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The use of stable isotopes to measure the ingestion rate of 1 potentially toxic benthic dinoflagellates by
harpacticoid copepods
Aurélie Boisnoir, Anne-Sophie Pavaux, Nikolaos Schizas, Sophie Marro, Thierry Blasco, Rodolphe Lemée, Pierre-Yves Pascal
To cite this version:
Aurélie Boisnoir, Anne-Sophie Pavaux, Nikolaos Schizas, Sophie Marro, Thierry Blasco, et al.. The use of stable isotopes to measure the ingestion rate of 1 potentially toxic benthic dinoflagellates by harpacticoid copepods. Journal of Experimental Marine Biology and Ecology, Elsevier, 2020, 524, pp.151285. �10.1016/j.jembe.2019.151285�. �hal-02391239�
The use of stable isotopes to measure the ingestion rate of
1
potentially toxic benthic dinoflagellates by harpacticoid copepods
2
3
Aurélie BOISNOIR1&2, Anne-Sophie PAVAUX2, Nikolaos V. SCHIZAS3, Sophie MARRO2, 4
Thierry BLASCO2, Rodolphe LEMÉE2, Pierre-Yves PASCAL1 5
6
1 Institut de Systématique, Évolution, Biodiversité, ISYEB, UMR 7205, Université des 7
Antilles. Équipe - -Pitre. France 8
2 Sorbonne Université, INSU-C , L b ’Océ ph V f ch , 9
Villefranche-sur-Mer. France 10
3 Department of Marine Sciences, University of Puerto Rico at Mayagüez, CALL Box 908, 11
Mayagüez. 00681, USA 12
13
*Corresponding author: [email protected] 14
15
Keywords: Ostreopsis cf. ovata, Amphidinium cf. carterae, grazing rate, isotope, meiofauna, 16
food web 17
Running title: Dinoflagellate ingestion by copepods 18
19
Abstract 20
Phycotoxins synthesized by benthic dinoflagellates are known to bioaccumulate in 21
macrofauna and hence represent a risk for human health. However, the presence of toxins 22
synthesized by benthic dinoflagellates in smaller marine organisms than macrofauna has not 23
been considered despite the fact that such small organisms have an important ecological role 24
in the benthic food web. This present study quantified, for the first time, the trophic 25
relationship between benthic dinoflagellates and meiofauna by using stable isotope enriched 26
dinoflagellates during ingestion experiments. Results showed that harpacticoid copepods were 27
not able to discriminate, during ingestion, between the potentially toxic cells of Ostreopsis cf.
28
ovata and the non-toxic cells of Amphidinium cf. carterae, even when another food resource, 29
such as diatoms (e.g. Odontella sp.), was provided (Kruskal Wallis test, p > 0.05).
30 31
Introduction 32
Over the last ten years, the frequency and the geographic extent of harmful algal blooms 33
(HAB) has increased worldwide (Hallegraeff, 1993; Van Dolah, 2000; Cohu et al., 2011).
34
Among the species which are able to generate HAB, around twenty benthic marine species 35
have been identified to produce a wide variety of toxins including the most potent toxins 36
occurring in nature (Accoroni et al., 2016; Chomérat et al., 2019; Hoppenrath et al., 2014;
37
Litaker et al., 2017; Rodríguez et al., 2018; Tubaro et al., 2011; Verma et al., 2016; Yasumoto 38
et al., 1987).
39
Benthic dinoflagellates synthesizing these toxins are particularly dangerous due to 40
their impact on marine life (Shears and Ross, 2009) and human health (Alcala et al., 1988;
41
Ciminiello et al., 2006; Friedman et al., 2008). Indeed, herbivorous and filter-feeding marine 42
organisms can ingest and accumulate toxins produced by benthic dinoflagellates throughout 43
their life (Chungue et al., 1977; Gleibs and Mebs, 1999; Yasumoto et al., 1976, 1971).Toxins 44
produced by benthic dinoflagellates can transfer between different trophic levels, through 45
predation and bioaccumulation processes thereby reaching high concentrations in top 46
predators which can be consumed by humans (Lewis and Holmes, 1993; Randall, 1958;
47
Vernoux, 1988). Ingestion of seafood, previously contaminated by these toxins, can lead to 48
mass mortalities of marine organisms involving important ecological impacts over large 49
spatial scales (Shears and Ross, 2009) as well as economic impacts affecting the shellfish 50
farming sector (Shumway, 1990). Due to the thermostability of these toxins (Kohli et al., 51
2015), each benthic toxic genus is responsible for specific human health issues, nevertheless 52
leading in rare cases to death (Bagnis et al., 1979; Alcala et al., 1988; Onuma et al., 1999).
53
The genera Gambierdiscus, Fukuyoa, Ostreopsis, Prorocentrum, Coolia, and 54
Amphidinium are frequently involved in benthic dinoflagellate blooms of (Hoppenrath et al., 55
2014; Leung et al., 2018; Smith et al., 2017).
