Marine Plankton
Contents
Plankton in the open ocean
Introduction
Take a second to consider the scale of our oceans. Approximately two thirds of our planet is covered in seawater. The oceans teem with life, yet humans are ill-equipped to explore this environment without specialist equipment. The fascinating world of plankton lies hidden beneath the surface of our seas.
Plankton consists of a diverse range of living organisms that spend at least a part of their life cycle suspended in water. The term ‘plankton’ is derived from the Greek word “plankton”: ‘that which is made to wander or drift’. This term is further divided into the phytoplankton and zooplankton, meaning plant- (Gk. “phyto”) and animal- (Gk. “zoön”) drifters respectively.
Planktonic organisms may have a limited ability to control their fine-scale distribution in the water column, but are otherwise at the mercy of oceanic currents and water movements. Holoplantkon refers to those organisms that spend their entire life in the plankton, as opposed to the meroplantkon, which are only planktonic for a part of their lives. Organisms that are capable of resisting the powers of currents, such as fish and squid, are referred to as neckton.
Planktonic organisms are typically classified into broad size categories according to the ‘Sieburth-scale’, originally proposed in 1978. Viruses and jelly fish sit at opposite ends of this scale, which runs from fractions of a millimetre to metres.
Plants of the ocean
Photosynthesis is the process by which inorganic building blocks such as carbon dioxide (CO2) and water (H2O) are combined using energy from the Sun to produce organic compounds. Organisms that are capable of this process can be referred to as photoautotrophs (Gk. “Photon”, “auto”, “trophe”; light-self-nutrition) and primary producers. Chlorophyll and other similar pigments are found in light-harvesting organelles called chloroplasts. These are present in all species of phytoplankton. Organic matter formed by these organisms forms the basis of almost all food chains. Approximately 50% of global primary production occurs in the oceans.
On land, plants are typically large, conspicuous organisms; trees, herbs, bushes and grasses etc. They have root structures to take up water and nutrients from the soils beneath them, and also to provide anchorage. Marine plants, the phytoplankton, are fundamentally different. They are microscopic, single-celled organisms. Smaller objects have a greater surface area relative to their volume, and hence mass. Being small in the oceans confers several advantages: Phytoplankton cells are so small that their weight is, to a large extent, offset by the frictional drag exerted on them by the water. Thus, they sink very slowly, enabling them to stay within the surface, sunlit waters. Many species of phytoplankton have ornate spines and appendages which appear to both increase drag, and serve as defence mechanisms. Phytoplankton cells do not have roots, and must take up nutrients from their surrounding environment. Having a large surface area relative to their volume ensures that they maximise their chances of attaining enough resources for growth.
Diatoms
Perhaps the most conspicuous group of phytoplankton are the diatoms. Their name is derived from the Greek words “dia” and “temnein”, literally ‘cut in half’. This seemingly bizarre name arose because of the nature of their cell wall, or frustule, which is made up of two halves or valves like that of a laboratory petri dish. The silicon-rich frustule is perforated by numerous pore-like structures which can give the cell a beautiful appearance when viewed under high magnification. These connect the cell to the outside seawater. They also help strengthen the cell wall whilst reducing its mass.
In temperate latitudes, diatoms are well known for their capacity to form immense blooms in spring as the sun’s energy infiltrates nutrient rich surface waters. Their rapid growth rates are thought to confer them a competitive advantage during these times, only to be thwarted as silicate concentrations become depleted. Diatoms produce large quantities of mucus when nutrient stressed, causing them to coagulate and sink into the deep ocean. The ecological significance of this is still not fully understood as it appears to represent a genetic dead-end i.e. once in the deep sea, they do not possess a mechanism to enable them to return to the surface. Diatoms are one of the major producers of omega-3 fatty acids, which are now widely reputed as being beneficial for human health.
Flagellates
Much of the remainder of marine plants belong to a diverse assemblage of unicellular organisms called flagellates. These cells possess a whip-like flagellum. It is at this end of the plankton size spectrum that the distinction between plant and animal becomes blurred. The flagella are used to propel the cell, enabling them to ‘swim’ upwards towards the sunlit waters. Some flagellates, although mobile, contain photosynthetic pigments and are thus autotrophic. Others are devoid of pigments and hence are heterotrophic: they either absorb organic matter directly from their surroundings, or actively capture and ingest bacterial prey. Others still are capable of both autotrophic and heterotrophic nutrition: they are mixotrophic.
Dinoflagellates
Dinoflagellates are comparatively large relatives and have two flagella. They are so-called because of the way that they ‘whirl’ or corkscrew through the water (Gk. “Dinos”; whirling). Approximately half of all the dinoflagellates are autotrophic and those belonging to the other half are either heterotrophs or mixotrophs (see below). Many armour their cell wall with cellulose plates, forming the theca. In some species of thecate dinoflagellates the plates form long spines which serve to increase the cell’s frictional drag and also act as a grazing deterrent. The ‘naked’ dinoflagellates are lacking in any such extracellular armour.
Several species of dinoflagellates bioluminesce: They produce a flicker of light when disturbed. This is frequently observed by SCUBA- (Self Contained Breathing Apparatus) divers at night when they turn off their torches and disturb the water with their hands. Some species of dinoflagellates can produce virulent neurotoxins such as saxitoxin and brevetoxin. These can cause serious harm if they enter the human food chain. For this reason, monitoring programmes exist to ensure that the fish and shellfish that we consume are safe.
Phaeocystis
Phaeocystis is a particularly interesting genus of flagellate. It can exist as small, individual cells or large gelatinous colonies. The latter are long since known for fouling fishermen’s nets. Large blooms of these organisms can occur throughout the world, particularly in the temperate and polar seas. These are problematic for fish as the gelatinous colonies can clog their gills. Indeed, blooms of Phaeocystis can trigger dramatic changes in the structure of marine ecosystems owing to its seemingly unpleasant nature. It produces a strong smelling compound called dimethylsulfupropionate (DMSP) which is thought to serve as a grazing deterrent. It is also suggested to play a role in climate regulation by stimulating the formation of clouds. There is growing concern that the frequency and magnitude of Phaeocystis blooms in coastal waters are increasing as a result of eutrophication.
Coccolithophorids
Coccolithophorids are another important group of flagellates. Each cell is covered by an array of intricate plates or coccoliths. These are made from calcium carbonate, the material that teachers use to write on black boards. The exact function of the coccoliths is unknown. The coccoliths have been suggested to serve as a grazing deterrent, to help maintain buoyancy and to act as an ultra-violet radiation filter. Certain species produce enormous blooms that are clearly visible from space, causing the water to have a milky-white appearance that fishermen refer to as ‘white water’.
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