This authors' personal copy may not be publicly or systematically copied or distributed, or posted on the Open Web, except with written permission of the copyright holder(s). It may be distributed to interested individuals on request. Vol. 80: 193–207, 2017 AQUATIC MICROBIAL ECOLOGY Published online October 5 https://doi.org/10.3354/ame01849 Aquat Microb Ecol Grazing of the heterotrophic dinoflagellate Noctiluca scintillans on dinoflagellate and raphidophyte prey Beth A. Stauffer1,*, Alyssa G. Gellene2, Diane Rico3, Christine Sur4, David A. Caron2 1Department of Biology, University of Louisiana at Lafayette, Lafayette, LA 70403, USA 2Department of Biological Sciences, University of Southern California, Los Angeles, CA 90089, USA 3School of Oceanography, University of Washington, Seattle, WA 98105, USA 4Graduate Group in Ecology, University of California, Davis, Davis, CA 95616, USA ABSTRACT: Noctiluca scintillans is a bloom-forming heterotrophic dinoflagellate that can ingest (and grow on) a number of phytoplankton prey, including several potentially toxic phytoplankton species. The current study documented (1) a range of N. scintillans growth rates (μ = −0.09 to 0.83 d−1) on several species of harmful dinoflagellates and raphidophytes, including Heterosigma akashiwo and Akashiwo sanguinea, and (2) the first published growth rates on Lingulodinium polyedrum, Chattonella marina, and Alexandrium catenella. N. scintillans attained maximum growth rates (μ = 0.83 d−1) on the raphidophyte H. akashiwo and negative growth rates (i.e. signif- icant mortality) on the dinoflagellates A. catenella (μ = −0.03 d−1) and A. sanguinea (μ = −0.08 d−1) and the raphidophyte C. marina (μ = −0.09 d−1). Toxin production by A. catenella did not appear to be responsible for negative effects on N. scintillans growth, as indicated by feeding experi- ments using mixed algal assemblages and the addition of high concentrations of purified dissolved saxitoxin (up to 16.73 ng ml−1). However, growth of both N. scintillans and H. akashiwo was neg- atively affected when exposed to A. catenella culture and cell-free filtrate. These results suggest (1) a species-specific role of N. scintillans in top-down control of toxic bloom-forming dinoflagel- lates and raphidophytes, (2) direct, though not necessarily saxitoxin-dependent, inhibition of N. scintillans growth by A. catenella, and (3) indirect effects of A. catenella on N. scintillans growth through reduction in the availability of high-quality prey. Together, these results provide insights into the potentially significant role of N. scintillans as a grazer of blooms of these species. KEY WORDS: Noctiluca scintillans · Alexandrium catenella · Heterosigma akashiwo · Microzoo- plankton · Saxitoxin · Grazer deterrence Resale or republication not permitted without written consent of the publisher INTRODUCTION Ramsdell et al. 2005). In addition to numerous studies of the abiotic (e.g. bottom-up; Heisler et al. 2008) Red tides and harmful algal blooms (HABs) are controls on HAB populations, the important role of characterized by their deleterious effects on eco - biotic (e.g. top-down) controls on population growth system health, food web structure, and/or human and bloom dynamics has more recently received health (Smayda 1997, Sellner et al. 2003, Anderson et attention (e.g. Turner & Tester 1997, Smayda 2008). al. 2008). Estimated annual costs of HABs in the The role of top-down controls such as grazing on United States total $50 million (Ramsdell et al. 2005) initiation and termination of HABs is complex and and are likely to continue to rise as the frequency and species specific (Turner & Tester 1997). Some grazers extent of HABs increase in the United States and are able to avoid ingestion of HAB species (Colin & worldwide (Anderson et al. 2002, Glibert et al. 2005, Dam 2003, Cassis & Taylor 2006), while others seem *Corresponding author: [email protected] © Inter-Research 2017 · www.int-res.com 194 Aquat Microb Ecol 80: 193–207, 2017 incapable of avoiding these species and suffer mor- up to 0.52 d−1 on diets of the diatom Thalassiosira sp. tality (Avery et al. 2008), reduced reproduction (Buskey 1995) and 0.66 d−1 on the chlorophyte Platy- copy (Colin & Dam 2002a, Haberkorn et al. 2010), and monas helgolandica (Zhang et al. 2016). Lower other physiological effects (Colin & Dam 2003) as a growth rates of N. scintillans (0 to 0.35 d−1) have been result. Those grazers that survive and are sub- observed on diets of the harmful raphidophytes Chat- sequently consumed also represent a mode of toxin tonella antiqua and Heterosigma akashiwo (Naka- Author transfer through food webs to higher organisms mura 1998, Kim et al. 2016) and the dinoflagellates (Doucette et al. 2005, Escalera et al. 2007). Grazer