56
The most serious human poisoning events are related to the occurrence of Gambierdiscus, 57
Fukuyoa (Bagnis et al., 1979; Chinain et al., 2019, 2014; Friedman et al., 2017), and 58
Ostreopsis (Alcala et al., 1988; Randall, 2005) genera which are known to synthesize potent 59
neurotoxins (Alloisio et al., 2016; Dechraoui et al., 1999). The species of Gambierdiscus and 60
Fukuyoa are known to produce ciguatoxins (Yasumoto et al., 1977; Pearn, 2001; Litaker et 61
al., 2017; Munday et al., 2017) which are potentially lethal for humans, identified as ciguatera 62
fish poisoning (Chinain et al., 2019, 2014; Yasumoto et al., 1977). Toxic Ostreopsis species 63
synthesize ovatoxins and palytoxins (Ciminiello et al., 2008; Patocka et al., 2018; Rossi et al., 64
2010) causing human palytoxicosis and clupeotoxism after the consumption of certain 65
tropical crustaceans and fish species which are able to bioaccumulate these toxins (Alcala et 66
al., 1988; Onuma et al., 1999). In temperate regions, toxins synthesized by Ostreopsis spp. are 67
involved in irritations by direct contact (Tichadou et al., 2010) and poisoning by inhalation 68
(Gallitelli et al., 2005; Durando et al., 2007), however no incident has yet been reported by 69
ingestion, even though these toxic compounds were detected in seafood.
70
Toxins produced by Prorocentrum, Coolia, and Amphidinium are less dangerous for 71
humans than those synthesized by the genera Gambierdiscus, Fukuyoa and Ostreopsis. The 72
genus Prorocentrum is distributed worldwide (Rodriguez et al., 2010; Richlen and Lobel, 73
2011) and is able to produce okadaic acid (Valdiglesias et al., 2013). This phycotoxin 74
accumulates in shellfish and crustaceans (Kumagai et al., 1986; Vale and Sampayo, 2002) is 75
responsible for human diarrhetic shellfish poisoning (Tripuraneni et al., 1997). Species of 76
Coolia synthesize lipophilic toxins but these organisms have not yet been related to human 77
poisoning events (Ben-Gharbia et al., 2016). Some Amphidinium species synthesize 78
amphidinols and amphidinolides (Paul et al., 1997; Kobayashi, 2008). Laboratory scale 79
experiments have shown that toxins produced by the genus Amphidinium can affect marine 80
organisms (Pagliara and Caroppo, 2012), but there is still no evidence showing that the 81
bioaccumulation of these toxins throughout trophic levels, including humans, is harmful 82
(Botana, 2014).
83 84
The effects of toxins on benthic macrofauna consumed by humans have been intensively 85
studied for several decades, for instance on filter feeders, (Lee et al., 1988; Amzil et al., 86
2012), macrophyte grazers (Amzil et al., 2012; Biré et al., 2015, 2013; Chungue et al., 1977;
87
Yasumoto et al., 1976) and carnivorous fish (Lewis and Endean, 1984), and this due to the 88
presence of toxic benthic dinoflagellates in coastal areas. However, such effort focusing on 89
macrofaunal organisms has led to overlook the potential transfer of benthic dinoflagellates 90
toxins to higher trophic levels through smaller size organisms representing the meiofauna.
91
Meiofauna are a prominent component of the benthos and consist of marine metazoans 92
with variable sizes ranging from 40 µm to 500 µm (Giere 2009). Meiofauna play two 93
important roles in the benthic marine food web. They i) actively graze microalgae (Blanchard 94
1991, Montagna et al. 1995) and to a certain extent bacteria (Pascal et al., 2009), and ii) 95
constitute the predominant food source for a variety of benthic and pelagic predators (Gee 96
1989, Coull 1990). In the pelagic environment, zooplankton copepods can ingest 97
dinoflagellates (Klein Breteler et al. 1990, Klein Breteler et al. 1999) and can consequently 98
transfer their toxins throughout the pelagic food web (Maneiro et al. 2000, Jansen et al. 2006).
99
Harpacticoid copepods associated to macrophytes can dominate the meiofauna (Beckley and 100
McLachlan, 1980; Guidi-Guilvard et al., 2012; Johnson and Scheibling, 1987) and mainly 101
graze on epiphytic algae (De Troch et al. 2007, Pavaux et al., 2019). Benthic copepods are 102
ingested by benthic and pelagic predators (Gee 1989, Coull 1990) and could consequently 103
create an important link between benthic toxic dinoflagellates and higher trophic levels. To 104
our knowledge, this trophic link has never been measured.
105
The aim of the present study is to quantify ingestion rates of harpacticoid copepods fed 106
with two species of benthic toxic dinoflagellates (Ostreopsis cf. ovata and Amphidinium cf.
