of phytoplankton is higher than water, then they are called negatively buoyant phytoplankton. These planktons lack motility. With the higher density, they keep sinking gradually. So how do they survive in deeper waters?
Well, they typically possess chlorophyll and other photosynthetic pigments that help them to utilize light energy even in deep and dark waters!
Now, as these phytoplankton move into deeper layers, they transport organic materials and various nutrients with them. In this way, they play a significant role in nutrient cycling among the upper and lower water layers.
Example: Asterionella formosa, Melosira sp etc.
2. Positively Buoyant Phytoplankton
If the density of phytoplankton is lower than water, then they are called positively buoyant phytoplankton. These planktons are fully motile. In general, they keep floating at or near the upper water layers.
In the case of growth and survival, this type of adaptation seems to be very beneficial. This is because they can easily uptake essential nutrients as their residence is in the upper parts of water.
Well, one of the fundamental features attributing to their buoyancy is gas vacuoles. They produce gas-filled protein membrane structures which reduce their density and make them efficient floaters!
Talking about their roles, I must say these planktons are an amazing food source for marine organisms, especially zooplankton. It indicates their role in the marine food web. Also, they fix carbon dioxide in photosynthesis and supply organic carbon in marine ecosystems.
Example: Microcystis, Aphanizomenon, Gloeotrichia, Coelosphaerium, and Gomphosphaeria.
3. Neutrally Buoyant Phytoplankton
If the density of phytoplankton is equal to water, then they are called positively buoyant phytoplankton. These planktons may be either motile or non-motile. They do not sink or float. Rather maintain a stable position at their preferred depth.
They do also have gas vacuoles like positively buoyant plankters. Additionally, they can increase or decrease the size of their vacuoles and thus control their buoyancy.
Well, these planktons do have quite similarities with the previous one. They are also available as a food source for zooplankton and other marine organisms. You can see both positively and neutrally buoyant plankters are primary producers in the marine food web!
Example: Thalassiosira, Ceratium, etc.
Morphological Adaptation of Phytoplankton
Phytoplankton modify and build special morphological features so that they can keep afloat in the water. To achieve long-term floatation, phytoplankton are variously modified as follows:
In general, most of the planktons make their adaptation to remain floated in the water surface through their small size. Small size occupies a low mass to surface area. As a result, it happens to be in the boundary of density, that prefers it to remain afloat. e.g. Pyramimonas sp.
2. Cellular Shape
There are some planktons, those show certain change in their cellular shape. The shape is changed in such a way that it prevents their vertical sinking.
For example, Rhizosolenia’s needle-like shape provides buoyancy to it because it reduces the density of the organism. The shape allows for a larger surface area compared to its volume, which helps it stay afloat in water.
Some of the adapted shapes of phytoplankton are:
- Discoid shape (Coscinodiscus)
- Needle shape (Rhizosolenia)
- Ribbon shape (Skeletonema, Melosira)
- Siphonous (Caulerpa)
As you already know, phytoplanktons modify their existing structure or develop some special structure for the sake of adaptation. Those special structures are termed as appendages. For example, bristles, spines, horns, and chaete are some of the prominent appendages.
Appendages perform two types of functions in the adaptation of phytoplankton. These are:
- Keep the plankters according to their suitable depth in water.
- Prevent sinking effectively.
Example: The frustules of Chaetoceros produce long chaetae. These chaete are specialised to prevent sinking and maintain proper position of Chaetoceros.
4. Colonial life
Certain planktons exhibit colonial behavior as a result of adaptation. To evolve into colonies, single planktons cluster together and turn into a large colony.
Here is an interesting note! Surface area is highly proportional to sinking resistant capacity. The formation of a colony is equivalent to increased surface area. This expanded area provides increased resistance to sinking.
For example, Asterionella produces star-like colonies for their adaptation.
5. Oil droplets
Many phytoplankton species deposit oil as reserved food. These oils can be utilized to meet nutrient deficiency.
Well, in case of adaptation, oil droplets help plankters to become lightweight by reducing their specific gravity. The lightweight phytoplankton will easily float at or near the water level.
For example, Diatom species Pinnularia deposit oil droplets for their adaptation.
6. Gas vacuoles
Among the phytoplankton, Cyanobacteria produce gas-filled structures consisting of protein. These structures are stacked into vacuoles and thus form gas vacuoles. The main function of these gas vacuoles is to keep the plankters floating. It functions almost similarly to oil droplets. Both gas and oil make the density of the organism lower. And the lower the density, the lower the chance of sinking!
The cyanobacterial cells regulate the gas contents inside vacuoles. When the gas content increases, those vacuoles become filled with gases. Only completely filled gas vacuoles serve the maximum possible buoyancy to cyanobacteria. So, by adjusting the gas quantity, cyanobacteria can easily remain afloat in water!
7. Air bladder
Air bladder is basically a container filled with air i.e. various gases present in air. Some particular algae produce air bladders to maintain their survival and adaptation.
Because algae perform photosynthesis, they need to stay close to the water surface. Here comes the air bladders which help algae by increasing buoyancy and remain in the upper water column.
Example: Sargassum fluitans