The theca consists of two large thecal plates (Figures 1-4) of various shapes (nearly circular, oval, triangular, heart-shaped) and eight tiny plates in the apical flagellar area (Figure 6 & Monti et al. 2010) around the larger flagellar pores and smaller auxiliary pore. The anterior margin is straight or slightly convex, with a small depression in the middle. The large thecal plates are covered with minute spines (Figures 5 & 6) or papillae (600-700 per plate, according to Faust & Gulledge 2002) and a number of pores, which may serve as exits for trichocyst threads. Cells are 14-22 µm long and 10-15 µm wide. Although in a recent study, Monti et al. (2010) found near total coincidence between sizes of P. minimum and P. balticum.
P. minimum is primarily photosynthetic, with several yellow-brown chloroplasts (Figure 2), although mixotrophy has been reported (Stoecker et al. 1997). A number of interspecific and intraspecific genetic studies have taken place, and several hundred nucleotide and protein sequences associated with P. minimum are available in GenBank:
POTENTIALLY MISIDENTIFIED SPECIES:
When occurring in its round or circular form, P. minimum is very similar to P. balticum. The main differences are the smaller size of P. balticum (about 9-15 µm vs. 14-22 µm in P. minimum) and the presence of two minute apical spines in P. balticum. Both have been recorded in the IRL, and there is some uncertainty as to the details of their distribution here.
HABITAT & REGIONAL OCCURENCE:
Prorocentrum minimum is mostly found in brackish water and estuaries of temperate and tropical areas, though it sometimes appears in open coastal waters. Most records are from the northern hemisphere, but that may be an artifact of sampling intensity. It is regarded as an invasive species in some areas (Hajdu et al. 2005). In a review of its biology, Berland & Grzebyk (1991) reported a salinity range of 5-37 PSU and a temperature range of 4-31 °C.
INDIAN RIVER LAGOON DISTRIBUTION:
This species is common in the IRL, and may form blooms in the summer of >2 mm3 L-1 in biovolume (Badylak & Phlips 2004). Blooms elsewhere can reach a population density above 106 cells per liter.
Asexual reproduction is by binary fission. No record of sexual reproduction has appeared to date, even when the cell cycle was studied in detail by Pan & Cembella (1998).
It is questionable as to whether or not P. minimum produces toxins. A specific toxic compound has not been characterized (Saba et al. 2011), although a number of marine invertebrates have shown inhibition of life processes (allelopathy) or mortality (Grzebyk et al. 1997; Heil et al. 2005; Wikfors 2005) when in the presence of P. minimum. Marine mortalities may also occur when bacterial metabolism de-oxygenates the water during bloom decay. Reports of human illness or death from P. minimum toxin via ingestion of shellfish have not been confirmed (Landsberg 2002). Blooms in excess of 106 cells per liter have occurred in several places, with associated marine mortalities. In the IRL, cell concentrations of 105 cells per liter are not uncommon (pers. obs. & Phlips et al. 2010). Differing environmental conditions during blooms suggest variation in the expression of ‘toxicity’ in this species, although a specific chemical compound has not been identified as ‘the toxin’. Several recent studies have provided strong evidence that variation in toxic effects is dependent upon the environment and its effect on the physiology of P. minimum. Mortalities attributable to P. minimum have not been documented in the IRL.
Badylak, S & EJ Phlips. 2004. Spatial and temporal patterns of phytoplankton composition in a subtropical coastal lagoon, the Indian River Lagoon, Florida, USA. J. Plankton Res. 26: 129-1247.
Berland, B & D Grzebyk. 1991. Prorocentrum minimum (Dinophycées). 101-113. In: Sournia, A et al. (Eds.). Le phytoplankton nuisible des côtes de France. Institut francais de recherché pour l’exploitation de la mer. Brest, France.
Faust, MA & RA Gulledge. 2002. Identifying harmful marine dinoflagellates. Smithsonian Contrib. U.S. Nat. Herb. 42: 1-144.
Grzebyk, D, Denardou, A, Berland, B & YF Pouchus. 1997. Evidence of a new toxin in the red-tide dinoflagellate Prorocentrum minimum. J. Plankton Res. 19: 1111-1124.
Hajdu, S, Pertola, S & H Kuosa. 2005. Prorocentrum inimum (Dinophyceae) in the Baltic Sea: morphology, occurrence – a review. Harmful Algae 4: 471-480.
Hallegraeff, GM. 2003. Taxonomic Principles. 383-432. In: Hallegraeff, GM, Anderson, DM & AD Cembella (Eds.). Manual on Harmful Marine Microalgae. UNESCO Publishing, Paris. 793pp.
Hallegraeff, GM, Bolch, CJS, Huisman, JM & MF de Salas. 2010. Planktonic Dinoflagellates. 145-212. In: Hallegraeff, GM et al. Algae of Australia: Phytoplankton of Temperate Coastal Waters. CSIRO Publishing, Melbourne, Australia.
Heil, CA, Glibert, PM & C Fan. 2005. Prorocentrum minimum (Pavillard) Schiller. A review of a harmful algal bloom species of growing worldwide importance. Harmful Algae 4: 449-470.
Hoppenrath, M, Elbrächter, M & G Drebes. 2009. Marine Phytoplankton. Kleine Senckenberg-Reihe 49. E. Schweizerbart Science Publishers, Stuttgart Germany. 264 pp.
Landsberg, JH. 2002. The effects of harmful algal blooms on aquatic organisms. Rev. Fish. Sci. 10: 113-390.
Monti, M, Stoecker, DK, Cataletto, B & L Talarico. 2010. Morphology of the flagellar pore complex in Prorocentrum minimum (Dinophyceae) from the Adriatic and Baltic Sea. Botanica Marina 53: 357-365.
Pan, Y & AD Cembella. 1998. Flow cytometric determination of cell cycles and growth rates of Prorocentrum spp. 173-176. In: Reguera, B et al. [Eds.]. Harmful Algae. Xunta de Galicia and Intergovernmental Oceanographic Commision, UNESCO. GRAFISANT, Santiago de Compostela, Spain.
Phlips. EJ, Badylak, S, Christman, M, Wolny, J, Garland, J, Hart, J, Landsberg, J & six others. 2011. Scales of temporal and spatial variability in the distribution of harmful species in the Indian River Lagoon, Florida, USA. Harmful Algae 10: 277-290.
Saba, GK, Steinberg, DK, Bronk, DA & AR Place. 2011. The effects of harmful algal species and food concentration on zooplankton grazer production of dissolved organic matter and inorganic nutrients. Harmful Algae 10: 291-303.
Stoecker, DK, Li, AS, Coats, DW, Gustafson, DE & MK Nannen. 1997. Mixotrophy in the dinoflagellate Prorocentrum minimum. Mar. Ecol. Prog. Ser. 152: 1-12
Throndsen, J, Hasle, GR & K Tangen. 2007. Phytoplankton of Norwegian Coastal Waters. Almater Forlag AS, Oslo, Norway. 343 pp.
Velikova, V & J Larsen. 1999. The Prorocentrum cordatum/Prorocentrum minimum taxonomic problem. Grana 38: 108-112.
Wikfors, G. 2005. A review and analysis of trophic interactions between Prorocentrum minimum and clams, scallops, and oysters. Harmful Algae 4: 585-592.