Plankton

Plankton (Gr. planktos, wandering) refers to a diverse group of microscopic organisms that exist in vast water bodies, drifting in the water column and unable to swim upstream. In the aquatic and marine system, planktons constitute the base of a large number of food webs. Depending on the size and taxonomy, planktons can be broadly categorised in several ways.

Classification of Planktons

On the basis of their size, planktons can be

Types	Size
Megaplankton	over 20 cm
Macroplankton	2 to 20 cm
Mesoplankton	0.2 to 20 mm
Microplankton	20 to 200 µm
Nanoplankton	2 to 20 µm
Picoplankton	0.2 to 2 µm
Femtoplankton	less than 0.2 µm

On the basis of trophic levels, planktons can be

Types	Short Description
Phytoplankton	Autotrophs generate their food through photosynthesis or chemosynthesis
Zooplankton	Primary consumers, which feed on other planktons
Bacterioplankton	Recycle nutrients and break down organic material
Mycoplankton	Decomposes organic matter in the aquatic environment
Viroplankton	Influence microbial communities in the aquatic environment.
Mixotrophs	Organisms that can perform photosynthesis as well as feed on other plankton, depending on conditions.

Phytoplanktons

Phytoplanktons are autotrophic organisms that prepare their food and produce approximately 40-50% of the total oxygen on the planet. Moreover, phytoplankton is the main source of nutrition for most marine animals, ranging from small zooplankton to big fish.

Some common phytoplankton are-

Diatoms: A group of mostly unicellular algae that exhibits a ‘bloom and bust’ lifestyle. When environmental conditions are favourable, they dominate the phytoplankton community (bloom), but with depletion of nutrients, diatom cells increase the sinking rate and exit (bust) the aquatic upper layer.

Dinoflagellates: A single-celled protist with two whiplash flagella. Bioluminescent in nature, and can create glowing waves in the sea and ocean during nighttime.

Cyanobacteria: The oldest photosynthetic organisms, which also perform nitrogen fixation.

Zooplanktons

Zooplankton are heterotrophs that acquire energy by consuming other organisms (phytoplankton, bacterioplankton and smaller zooplankton). Zooplankton range from protozoans to larvae of fish, molluscs, and cnidarians (jellyfish in larval stages). They can be categorised on the basis of their life cycle-

Holoplankton: These organisms remain as zooplankton throughout their lives, like tiny crustaceans, jellyfish, etc.

Meroplankton: At some stage in their life cycle (primarily as larvae), these organisms thrive as zooplankton. Larvae of fish, arthropods, molluscs, and sea urchins are meroplanktons.

Bacterioplanktons

Bacteria that take part in the nutrient cycle within aquatic environments are included as bacterioplanktons. They are involved in the decomposition and recycling of organic material, as well as regulating the energy flow throughout the ecosystem. Bacterioplankton assists in the decomposition of detritus to release important nutrients to other marine life.

Ecological Significance of Planktons

As producers, phytoplanktons account for half of the total amount of oxygen evolved.
Planktons are used as fish food in fisheries.
Phytoplanktons are used as a source of food in space travel.
Plankton helps in carbon sequestration.
Dead and decaying organic matter can be recycled into inorganic nutrients by planktons.
CO₂ uptake by the ocean can be increased by phytoplanktonic growth
In the presence of abundant nutrients, unicellular dinoflagellates multiply rapidly to cause a bloom, known as the red tide. They produce toxins and are harmful to fish and other organisms that feed on fish.

Summary

Plankton form the base of aquatic food webs. They play an important role in oxygen production and nutrient cycling. Planktons regulate the carbon cycle and support marine biodiversity. Their presence is essential for life on Earth.

Frequently Asked Questions (FAQs)

Q1. How do planktons reproduce?

Most planktons reproduce asexually by binary fission, while zooplankton use either sexual or asexual methods.

Q2. What is the effect of planktons on the carbon cycle?

Phytoplankton absorbs carbon dioxide (CO₂) from the atmosphere during photosynthesis. When they die, part of this carbon ends up in the deep ocean, where it counterbalances the harmful effects of CO₂ in the atmosphere, thus controlling the climate.