Cyanobacteria are primitive algae characterised by the absence of the membrane-bound cell components (they are prokaryotes). Cyanobacteria are often blue-green in colour. They have unicellular, colonial and filamentous forms and do not have flagellate cells at any stage in their life cycle.
Blue-green algae include benthic and planktonic forms. Many species have adaptations to aid survival in extreme and diverse habitats, such as gas vacuoles for buoyancy control, akinetes (resting stages) and heterocysts (specialised cells which can fix atmospheric nitrogen) for survival in waters where the nitrate and ammonia levels are relatively low. Not all taxa have these features. In marine and brackish waters, blue-green algae have produced toxins that have resulted in neuromuscular and organs distress as well as external contact irritation.
Six genera of blue-green algae have been implicated in blooms in Australian coastal waters: Anabaena, Microcystis, Amphizomenon, Nodu-laria, Trichodesmium and Lyngbya. Trichodesmium erythraeum is the most common blue-green in temperate coastal waters of NSW (Box 6.5). This tropical/subtropical species produces episodic 'red tides' that were historically reported as 'sea sawdust' during Captain Cook's voyage through the
Figure 6.4 Common bloom species in New South Wales marine and estuarine waters. a) SEM of the red-water dinoflagellate Scripsiella trochoidea, 16-36 pm long. Note tube-shaped apical pore on top of the cell and nearly equatorial (not displaced) girdle groove, b) LM of the chain-forming dinoflagellate Alexandrium catenella - the causative organism of paralytic shellfish poisoning. Individual cells 20-22 pm long, c) SEM of the red water dinoflagellate Alexandrium minu-tum - the causative organism of paralytic shellfish poisoning. Individual cells 24-29 pm diameter. Note the hook-shaped apical pore on top of the cell and characteristic shape of the first apical plate, d) LM of the ciliate Mesodinium rubrum, with two systems of cilia arising from the waist region, 30 pm diameter, e) LM of the 'raspberry-like' cell of the fish-killing flagellate Hetersosigma akashi-wo ('Akashiwo' = red tide), containing numerous disc-shaped chloroplasts, cell 11-25 pm long, f) LM of an undescribed flagellate resembling Haramonas. The cell surface is covered by numerous mucous-producing vesicles, cells 30-40 pm long, g) SEM of the small armoured dinoflagellate Dinophysis acuminata - the causative organism of diarrhetic shellfish poisoning, cells 38-58 pm long, h) SEM of the siliceous skeleton of the silicoflagellate Dictyocha octonaria, 10-12 pm diameter, i) SEM of the small unarmoured, fish-killing dinoflagellate Karlodinium micrum, 15 pm diameter. (NSW DECC.)
A particularly large bloom of T. erythraeum occurred once off southern New South Wales. Sea-surface-temperature imagery showed the bloom was associated with unusually warm water throughout the area. Perhaps strong warming from the East Australian Current transported and triggered the bloom in local estuaries such as Batemans Bay, Clyde River estuary. The association with warm water is evident in this species' annual distribution (Ajani et al. 2001a) from the Port Hacking 100 m station (off Sydney), where peak concentrations occur in mid-April when surface waters exceed around 220C.
Coral Sea in 1778. The filaments of this alga are united (parallel) into small raft-like bundles that are just visible to the naked eye (around 1 mm). The filaments are generally cylindrical, uniformly broad or slightly tapering at the tips, and are straight or slightly curved. Trichodesmium filaments do not have any specialised cells such as heterocysts or akinetes (Figure 6.3a).
Blooms of Trichodesmium erythraeum are most commonly seen in northern NSW waters in spring, summer and early autumn when the East Australian Current (EAC) transports these algal masses into NSW from Queensland waters (Box 6.5). These blooms appear yellow-grey in their early stages, while they become a reddish-brown later (Figure 6.5e, page 136).
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