Role of dissolved nitrate and phosphate in isolates of Mesodinium rubrum and toxin-producing Dinophysis acuminata

Abstract

Dinophysis acuminata, a producer of toxins associated with diarrhetic shellfish poisoning (DSP) and/or pectenotoxins (PTXs), is a mixotrophic species that requires both ciliate prey and light for growth. Linkages have been described in the literature between natural abundances of the predator Dinophysis and its prey, Mesodinium rubrum, and culture experiments have demonstrated that prey, in addition to light, is required for toxin production by Dinophysis acuminata; together these suggest Mesodinium is a critical component for Dinophysis growth and toxicity. However, little is known about the role of dissolved inorganic nutrients on Mesodinium growth or that of toxin-producing Dinophysis. Accordingly, a series of experiments were conducted to investigate the possible uptake of dissolved nitrate and phosphate by 1) Dinophysis starved of prey, 2) Dinophysis feeding on Mesodinium rubrum, and 3) M. rubrum grown in nutritionally-modified media. All single-clone or mixed cultures were monitored for dissolved and particulate nutrient levels over the growth cycle, as well as growth rate, biomass, and toxin production when appropriate. D. acuminata did not utilize dissolved nitrate or phosphate in the medium under any nutrient regime tested, i.e., nutrient-enriched and nutrient-reduced, in the absence or presence of prey, or during any growth phase monitored, i.e., exponential and plateau phases. Changes in particulate phosphorus and nitrogen in D. acuminata, were instead, strongly influenced by the consumption of M. rubrum prey, and these levels quickly stabilized once prey were no longer available. M. rubrum, on the other hand, rapidly assimilated dissolved nitrate and phosphate into its particulate nutrient fraction, with maximum uptake rates of 1.38 pmol N/cell/day and 1.63 pmol P/cell/day. While D. acuminata did not benefit directly from the dissolved nitrate and phosphate, its growth (0.37±0.01 day−1) and toxin production rates for okadaic acid (OA), dinophysistoxin-1 (DTX1) or pectenotoxin-2 (PTX2), 0.1, 0.9 and 2.6 pg /cell/day, respectively, were directly coupled to prey availability. These results suggest that while dissolved nitrate and phosphate do not have a direct effect on toxin production or retention by D. acuminata, these nutrient pools contribute to prey growth and biomass, thereby indirectly influencing D. acuminata blooms and overall toxin in the system.

The Importance of Mesodinium rubrum at Post-Spring Bloom Nutrient and Phytoplankton Dynamics in the Vertically Stratified Baltic Sea

“…Also, the excretion of nutrients through mineralization and cell explosion can be a significant source of nitrogen (Lindholm, 1985;Miller et al, 1995) to phytoplankton species present in the surface layer community. High nitrate and phosphate assimilation rates reported in previous studies (Dugdale et al, 1987;Jiang, 2011;Tong et al, 2015), support the assumption that inorganic nitrogen available in spring-summer continuum, either brought close to the surface through pycnocline rise or from adjacent areas, will be mostly assimilated by dominating M. rubrum if the other environmental conditions support its growth. Also, the dominance of M. rubrum in May-June, its migration behavior, and phosphate utilization in the surface layer is strongly influencing the amount of excess PO 3− 4 that is usually regarded to support the summer cyanobacterial bloom development (e.g., Janssen et al, 2004;Raateoja et al, 2011).…”

“…Increased temperature and water column stability, decreased salinity and depletion of dissolved inorganic nitrogen from the surface layer are known to have positive influence to the occurrence and abundance of M. rubrum (Lindholm and Mörk, 1990;Cloern et al, 1994;Montagnes et al, 2008;Johnson et al, 2013) in different locations worldwide. In several studies, the ability of directly utilize nitrate, ammonium, dissolved organic nitrogen (Lindholm and Mörk, 1990;Wilkerson and Grunseich, 1990;Tong et al, 2015) and phosphates (review by Lindholm, 1985;Tong et al, 2015) have been reported. M. rubrum mass occurrences tend to develop in a chemical environment where competing photosynthetic species are a resource (nutrient) limited or are not able to migrate vertically to exploit the pools of dissolved inorganic nutrients below the euphotic layer.…”

Physiological and behavioral responses of phytoplankton communities to nutrient availability in a disturbed Mediterranean coastal lagoon

“…Mixotrophy is also stimulated by nutrient limitation, and would have been particularly competitive in Biguglia lagoon during the three sampling periods. This may explain the bloom of the mixotrophic Mesodinium rubrum (Tong et al, 2015;Seong et al, 2017) in the southern basin in the autumn and spring samplings ( Fig. 5).…”

Uptake of Inorganic and Organic Nitrogen Sources by Dinophysis acuminata and D. acuta

“…Several field studies indicated a preference for regenerated forms of nitrogen in Dinophysis acuminata (ammonium and organic molecules; [15][16][17]). These trends have been confirmed in laboratory incubations of D. acuminata [18,19] which yielded no uptake, or fairly low rates, of inorganic forms of N and P, but rapid assimilation of urea. All previous results suggested that Dinophysis is either a III B1 type of mixotroph (sensu [8]), i.e., "protists that retain chloroplasts and sometimes other organelles from one algal species or very closely related algal species", or a pSNMC (plastidic Specialist Non-Constitutive Mixotroph) [7].…”