107
carterae), each having a distinct level of toxicity. Feeding experiments were also undertaken 108
with and without a non-toxic diatom, Odontella sp., in order to evaluate the impact of another 109
food resource on the ingestion rate of dinoflagellates.
110 111
Material and method 112
Micro-algal culture conditions 113
The dinoflagellate clonal cultures of Amphidinium cf. carterae (MCCV092), Ostreopsis 114
cf. ovata (MCCV070) and the clonal culture of the diatom Odontella sp. (MCCV081) were 115
initiated using specimens collected in Bois Jolan ( 6° 4’ 8 2’’ — 6 °2 ’5 8W, 116
Guadeloupe, Caribbean Sea). These strains are maintained in the Mediterranean Culture 117
Collection of Villefranche, France (MCCV strain number). Non-axenic cultures were grown 118
in L1 medium (Guillard and Hargraves, 1993) using autoclaved, aged 0.2 µm filtered 119
seawater with a salinity of 35. The cultures were maintained at 27°C with a 12:12 light: dark 120
cycle provided by cool-white fluorescent tubes in a Memmert incubator. Stock cultures were 121
grown in 15 mL culture medium in flat culture flasks in order to optimize the surface culture 122
area for gas exchange and growth of benthic dinoflagellates. During the exponential growth 123
phase, clonal cultures were successively diluted in order to scale up the culture volume from 124
15 mL (in tissue culture flask of 25 cm2 surface area, Corning) to 1 L (in fernbach culture 125
flasks of 314 cm2 surface area, Scott/Duran).
126
The biovolume of cells sampled from each cultivated strain was evaluated under an 127
inverted microscope (Zeiss Axiovert 40 C) using the approximate geometrical shapes of the 128
dinoflagellates, and the mathematical equation suggested for each genus (Hillebrand et al., 129
1999). The ellipsoid shape was chosen for Amphidinium cf. carterae, the cone and half sphere 130
shape was chosen for Ostreopsis cf. ovata and the cylinder shape for Odontella sp. Five cells 131
of each genus were used to determine the biovolume.
132
Dinoflagellates identification 133
A volume of 5 mL of each clonal culture was centrifuged for 6 min at 2000 rpm. The 134
supernatant was removed and cell cultures were resuspended with 1 mL of sterile water 135
(MilliQ, Millipore). After a second centrifugation step (2 min at 2000 rpm), only the cell 136
pellets were kept and homogenized with 40 µL of sterile water and a fraction of 10 µL of cell 137
pellet was transferred to 0.2 mL polymerase chain reaction (PCR) tubes. Then, PCR tubes 138
were stored at —20°C until further analysis.
139
Approximatively 400 base pairs of the internal transcribed spacer region (ITS1-5.8s- 140
ITS2) ribosomal DNA (rDNA) were amplified by PCR using the primer 32 (5’- 141
GTGAACCTGCRGAAGGATCA-3’) wh ch s h s c p y s qu c f h 142
universal eukaryote primer 329-R (Moon-van der Staay et al., 2001) and the designed primer 143
D1R- (5’-TATGCTTAAAATTCAGCAGGT-3’) wh ch s h s c p y 144
sequence of the primer DIR-F (Scholin et al., 1994).
145
Each PCR tube containing the cell pellets was resuspended in 1 µL of each primer at 146
10 µM, 1 µL of dNTP at 10 mM, 1 µL of the 50X Taq Advantage 2 DNA polymerase 147
(Clontech), 5 µL of 10X Advantage 2 PCR Buffer and 31 µL of sterile water in order to 148
perform PCR reactions in a final volume of 50 µL. The PCR was performed using a 149
MasterGradiant thermocycler (Eppendorf) with the following conditions: one initial 150
denaturation at 94°C for 10 min followed by 35 cycles each consisting of 1 min at 94°C, 1 151
min at 53°C, 1 min at 68°C and a final elongation for 10 min at 68°C. The PCR products were 152
purified using QIAGEN MinElute PCR Purification kit according to the recommendations.
153
After sequencing by Genewiz, sequences were treated using BioEdit software and were 154
compared to the National Center for Biotechnology Information database using BLASTn tool.
155
The rDNA sequences obtained have been deposited on GenBank with the GenBank accession 156
nos. MK543271 and MK543258 for the strains MCCV092 and MCCV070 respectively.
157
Labeling of micro-algal cultures 158
The experiments were run using a dinoflagellate culture (Amphidinium cf. carterae or 159
Ostreopsis cf. ovata) and a diatom culture (Odontella sp.) in the exponential growth phase 160
characterized by a cell density above 1000 cells mL—1. Cultures of Amphidinium cf. carterae 161
and Ostreopsis cf. ovata were labeled with 15N a week before the beginning of feeding 162
experiments.
