Urease activity in cultures and field populations of the toxic dinoflagellate Alexandrium

Abstract

Nitrogen availability is an important factor controlling phytoplankton abundance and species composition in marine waters. In addition to inorganic nitrogen, some phytoplankton species can use dissolved organic nitrogen sources such as urea for growth. Herein we demonstrate that axenic laboratory cultures of the toxic dinoflagellate, Alexandrium fundyense strain CB301A and A. catenella strain TN9A were able to grow on urea as a sole nitrogen source in the presence of nickel. This nickel dependence suggests that these Alexandrium species hydrolyze urea into ammonia with the enzyme urease rather than adenosine triphosphate urea amidolyase. Cells grown on urea had lower toxin content (15%-30%) than f/2-grown cells. In A. fundyense the urease enzyme appears to be nitrogenregulated. In culture experiments, enzyme activity was highest in nitrate-starved and urea-grown (replete) cells, whereas activity was undetectable in f/2-grown (replete) and phosphate-starved cells. Urease activity in ammoniagrown (replete) cells was also depressed. Urease activity also appeared to increase with decreasing nitrate-limited growth rate in semicontinuous cultures. May and June cruises in the Gulf of Maine followed the yearly bloom of A. fundyense. On average, inorganic nitrogen concentrations in May were higher than in June, whereas cell abundances, urea concentrations, and urease activity in May were lower than in June. The latter measurements relied on an immunomagnetic bead separation to isolate living A. fundyense cells from mixed phytoplankton samples for analysis. The differences between May and June suggest that urea may be important for Alexandrium nutrition as inorganic nitrogen concentrations in surface water decline.Harmful algal blooms cause serious public health and economic impacts on a global scale (Hallegraeff 1993;Anderson et al. 2000). For example, species within the marine dinoflagellate genus Alexandrium are responsible for paralytic shellfish poisoning (PSP) along the coastlines of the United States, Canada, and many other countries (Hallegraeff 1993;Anderson 1997). PSP can result in serious illness or death if humans ingest sufficient shellfish contaminated with the dinoflagellate toxins (Van Dolah 2000). Observations of Alexandrium fundyense and PSP outbreaks in the northeastern United States indicate that populations accumulate in a variety of oceanographic habitats (Anderson 1997). There is considerable interest in the environmental factors that affect the growth of this genus in field populations. For example, A. fundyense populations in the western Gulf of Maine are often found in surface waters where inorganic nitrogen concentrations are low (Ͻ2 mol L Ϫ1 ) to undetectable, particularly in the late spring (Martorano 1997;Poulton 2001). In this system it is unclear whether A. fun-1 Corresponding author (sdyhrman@whoi.edu). AcknowledgmentsWe thank Dave Kulis, Jody Donahue, Mike Lomas, Bruce Keafer, Liz Jablonski, and Dave Townsend for technical assistance and sample processing. Special thanks also go t...

Heavy metal incorporation in foraminiferal calcite: results from multi-element enrichment culture experiments with <i>Ammonia tepida</i>

“…Accordingly, Ni must be considered an important prerequisite for diatom growth (Syrett and Peplinska, 1988). Dyhrman and Anderson (2003) made similar observations with respect to Ni for some dinoflagellate species. Apart from this, Ni is also essential for microbiologically-induced calcite precipitation.…”

“…However, interactions between trace metals, in which one element becomes toxic due to the limitation of another, have been observed. For example, Egleston and Morel (2008) reported that Ni became toxic due to the limitation of Zn, resulting in toxicity to diatoms. Bruland et al (1991) found that synergistic and antagonistic interactions between Cu and potential biolimiting metals, such as Fe, Mn, and Zn, might have large effects on the production of marine biota.…”

Evidence for dissolved organic nitrogen and phosphorus uptake during a cyanobacterial bloom in Florida Bay

“…There is some evidence that urease may be constitutively expressed in some cyanobacteria, dinoflagellates and diatoms (Collier et al 1999, Peers et al 2000, Fan et al 2003. Relatively high rates of urease activity have been previously reported for the dinoflagellates Alexandrium fundyense and A. catenella (Dyhrman & Anderson 2003), as well as for Prorocentrum minimum (Fan et al 2003). Fan et al (2003) found that urease content per cell was significantly higher for the dinoflagellate P. minimum than for the either the diatom Thalassiosira weissflogii or the pelagophyte Aureococcus anophagefferens; however, on a per cell volume basis, A. anophagefferens had the highest activity.…”

Role of urea in microbial metabolism in aquatic systems: a biochemical and molecular review

“…In phytoplankton that have urease, a basal level of urease activity is usually always detectable, but the level of activity varies with N source (Antia et al 1991, Collier et al 1999, Peers et al 2000, Dyhrman & Anderson 2003, Lomas 2004, Solomon & Glibert 2008, suggesting that urease activity is regulated by external or internal factors, as is urea uptake. However, the pattern of regulation may differ not only among but also within phytoplankton taxonomic groups (Fig.…”