Prorocentrum micans (Buskey 1995, Strom & Morello deterrence and the consequences of ingestion of 1998), Pyrodinium bahamense (Han sen et al. 2004), HAB species, specifically of toxic dinoflagellates, Alexandrium minutum (Frangópulos et al. 2011), and have primarily been investigated in mesozooplank- Alexandrium pohangense (Kim et al. 2016). Field ob- ton (Turner & Tester 1997, Lonsdale et al. 2000, servations have also documented ingestion by N. Turner et al. 2000, Colin & Dam 2005, Turner 2010, scintillans of the toxic species Gymnodinium catena- Haley et al. 2011), shellfish (Cassis & Taylor 2006, tum (Alonso Rodríguez et al. 2005), Pseudo-nitzschia Hégaret et al. 2007, Abi-Khalil et al. 2016, Lassudrie spp., and Dinophysis spp. (Escalera et al. 2007), though et al. 2016, Navarro et al. 2016), and larval fishes and growth rates on these species were not documented in crustaceans (Lefebvre et al. 2005, MacKenzie & Har- either of the studies. Negligible or negative growth wood 2014). Effects of HABs, specifically those in the rates of N. scintillans have been observed when fed the Alexandrium genus of toxic dinoflagellates, on graz- pelagophyte Aureoumbra lagu nensis, the causative or- ing by protistan members of the microzooplankton ganism of brown tides off the Texas coast (Buskey have been less well documented, with a few notable 1995). Finally, both positive (Azanza et al. 2010) and exceptions focused on ciliates (Jeong et al. 2002, negative (Jeong & Shim 1996) growth rates of N. scin- Schoener et al. 2007, Tillmann et al. 2007) and het- tillans (Jeong & Shim 1996) have been reported when erotrophic dinoflagellates (Tillmann & John 2002, fed the dinoflagellate Akashiwo sanguinea (Jeong & Tillmann et al. 2007, 2008). In contrast to their meso- Shim 1996, Azanza et al. 2010). or macrozooplankton counterparts, unicellular micro - N. scintillans co-occurs with several raphidophyte zooplankton grazers are capable of growth rates on and dinoflagellate HAB species in waters of the east- the same order as their phytoplankton prey. As such, ern North Pacific Ocean. The current study investi- microzooplankton have the potential to exert signifi- gated growth and grazing dynamics of N. scintillans cant control on phytoplankton populations on time on bloom-forming raphidophyte (H. akashiwo, Chat- scales relevant to bloom dynamics. tonella marina) and dinoflagellate species (Alexan- Noctiluca scintillans (Macartney) is a heterotrophic drium catenella, Lingulodinium polyedrum, A. san- dinoflagellate with a wide geographical distribu - guinea), with which it is known to co-occur in waters tion throughout temperate, subtropical, and tropical off southern California, USA. Documentation of high coastal waters (Elbrächter & Qi 1998, Mohamed & growth rates of N. scintillans when fed H. akashiwo Mesaad 2007, Gomes et al. 2008, Hallegraeff et al. and positive growth rates when fed L. polyedrum 2008, Harrison et al. 2011). Massive blooms of N. scin- challenges the idea of toxic or harmful effects of tillans characterized by surface accumulations associ- these species on this protistan grazer. Zero or nega- ated with reduced vertical mixing and/or frontal tive growth rates on the other prey species tested boundaries (Le Fèvre & Grall 1970, Holligan 1979, suggest species-specific variability in the extent to Uhlig & Sahling 1990) have been associated with fish which N. scintillans may only be able to exert top- kills, the underlying causes of which involve oxygen down control on the bloom-forming prey with which depletion by the decomposition of large accumula- it co-occurs. The current study is the first to quantify tions of the dinoflagellate or release of ammonia as N. scintillans grazing on toxic A. catenella, L. poly - the bloom subsides (as reviewed in Elbrächter & Qi edrum, and C. marina. Results suggest that the neg- 1998). N. scintillans has been documented to grow to ative effects of A. catenella on N. scintillans growth varying degrees on a wide range of phytoplankton were likely due to both direct, though not necessarily prey species. As such, N. scintillans has been charac- toxin-dependent, inhibition of N. scintillans growth terized as a non-discriminate feeder (Elbrächter & Qi and indirect effects through reduction in the avail- 1998) and unlikely to be prey limited during blooms ability of high-quality prey. The results suggest a (Uhlig & Sahling 1990). Highest rates of N. scintillans potentially significant but highly species-specific role growth have been observed on prey 5 to 25 µm in size for N. scintillans in top-down control of harmful dino- (Chen & Qi 1991, Nakamura 1998), with growth rates flagellate and raphidophyte blooms. Stauffer et al.: Noctiluca growth on dinoflagellate and raphidophyte prey 195 MATERIALS