PHYTOPLANKTON- MICROSCOPIC YET MAMMOTH FUNCTIONING PLANTS OF THE AQUATIC WORLD:
Phytoplankton, the unicellular microscopic form of plant, is the dominant floral component in the marine environment. These are represented by about 5000 known species identified till date and yet numerous are there to be identified. They contain the pigments of various kind that enable them to carry on photosynthesis and thus giving them the identify of plant. Phytoplankters belong to 13 classes of algae out of which Bascillariophyceae and Dinophyceae are found to be dominant in the marine environment where as, the Chlorophyceae and Cyanophyceae are the predominating classes in the fresh water habitats. The shape of these organisms very from species to species, which is the main identifying feature. Phytoplankton can be found at all latitudes of the world ocean and their presence has been noticed in the ice pores as well. Their distribution is mainly of two types such as spatial distribution (the distribution with respect to space) and temporal distribution (the distribution with respect to time). The spatial distribution again can be categorised into (i) Horizontal distribution which includes the estuarine, coastal and offshore regions (ii) Vertical distribution according to depth of water column; such as surface, subsurface layers including epipelagic, mesopelagic and so on and (iii) latitudinal distribution comprising of tropical, temperate, boreal habitats. The temporal distribution can be depicted under two major heads; the seasonal distribution and diurnal distribution. The role of phytoplankters in the marine environment is not less in any means, than that of the land plants in the terrestrial habitat. Instead of their microscopic size, the phytoplankters have a tremendous impact on the ecology of the marine biosphere. They are the chief primary producers in the marine habitat and thus, they are the initiator of the food chain. It has been estimated that the phytoplankton primary productivity of the marine environment contributes to 90-95% of the gross primary productivity of the world oceans. When the global primary productivity is concerned, the phytoplankton productivity contributes to about 40% of the global production. Corals, the coelenterates that form the most beautiful and colourful under water structures, have also the algal component in them, generally the calcareous alga zooxanthellae
, which makes them autotrophic in nature. The rapid growth and multiplication of phytoplankters by asexual reproduction, generally by cell division, due to eutrophication (enrichment of water by growth promoting substances) is known as phytoplankton blooming. The dense accumulation of these microscopic plant cells causes the change in colour of the water due to their pigment contents. This phenomenon is generally known as ‘Red Tide’ or ‘Brown tides’. It is found that only about 2% of the about 5000 known phytoplankters forms the harmful blooms. The bloom phenomenon that contains toxins or having negative impacts is generally termed as “Harmful Algal Bloom” (HAB). The impacts of HABs are far reaching. The toxins released by this blooms move through the food chain and food web and reaches the higher trophic levels through Bioaccumulation and Biomagnifications.
They affect the general life style and even can kill the higher organisms like zooplankton, shellfishes, fishes, birds, marine mammals and human that feed upon them either directly or indirectly. Some of the toxins released in HABs are Saxotoin, Neogonyautoxin or gonytoxin, ciguaoxin, Maitotoxin, Brevertoxin, and Yessotoxin etc. Various illnesses found in human due to the above toxins are Ciguatera Fish Poisoning (CFP), Diarrhetic Sheltfish Poisoning (DSP), Neurotoxic Shell-fish Poisoning (NSP), Paralytic Shell-fish Poisoning (PSP), Amnesic Shell-fish Poisoning (ASP), Domoic Acid Poisoning etc. Some toxins affect the central nervous system causing temporary or permanent memory loss, paralysis etc. while some of the toxins affect the respiratory system causing suffocation, malfunctioning of lungs etc. Tumour promoting toxins has also been reported from some HABs. Sometimes, air born symptoms are seen among the beach dwellers when irritation occurs in nose and eyes due to toxins carried by air from the toxic blooms on the sea surface. The other impacts involve the lowering of oxygen during the end phase of bloom (senescent phase) due to high rate of decomposition, causing the suffocation and death of other organisms. The decrease in light penetration deep into the water column due to the surface screening of the bloom forming species, causes the lower primary production. Sometimes the phytoplankters with more delicate and serrated structures get stuck onto the gills of fishes causing irritation, overproduction of mucous and eventually death. Besides, these negative impacts blooms that are not harmful have some positive affects like increase in primary productivity for a short period thus increasing the fish and shellfish catch for that period. Also it has been reported that today’s oil deposits in the North Sea and elsewhere are the remains of bygone blooms that settled and accumulated into massive amount of carbon on the ocean floor. In recent years, the incidence of harmful algal blooms has increased in frequency, severity and duration in global scale. Some algal species, which were previously considered harmless and others that were unknown to science until their initial outbreak has been a problem. The causes for this apparent expansion are unknown, but some believe that human alteration of the water quality of the coastal zone is an important factor. So time has come to develop general awareness on HABs and their impacts, in people, mainly living in coastal areas that will prevent the problem into some extent.