163
Before deploying the stable isotope enrichment experiment, cultures of dinoflagellates (>
164
1000 cells mL—1) were filtered through a 10 µm mesh and rinsed with autoclaved 0.2 µm 165
filtered aged seawater at a salinity of 35. Dinoflagellates remaining on the mesh were 166
collected with a pipette. The collection, rinsing, and suspension steps were performed on 167
successive small aliquots of culture (35-40 mL) in order to avoid net clogging due to mucus 168
accumulation. Rinsed dinoflagellates (1500 cells mL—1) were placed in 1 L of L1 culture 169
medium with enriched sodium nitrate (Na15NO3, 99%, MERCK) for one week. During this 170
period, the majority of N available for dinoflagellate intake was in the form of 15N. The 171
suspension and rinsing steps were used to start cultures in low nitrogen medium (Jauzein et 172
al., 2017) and to remove the majority of bacteria present in the growth medium (Rausch de 173
Traubenberg and Soyer-Gobillard, 1990). The same method was used to rinse dinoflagellates 174
and diatoms from their culture medium before the beginning of the feeding assays. In order to 175
evaluate the cell abundances for each culture, suspensions at adequate concentrations were 176
done by using a 1 mL Sedgewick Rafter© counting cell under a standard light microscope.
177
Harpacticoid copepods culture conditions 178
Copepods were collected from stands of Penicillus sp. (Ulvophyceae) growing in the 179
region of Bois Jolan (Guadeloupe, Caribbean Sea). They were identified as Canthocamptus 180
sp. based on morphological characteristics. To obtain a monospecific culture of copepods, a 181
unique female carrying eggs was isolated in 5 mL of GF/F filtered seawater. The volume of 182
the culture was increased gradually until reaching a final volume of 1 L and the cultures were 183
kept at 25-27 °C with a natural day/night cycle. Copepods were fed weekly with a mixture of 184
2/3 canned spinach and 1/3 dried fish food. Once a month, 2/3 of the culture volume water 185
was removed and replaced by the same volume of 0.22 µm filtered seawater.
186
Feeding assay 187
Three different controls were designed with: a) 150 live copepods in 0.2 µm filtered 188
seawater, b) 150 dead copepods (frozen and then thawed) in contact with dinoflagellates and 189
c) 150 live copepods trapped in a tube closed by a GF/F filter and immersed in the microcosm 190
containing dinoflagellates. Feeding experiments were performed with d) 150 live copepods in 191
contact with 15N enriched dinoflagellates only (Amphidinium sp. or Ostreopsis sp.), and e) 192
150 live copepods in contact with a mixture of enriched 13C diatoms and 15N dinoflagellates 193
(Amphidinium sp. or Ostreopsis sp.), see Figure 1. Each treatment was performed in triplicate 194
conditions (n=3) where 150 adult harpacticoid copepods were placed in a 100 mL microcosm 195
(64 cm2).
196
Concentrations of diatoms and dinoflagellates were determined in order to reach an 197
equal total biovolume of diatoms and dinoflagellates in each experiment (Table 1). Feeding 198
experiments lasted 4h with a salinity of 35 and a temperature of 27 °C (Memmert incubator).
199
Experiments were stopped by removing all copepods through a sieve (50 µm mesh).
200
Copepods were preserved in a —80 °C freezer for subsequent chemical analysis.
201 202
Isotope analysis and calculations 203
The δ15N of prey (Amphidinium cf. carterae and Ostreopsis cf. ovata) and predators 204
(copepods) were measured by EA-IRMS (Elemental Analysis – Isotope Ratio Mass 205
Spectrometry). Nitrogen isotope composition is expressed in the delta notation (δ15N) relative 206
to air N2: δ15N=[((15N/14N)sample / (15N/14N)reference)-1]×1000. For each experimental condition, 207
copepod sample was composed by 150 specimens representing a total of 109 7 µg (
208
standard error).
209
Incorporation of 15N is defined as excess levels which are above the background 15N level 210
(i.e. copepods in control treatment incubated without 15N enriched dinoflagellates) and is 211
expressed in terms of specific uptake (I). I was calculated as the product of excess 15N (E) and 212
biomass of N per grazer. I was converted in dinoflagellate carbon grazed using C/N ratio of 213
each dinoflagellate species measured with EA-IRMS. This C/N ratio was obtained from stable 214
isotope measurements (30 samples of 150 specimens). E is the difference between the 215
background (Fbackground) and the sample (Fsample) 15N fraction: E = Fsample - Fbackground, with F = 216
15N/(15N+14N) = R / (R + 2) and R = the nitrogen isotope ratio. For the Fbackground, we used 217
control values measured with control grazers. R was derived from the measured δ15N values:
218
R = [(δ15N/ 1000) + 1] × RairN2. The ingestion of dinoflagellates was calculated as [I × (C/N 219
ratio of enriched dinoflagellate)/ (Fenriched dinoflagellate × incubation time)] (Pascal et al., 2008) 220
Individual weights were derived from stable isotope samples and used in ingestion rate 221
calculations.
