Dinophyceae Dinoflagellates

Dinoflagellates are a group of unicellular algae with membrane bound organelles (they are eukaryotes) and flagella. There are approximately 2000 living species known (130 genera, Figure 6.2, page 135). About half the dinoflagellates feed on organic matter only (that is, they are heterotrophs, including some carnivores) and the other half either photo-synthesise or are partly autotrophic and partly heterotrophic (that is, part animal, part plant).

Dinoflagellates are motile at some stage of the lifecycle - having two different flagella. One flagellum is situated in a girdle groove around the middle of the cell (for rotation) and the other projects from the sulcus groove (at one end) for propulsion. Careful use of a microscope is required to see these flagella.

Dinoflagellates may be armoured (thecate - with cellulose cell walls made of plates) or unarmoured (non-thecate). Armoured dinoflagellates are usually irregular in shape, bearing horns, ridges and wings.

Over 80 species of marine dinoflagellates are known to produce cysts (more than 16 of these species are known to cause red tides and seven

Table 6.2. Factors affecting the growth, abundance and species composition of phytoplankton populations (adapted from Jeffrey and Hallegraeff 1990).

Physical

• Temperature - growth is possible within range; effect on rate of growth, on nutrient demands and on enzymatic processes; thermal stratification

• Light - length and brightness of day; spectral composition; light saturation; inhibitory or lethal intensities; IR absorption; UV effects

• Water movements - horizontal and vertical transport into and out of an area or depth zone; invasions; eddy diffusion

• Density distribution - effects of salinity, temperature, metabolism or gas production in relation to the sinking or rising rate (buoyancy) of organisms

Chemical

• Inorganic substances - nitrogen compounds, phosphates, silicates, sulphides, iron, trace elements, oxygen, ironic ratios and salinity, redox potentials, pH

• Organic substances - vitamins (B12, biotin and thiamine), acids (glycolic and glutamic), chelates, unknown or imperfectly known compounds such as 'humus', natural chelates and most extracellular compounds

• Light-absorptive capacity of algal pigments

Biological

• Inhibitory or stimulatory substances - through the activities of previous populations or the organisms own extracellular products (e.g. lag phases, toxin production)

• Intrinsic factors (phased cell division, diurnal and circadian rhythms); regenerative strategies (i.e. seeding ability)

• Cellular organisation and nutrition

• Life histories, reproductive strategies, resting stages

• Resource competition in relation to growth

• Symbiosis - bacteria on algal cells or in their mucilage; algal cells within algal cells

• Crazing pressure from zooplankton - quantitative and qualitative effects

• Parasitism

• Morphological diversity - cell structure (unicellular, colonial, filamentous); surface to volume ratios; mobility

species to be toxic). Cysts can be of two types - either temporary cysts (that is, the cell quickly re-established itself after a brief encystment) or resting cysts, which sink from the water column and often remain in the sediment anywhere from 6 weeks to 5 months, depending on the species.

BOX 6.2 THE 'SURF DIATOM': ANAULUSAUSTRALIS

The 'surf diatom' - Anaulus australis - has been reported as oily slicks at various NSW beaches. These cells are able to rise to the surface and form dense accumulations by attaching themselves to wave-generated bubbles in high-energy surf zones. In most cases, these accumulations disappear at night and reappear each morning. This species has been reported along the southern coasts of South Africa and Australia (McLachlan and Hesp 1984; Campbell 1996).

BOX 6.3 SPECIES IN THE PSEUDO-NITZSCHIA GENUS

Species belonging to the genus Pseudo-nitzschia have been implicated as the causative organisms of amnesic shellfish poisoning (UNESCO 1995; Hallegraeff 1994). Blooms of the toxic species P. multiseries (5% of total phytoplankton biomass) were detected over a 2-year period in Berowra Creek - in northern Sydney. A bloom predominantly of P. pseudodelicatissima (Figure 6.3h) has also been detected in Berowra Creek. Although this species has been found to be toxic elsewhere (UNESCO 1995), analysis results from oysters from nearby leases showed no detectable concentrations of domoic acid. Oyster leases in Wagonga Inlet, Narooma, have been closed for harvesting due to a bloom of P. pseudodelicatissima, P. pungens and P. australis (toxic species).

The purpose of cyst formation is probably a survival strategy, which is regulated by both physiological and environmental factors such as:

• protection from adverse conditions (such as temperature or nutrient availability)

• a refuge from predation

• alternation between planktonic and benthic habitats

• as part of the reproductive process

• to aid in dispersion/seed population for the subsequent bloom.

