Small but smart: the fascinating world of plankton

By Albini Dania

From the food we eat to the air we breathe, plankton help maintain and produce life on Earth. 

What is plankton?

The word ‘plankton’ derives from the Greek “planktos” (πλαγχτος), which in turn comes from the Greek verb “plazo” (πλαζω) meaning to “drift” or “wander” and is used to describe tiny plants – called phytoplankton, and small animals – called zooplankton, that live in all aquatic ecosystems (from oceans to freshwater ponds and streams).  However, marine plankton vary hugely in size, from few microns in diameter (phytoplankton) up to some exceptional large jellyfish weighing kilos! Plankton are crucial parts of aquatic ecosystems, but often they get forgotten or not given the recognition they deserve. 

Figure 1. Freshwater plankton (©Dania Albini)

Why plankton is important?

From the food we eat to the air we breathe, plankton help maintain and produce life on Earth. 

  1. They play a fundamental role in the global carbon cycle. Phytoplankton use sunlight to perform photosynthesis: they transform the CO2 dissolved in the water into organic substances used for their growth and reproduction. Oxygen is one of the products they excrete from this activity… Exactly like terrestrial plants. Marine phytoplankton undertake 50% of the photosynthesis on earth, producing 50% of the oxygen we breath. Plankton contribute to the carbon cycle not only through photosynthesis but also by transferring biomass (and so energy) to higher trophic levels. In fact, phytoplankton are the preferred food for zooplankton, which are in turn eaten by fish, fish are eaten by bigger predators such as birds, seals… and us humans.
  2. They are environmental bioindicators and they are very useful to study to see the effects of climate change on aquatic communities. This is because of their small size, short generation time and their ability to quickly respond to perturbations and environmental stressors (such as increase in water temperature and changes in pH).
  3. Zooplankton have a recreational role for humans. I am sure many of us can remember the excitement of when, in young age, we were introduced to pond dipping: the exploration of the unseen microscopic creatures ready to be discovered by kids! Many of those organisms in our nets were zooplankton (as well as many other interesting animals such as tadpoles, dragonfly larvae and colourful beetles).

How plankton can be used as environmental bioindicators?

The freshwater Daphnia magna, commonly known as water flea, is among the most widely chosen aquatic plankton for ecotoxicological studies since early 1900’s. Daphnia bioassays are especially used to monitor water quality and pollution events. Parameters such as growth, reproduction and immobilisation are used as reflections of its health: dose-response data (data which looks at different levels of a chemical’s presence and its effects on a system) predict the consequences of exposure at other dose levels and life stages, in other species, or in susceptible individuals.

Plankton are crucial parts of aquatic ecosystems, but often they get forgotten or not given the recognition they deserve. 

… And why smart?

Plankton, especially the freshwater groups, are generally quite small. However, small doesn’t mean simple or less intelligent. During my University career, I learnt that both zooplankton and phytoplankton can evolve many diverse and successful strategies to defend themselves against stressors. This ability is fundamental for their survival and thus is a main driving force in their life histories. Plankton are able to defend against predators and competitors (called biotic stressors) and also against physical abiotic environmental factors (such as prolonged light exposure, low nutrient availability and desiccation). Hairs, trichomes, spines and colonies are specialized morphological structures that can be formed by plankton to escape and survive predations. Dormant stages, such as resting eggs, are also fundamental for those zooplankton that live in unpredictable environments.

Can you explain more some of those defensive strategies?

A first example covers the defences of freshwater Daphnia sp., one of my favourite zooplankton. A behavioural defence consist in the “diel vertical migration”: during daylight, when predatory fish hunt visually on the upper level of the water, Daphnia hide in the darker depths, whereas during the night, when fish are in the lower layer of the water, they migrate upwards to safely feed on phytoplankton. Nevertheless, sometimes Daphnia coexist with their predators, and for this reason they can develop long head-spines (called helmets) and tail-spines that increase their size and may harm predators. Where this phenomenon happens on a seasonal/monthly basis, it is called “cyclomorphosis”.

Figure 2. Defended and undefended Daphnia. On the right, a Daphnia without predator-induced defences, on the left, a Daphnia with spined helmet and a tail spine (©Dania Albini).

Another interesting example is the formation of large colonies in phytoplankton as defence against herbivorous zooplankton. The freshwater Scenedesmus sp. is a green phytoplankton that is usually found as small, single cells. When a predator is detected in the water however, they reproduce as colonial forms. In this way, they increase their collective size, are more difficult to handle and less vulnerable to being eaten by the zooplankton.

Freshwater ostracods, small crustacean zooplankton, often inhabit temporary environments such as ephemeral ponds. For this reason, they adopt a “diversified bet-hedging” reproductive strategy, based on the idea of not “putting all of their eggs in one basket”. In fact, the eggs produced in one clutch are of two types: subitaneous – that hatch straight away after deposition (successful only in case the water body is inundated) and dormient – that don’t hatch and are “asleep” in the sediment (predicting a period of dry). This is a protection against the possibility of no offspring surviving to the next inundation period.

Can you tell a curiosity about plankton that is not well known?

Everyone knows that at a particular time of the year (usually summer months), marine phytoplankton can produce an emission of light called “bioluminescence”, an extraordinary phenomenon that colours the sea and the waves in a shade of bright blue. But not many knows that this effect is used by chefs… to create luminous phytoplankton-based cocktails and dishes! A Spanish chef from a Michelin three-star restaurant in Cadì (Spain) spent 5 years researching a way to extract the plankton’s bioluminescence. Now he has a full tasting menu based on bioluminescent dishes to be, of course, eaten in the dark!

Figure 3. Bioluminescent cocktail made with glowing phytoplankton ( ©Internet)

Images taken with permission from the author