222
Data analysis 223
The δ15N of copepods after all incubations was not normally distributed which led to the 224
use of non-parametric tests. Kruskal-Wallis tests were used to assess δ15N and average 225
ingestion rate of copepods. Dunn’s test is a multiple pairwise comparison method allowing 226
comparisons of the mean of the rank of each treatment after a Kruskal-Wallis test. Values are 227
presented as means ± standard deviations (SD), except when specified otherwise.
228 229
Results 230
The ITS sequence of 411 bp from the strain MCCV092 collected in Guadeloupe 231
matched with a sequence of Amphidinium cf. carterae (KY697961.1) through GenBank. No 232
data is available on the exact location where the strain was sampled. This match was made 233
with a E-value of 0, an identity percentage of 98.56% and on 100% of query coverage.
234
The ITS sequence of 370 bp for the strain MCCV070 collected in Guadeloupe matched in 235
GenBank with a sequence of Ostreopsis cf. ovata (MH844087.1) collected in Ecuador. This 236
match was made with a E-value of 0, a percentage of identity of 98,64% and on 100% of 237
query coverage.
238 239
The 15N enriched dinoflagellates Ostreopsis cf. ovata and Amphidinium cf. carterae used 240
during ingestion experiments presented a δ15N of 96,56 ‰ 555, 5 ‰ sp c y 241
Their individual ratios of C/N were 18.4 and 11.7, respectively. Ostreopsis cf. ovata and 242
Amphidinium cf. carterae cell biovolumes reached 17,933 7081 µm3 and 677 64 µm3 243
respectively. The clonal culture of Odontella sp. used during ingestion experiments presented 244
a biovolume of 3874 508 µm3. A total of 4500 harpacticoid copepod specimens were 245
isotopically determined from which weight, C and N contents were derived. The individual 246
weight of an adult copepod was estimated at 724 ± 99 ng copepod—1 with 30.9 % of C and 247
14.5 % of N.
248
Copepods were incubated in three control conditions a) copepods which were not fed 249
with 15N enriched dinoflagellates and presented a δ15 f ± ‰ (n = 3), b) copepods 250
which were euthanized (frozen and then thawed) before incubation and c) copepods trapped 251
in GF/F filter without having access to 15N enriched dinoflagellates. The δ15N of copepods 252
from both b) and c) control conditions were not significantly different from control a), 253
Kruskal-Wallis, p > 0.05, and this conclusion was validated with clonal cultures of both 254
Ostreopsis cf. ovata and Amphidinium cf. carterae (Figure 1 and letters associated).
255
In contrast, copepods incubated with 15N enriched dinoflagellates presented a 256
significantly higher δ15N compared to the control a). This difference was observed for both 257
dinoflagellate species (Kruskal-Wallis, p < 0.05). The δ15N of copepods incubated with 258
enriched Ostreopsis cf. ovata and Amphidinium cf. carterae was not affected by the addition 259
of diatoms (Kruskal Wallis, p > 0.05).
260
The average ingestion rate of copepods was not significantly different between 261
Ostreopsis cf. ovata (372 ± 412 pg C ind-1 h—1) and Amphidinium cf. carterae (49 ± 82 pg C 262
ind-1 h—1), Kruskal Wallis test, p > 0.05.
263
Discussion 264
Potential toxicity of benthic dinoflagellates in the Caribbean Sea 265
Investigating the toxicity of benthic dinoflagellates in the Caribbean Sea is recent and 266
studies generally focus only on several genera such as Gambierdiscus (Díaz-Asencio et al., 267
2019; Litaker et al., 2017), Prorocentrum (Moreira González, 2013) and Amphidinium 268
(Moreira González, 2013).
269
Despite a great diversity of Ostreopsis species observed in the Caribbean Sea 270
(Ballantine et al., 1988; Besada et al., 1982; Bomber et al., 1988; Faust, 2009, 1995) no 271
studies have yet focused on the actual production of toxins by Ostreopsis cf. ovata in this 272
area. In the Mediterranean Sea, environmental conditions have been shown to modify the 273
toxicity of Ostreopsis cf. ovata (Pezzolesi et al., 2012; Scalco et al., 2012). Indeed, high 274
toxicity levels were measured when water temperatures increased above 25° (Pezzolesi et al., 275
2012; Scalco et al., 2012) and with a salinity of 32 (Pezzolesi et al., 2012). These 276
environmental conditions, which seem to optimize the toxin production of Ostreopsis cf.
277
ovata in the Mediterranean Sea, were similar to those applied in this present study which 278
suggests that Caribbean strains could be toxic (Boisnoir et al., 2019, 2018). In addition, toxin 279
production by Ostreopsis cf. ovata strains collected in the Mediterranean Sea increased during 280
cell growth (Guerrini et al., 2010; Pistocchi et al., 2011). This trend was also observed for the 281
Brazilian strains (Nascimento et al., 2012). However, the increase of toxin production during 282
growth was not measured for all the Mediterranean strains of Ostreopsis cf. ovata (Scalco et 283
al., 2012). The risk of using aged cells, in post-exponential growth phase, is that the toxins 284
produced by Ostreopsis cf. ovata are released in the medium (Guerrini et al., 2010) which can 285
hence reduce the feeding efficiency of harpacticoid copepods (Pavaux et al., 2019).