Many dinoflagellates make daily diurnal migrations up and down the water column. During the day they migrate towards the surface of the water (for better light availability) and at night they move down to a depth of several metres (for better access to nutrients). This vertical migration is an important consideration when sampling or when analysing the results of sampling activities.

A regularly occurring red-tide on the south-east Australian coast is caused by the dinoflagellate Noctiluca scintillans (Figure 6.3b). Noctiluca are large (0.2-0.8 mm diameter) balloon-shaped, heterotrophic dinoflagellates, which consume other algae, some zooplankton and even fish eggs. They have no photosynthetic pigments, although in tropical waters they may appear green due to endosymbiotic flagellates. As Noctiluca blooms die off, the cells float to the surface forming dense red slicks. Ammonia stored as a waste product is often released at this stage, which is potentially dangerous to fish. Noctiluca are bioluminescent (they glow) at night, especially around a moving boat or breaking wave. Interestingly, the frequency of observation of this species off south-eastern Australia has increased during 1970s to 1990s. This may be due to a number of reasons, including a response to coastal eutrophication (Ajani et al. 2001a).

Figure 6.3 Common bloom species in New South Wales marine and estuarine waters. a) LM of the filamentous cyanobacterium Trichodesmium erythraeum producing raft-like bundles, up to 750 pm long, b) LM of the balloon-shaped, colourless dinoflagellate Noctiluca scintillans, 200-500 pm diameter, c) SEM of the dinoflagellate Gonyaulaxpolygramma, showing ornamented cellulose plates with longitudinal ridges, 29-66 pm long, d) LM of the large, unarmoured dinoflagellate Akashiwo sanguínea, 50-80 mm long, e) SEM of the calcareous nanoplankton Gephyrocapsa oceanica, 15 pm diameter, f) SEM of the triangular, armoured dinoflagellate Prorocentrum cordatum, 10-15 pm wide and covered with minute spinules, g) TEM of the weakly silicified cell of the centric diatom Thalassiosira partheneia, 10 pm diameter, h) TEM of the pennate diatom Pseudo-nitzschia pseudodelicatissima, 57-150 pm long. (NSW DECC.)

Figure 6.3 Common bloom species in New South Wales marine and estuarine waters. a) LM of the filamentous cyanobacterium Trichodesmium erythraeum producing raft-like bundles, up to 750 pm long, b) LM of the balloon-shaped, colourless dinoflagellate Noctiluca scintillans, 200-500 pm diameter, c) SEM of the dinoflagellate Gonyaulaxpolygramma, showing ornamented cellulose plates with longitudinal ridges, 29-66 pm long, d) LM of the large, unarmoured dinoflagellate Akashiwo sanguínea, 50-80 mm long, e) SEM of the calcareous nanoplankton Gephyrocapsa oceanica, 15 pm diameter, f) SEM of the triangular, armoured dinoflagellate Prorocentrum cordatum, 10-15 pm wide and covered with minute spinules, g) TEM of the weakly silicified cell of the centric diatom Thalassiosira partheneia, 10 pm diameter, h) TEM of the pennate diatom Pseudo-nitzschia pseudodelicatissima, 57-150 pm long. (NSW DECC.)

BOX 6.4 DINOPHYSIS ACUMINATA

Dinophysis acuminata (Figure 6.4g) - a producer of diarrhetic shellfish poisoning (DSP) - was implicated in the contamination of edible bivalves at two locations on the east coast of Australia, with over 80 cases of gastroenteritis being reported. A mouse bioassay revealed a positive result for an unidentified DSP toxin and D. acuminata were found in the guts of the bivalves. Pectenotoxin DSP toxins have now been fully characterised. Peak concentrations of D. acuminata at the Port Hacking 100 m station offSydney generally occur in January (Ajani et al. 2001a).

Dinoflagellates have the largest number of harmful species (around 40 species). They can produce toxic compounds that accumulate in filter-feeding bivalves and commercially important crustaceans and finfish. Consumption of these fisheries by humans can result a range of symptoms including gastroenteritis, headaches, muscle and joint pain, and, in extreme cases, paralysis and respiratory failure (PSP, DSP, NSP and ciguatera poisoning, Box 6.4). On a global scale, over 2000 cases of human-poisoning through fish or shellfish consumption are reported each year (Hallegraeffl995).

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