Factors Influencing the Toxicity, Detoxification and Biotransformation of Paralytic Shellfish Toxins - Reviews of Environmental Contamination and Toxicology - page 6

3 Factors that Infl uence HABs and their Toxicity

3.1 Eutrophication

Eutrophication is defi ned as the introduction of excessive nutrients from anthropo-
genic activities into the aquatic environment that increases algal biomass. Enrichment
by phosphorus (P) and nitrogen (N) in a habitat is the most important contributor to
eutrophication. In freshwater, phytoplankton production is strongly correlated with
total phosphorus inputs, whereas nitrogen most infl uences primary producers in
marine waters (Anderson et al. 2002 ). Over the last decade, human activities have
resulted in a signifi cant increase of nutrient releases to coastal ecosystems. In
general, the concentrations of P and N in the ocean are estimated to have increased by
three and fourfold, respectively, compared to levels recorded during pre-industrial and
pre-agricultural eras (Smil 2001 ). HAB species, like other photosynthetic organ-
isms, require nutrients for growth. The nutrients' source of HAB is not limited to
dissolved inorganic nutrients (nitrate and phosphate), but also include dissolved inor-
ganic and organic compounds, and particulate nutrients in the form of detritus
(Anderson et al. 2002 ). Nutrient enrichment enhances the impact of toxic HABs in
two ways. First, it increases the abundance of all phytoplankton, without altering the
relative fraction of the total phytoplankton biomass. Second, the harmful effects of
HAB increase as the abundance of toxic HAB species increases (Poulton et al. 2005 ).
Nutrient enrichment may favor one or a few species to become dominant, and thereby
to overwhelm other phytoplankton (Anderson et al. 2002 ).
Many photosynthetic bacteria and fl agellate species have been documented to
demonstrate physiological adaptation, which allows them to acquire both C and N via
particle ingestion, or to consume dissolved organic compounds (Al-Azad et al.
2013 ; Graneli and Carlsson 1998 ). Moreover, Alexandrium sp. is physiologically
adapted to take excess P up that is required for immediate growth at an early growth
stage (Lim et al. 2010 ). Harmful dinofl agellates tend to occur in water that has sea-
sonally high phosphate and nitrate level, high amounts of dissolved organic carbon
(DOC), and other forms of organic nutrients (Glasgow et al. 2001 ; Glibert et al.
2001 ). Bloom development coincides with at least a threefold reduction in DOC and
dissolved organic nitrogen (DON), which indicates that HAB cells have the ability
to acquire nutrients via extracellular oxidation or hydrolysis (Lewitus et al. 2001 ).
Nevertheless, brown tide species such as Aureoumbra lagunensis have been shown
to be incapable of nitrate uptake, and thus must use a reduced form of N (Deyoe and
Sutrle 1994 ). In fact, food-vacuole-containing prey fragments were found in
Dinophysis norvegica and D. acuminate (Jacobson and Anderson 1996 ), Heterosigma
carterae , A. tamarense ( Nygaard and Tobiesen 1998 ), and Gyrodinium galatheanum
(Li et al. 2000 , 2001 ), which confi rm these species to be mixotrophic (i.e., they have
the ability to ingest particulate food). In addition, the grazing rate of fl agellate
phytoplankton has been shown to increase when the dissolved phosphate concentra-
tion is low (<0.05 ʼ M). Several algal fl agellate species (viz., Alexandrium tamarense ,
Gyrodinium galatheanum , Chrysochromulina polylepis , Chrysochromulina Arizona ,
Prymnesium parvum , Ochromonas minima , Pseudopedinella sp., and Heterosigma