286
The toxicity of the genus Amphidinium from the Caribbean and inherent toxin content, 287
was studied on one single species A. massartii (Moreira González, 2013). To our knowledge, 288
no study has yet investigated the relationship between the environmental conditions and the 289
toxicity of Amphidinium cf. carterae. However, strains of Amphidinium cf. carterae collected 290
off the coasts of Bahamas, in the Mediterranean Sea and in China had hemolytic and 291
antilarval activities (Kong et al., 2016; Meng et al., 2010; Pagliara and Caroppo, 2012).
292
Moreover, cells of Amphidinium cf. carterae from Egypt collected during the 293
exponential growth phase were found to be toxic on Artemia salina, whilst cells collected in 294
the post-exponential phase had no effect (Ismael et al., 1999). On the other hand strains from 295
the Northern Arabian Sea did not show any significant toxicity on Artemia salina (Baig et al., 296
2006).
297
Methodological considerations 298
Identifying and quantifying copepod ingestion rates of specific microalgae species are 299
relevant practices when considering the context of harmful algal blooms (Haley et al., 2011).
300
Similarly to planktonic organisms, several methods can be used to quantify the ingestion rate 301
of benthic dinoflagellates by epiphytic copepods. However, it remains difficult to undertake in 302
situ copepod feeding experiments without depending on laborious, intrusive and potentially 303
biased incubation approaches (Nejstgaard et al., 2008). Methods used to quantify ingestion 304
rates of microalgae all present both strengths and weaknesses. Furthermore, methods which 305
are initially adapted for planktonic organisms can be difficult to implement on benthic 306
organisms.
307
Relationships between different trophic levels can be evaluated by identifying ingested 308
organisms in the feces or in the gut content and thus by undergoing microscope observations.
309
This method is nevertheless laborious even for a trained observer (Nejstgaard et al., 2008) and 310
a high fraction of food items remains impossible to identify (Gowing and Wishner, 1992). In 311
order to bypass direct microscope identification, another indirect approach based on the 312
measurement of specific pigments of dinoflagellates in the gut content of copepods can be 313
applied (Kleppel et al., 1988; Oechsler-Christensen et al., 2012). Actually, most studies 314
involving copepod feeding rely on this approach although several limitations have been 315
highlighted (Bustillos-Guzmán et al., 2002; Pandolfini et al., 2000) such as i) pigment 316
degradation during the digestion process (Pandolfini et al., 2000) and ii) limited specificity of 317
pigments for a given microalgal group (Antajan et al., 2004; Irigoien et al., 2004). However, 318
this method remains largely used as it is quick and inexpensive (Nejstgaard et al., 2008).
319
Biomarkers could be an alternative, however there is a lack of specificity for dinoflagellates, 320
particularly regarding amino acid (Guisande et al., 2002) and fatty acid (Desvilettes et al., 321
1994; Graeve et al., 1994) compositions. A new promising strategy is the use of prey-specific 322
DNA barcodes (Sheppard and Harwood, 2005). This approach was used to quantify specific 323
phytoplanktonic species present inside gut contents and fecal pellets of zooplankton 324
(Nejstgaard et al., 2008, 2003). The DNA-based approach may provide a way to rapidly 325
measure ingestion rates, even when the targeted microalgae are present in low abundances 326
within a mixed community (Haley et al., 2011). With this approach, cells of interest which are 327
used as prey must be previously sequenced in order to develop specific primer sets in order to 328
amplify their DNA (Sheppard and Harwood, 2005). Another inconvenience of this method is 329
the possible interference of a large amount of non-target DNA with the primer or the presence 330
of co-purifying material from the host copepod during quantification (Nejstgaard et al., 2008).
331
However, the use of high throughput DNA sequencing combined with better-designed 332
primers and improved databases will undoubtedly generate more studies employing DNA- 333
based approaches (Ho et al., 2017).
334
Alternatively, grazing experiments offer a straightforward method to estimate prey 335
consumption and do not present drawbacks associated with label specificities. The most 336
current grazing experiment method is based on the disappearance of prey cells over time 337
(Frost, 1972). This method is reliable for planktonic organisms (Campbell et al., 2005; Haley 338
et al., 2011; Turner, 2014) but presents limitations for benthic organisms. When microalgae 339
are present in low concentrations, the accuracy of cell counting decreases (Campbell et al., 340
2005; Haley et al., 2011) which leads to an overestimation of ingestion rates. Moreover, 341
benthic dinoflagellates can form agglomerates, which may sink and attach to the surface of 342
the cultivation flasks, thereby increasing this bias. Methods used to detach benthic cells can 343
damage cell integrity and introduce a bias in interpretations. Indeed, an empty theca can be 344
considered as a consumed cell with an ingested cytoplasmic content or as a cell which is not 345
grazed.
346
Grazing experiments can also be performed using pre-labeled prey. In this case, 347
dinoflagellates can be labeled using radioactive isotopes (Lampert and Taylor, 1985; Napp 348
and Long, 1989; White and Roman, 1991) but with the inconvenience to present legal 349
restrictions. Compared to radioactive isotopes, using stable isotopes to enrich dinoflagellates 350
is more appropriate especially for investigators who are limited by radioactive material 351
regulations. The method using pre-labeled dinoflagellates enriched with stable isotopes was 352
consequently chosen in this present study and controls were conducted to assess its efficiency.
353
For instance, passive adhesion of labeled dinoflagellates on the cuticle of copepods (Figure 354
1.b) could overestimate the ingestion of dinoflagellates. However, controls using dead 355
copepods revealed a limited bias due to such events. Caged copepods were not able to ingest 356
benthic dinoflagellates but could consume their soluble secretions through cage meshes.
357
Results showed a limited labeling for caged copepods which highlighted that the transfer of 358
dinoflagellate compounds through the dissolved form was limited (Figure 1.c). Even if 150 359
specimens were pooled for each sample, measured ingestion rates presented a high variability 360
potentially due to different feeding behaviors between specimens.
361
Ingestion rate 362
Copepod ingestion of toxic planktonic dinoflagellates have been previously measured 363
for genera such as Alexandrium (Lasley-Rasher et al., 2016; Sopanen et al., 2011; Teegarden 364
and Cembella, 1996), Karenia (Prince et al., 2006; Schultz and Kiørboe, 2009; Walsh and 365
O’ , 2 4), and Gymnodinium (da Costa et al., 2012; Koski et al., 1998; Paffenhöfer, 366
1971). These studies revealed that different factors can lead to difficulties in interpretation 367
when describing interactions between grazers and cell prey. Furthermore, different predators 368
and various clonal cultures of dinoflagellates which are often used during the feeding assay 369
experiments make the comparison between studies problematic (Teegarden and Cembella, 370
1996). The interaction between grazers and cell prey can be highly specific (Teegarden, 1999;
371
Teegarden and Cembella, 1996) and even site-specific when a single grazer is considered 372
(Teegarden and Cembella, 1996; Uye and Takamatsu, 1990). Indeed, within a species, trophic 373
interactions can be population-specific and linked with population history (Colin and Dam, 374
2002). For instance, a same species of grazer, present at two different geographic sites, and 375
fed with the same toxic dinoflagellate species, can present different ingestion rates, which 376
suggests that historical exposure of grazers can impact the ingestion (Colin and Dam, 2003, 377
2002). There are also ontogenetic considerations, such as different developmental stages (e.g.
378
nauplii vs. copepodides vs adult copepods) which certainly display diet differences in the type 379
and amount of preyed items.
380
Trophic interactions between dinoflagellates and the meiobenthos are less investigated 381
than with the macrobenthos and zooplankton. Harpacticoid copepods can graze on different 382
food items due to the large diversity of microalgal species present in the microphytobenthos 383
(Azovsky et al., 2013) however, they are usually selective in their ingestion (Azovsky et al., 384
2005; Buffan-Dubau et al., 1996). Indeed, in temperate regions, copepods were found to feed 385
on a broad diversity of diatoms (Cnudde et al., 2011; Decho, 1986; Rzeznik-Orignac and 386
Fichet, 2012; Wyckmans et al., 2007) and bacteria (Cnudde et al., 2013, 2011; Pascal et al., 387
2013, 2009).
388
In tropical areas, potentially toxic dinoflagellates can be a major component of the 389
microphytobenthic communities (MacIntyre et al., 1996). To our knowledge, the ingestion 390
rate of potentially toxic dinoflagellates by meiofauna has never been measured in benthic 391
environments.
392
The present study indicates that harpacticoid copepods can feed on potentially toxic 393
benthic dinoflagellates even when another food source is available. Indeed, ingestion rates of 394
dinoflagellates were not affected when adding diatoms as a food resource, which suggests that 395
dinoflagellates are i) part of the regular copepod diet and ii) are not neglected when another 396
food resource is available. Similar ingestion rates were found when copepods were fed with 397
Ostreopsis sp. and with Amphidinium sp. even though Ostreopsis cells have a cellulosic theca 398
(Schmidt, 1901) whilst Amphidinium cells have none (Fensome et al., 1993). This result 399
suggests that rigid cellulosic walls can be easily broken down by copepods in order to ingest 400
the cytoplasmic content. Moreover, the high 15N variability found in the present study when 401
applying feeding conditions d) and e) could be due to fluctuations of the body size explained 402
by i) sexual dimorphism in harpacticoid copepods where the females are larger than the males 403
and/or ii) different shapes of the mouthpart which determine food ingestion (Giere, 2009).
404
Observations showed that the body size of copepods could influence the ingestion rate of 405
planktonic copepods (Turner and Tester, 1989). Indeed, the body size is an important factor 406
for many physiological processes (Peters, 1985) since in general terms, maximal ingestion 407
rates are inversely correlated to the body size (Moloney and Field, 1989).
408
Ingestion role in regulation of blooms 409
Predation of dinoflagellates by meiofauna have received only little attention and this 410
mainly due to technical constraints associated to the quantification procedure (Danovaro et 411
al., 2007). Blooms of benthic toxic dinoflagellates are currently increasing worldwide 412
(Hallegraeff, 1993; Van Dolah, 2000; Cohu et al., 2011) and the position of the meiofauna 413
within these events needs further clarification mainly regarding the regulation of toxic 414
dinoflagellate abundances through ingestion and transfer of toxins in the food web. In the 415
global context of climate change now experienced by the oceans, quantification of predation 416
rates might be useful to understand the ecological role of benthic toxic dinoflagellates and to 417
forecast how these relationships will evolve.
418
Algal blooms are possible only if algal growth exceeds the loss by predation and 419
senescence (Buskey et al., 1997). The role of predation must hence be further assessed in 420
order to better describe the regulation of benthic toxic dinoflagellate blooms. Population 421
blooms of benthic dinoflagellates may be controlled by meiobenthic organisms in a 422
comparable way to those occurring in planktonic environments. Model simulations have 423
already shown that during the first stages of pelagic dinoflagellate blooms, growth can be 424
retarded or inhibited with a low pressure of predation by copepods since each grazed cell is, 425
in proportion, an important loss when considering such a small local population (Haley et al., 426
2011). However, the impact of low predation was minimal when simulations were carried out 427
with higher micro-algae abundances, above 100 cells L—1 (Haley et al., 2011), which 428
highlights the fact that predation pressure has an important role in bloom dynamics, especially 429
in the early development stages. Some dinoflagellates are able produce cysts (Anderson, 430
1998; Faust, 1992; Tian et al., 2017) which make them sink down to the seabed, thereby being 431
less available for planktonic copepods (Butman et al., 2014; Dale et al., 1978; Mohamed and 432
Al-Shehri, 2011). It has been suggested that a decrease in the abundance of dinoflagellates 433
due to grazing is relatively minor compared to the increase of abundance resulting from a 434
gradual release of germinated cells from the benthic cyst beds (Anderson et al., 2005).
435
Planktonic dinoflagellates are ingested by copepods even at low concentrations (Haley 436
et al., 2011). Therefore, benthic toxic dinoflagellates, which are found all year round in 437
Guadeloupe and Martinique at viable densities (Boisnoir et al., 2019), could consequently be 438
grazed and assimilated by copepods on a permanent basis. Meiofauna represent a key 439
component of coastal benthos, since over 75 % of the total meiofauna production is 440
transferred to higher trophic levels through predation (Danovaro et al., 2007), especially due 441
to macrofaunal and other epibenthic predators (Chardy and Dauvin, 1992). Harpacticoid 442
copepods ingesting toxic benthic cells could bioaccumulate the toxins synthesized by the 443
benthic dinoflagellates and contaminate secondary consumers when they are consumed.
444
Furthermore, the toxicity of dinoflagellate cysts has been shown to be higher in benthic cells 445
than in planktonic cells (Dale et al., 1978) which can expose meiobenthic grazers to a more 446
important toxic risk, able to induce mass mortalities and promote the development of 447
dinoflagellate blooms. Natural marine toxins are a considerable increasing threat when 448
bioaccumulation takes place within the food chain (Ramos and Vasconcelos, 2010). Indeed, 449
the ingestion of benthic toxic dinoflagellates is a way of introducing phycotoxins in the food 450
web, and to our knowledge, this transfer has been rarely considered. The role of the 451
meiofauna in this transfer of toxins has been considered as minor, however, more interest 452
should be given to estimate the bio-magnification potential through specific models since the 453
amplification effect of such toxins is liable to contribute to the emergence of diseases related 454
to toxic benthic dinoflagellates.
455 456
Acknowledgments 457
Th s s u y w s fu by h ‘C c é T M qu ’ and by the French 458
ANR project OCEAN 15 (2016-2020). We thank the molecular biology platform of Institut de 459
la Mer de Villefranche (IMEV), 181 Chemin du lazaret, 06230 Villefranche sur Mer, France.
460
The authors warmly thank Pr. Olivier Gros of “Université des Antilles” for his valuable 461
advices in molecular biology. Au h s p f h ch s ch G up “GD 462
HYCOTOX” We are grateful for Monique Ras for English corrections.
463 464
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