Influence of diatoms on copepod reproduction. II. Uncorrelated effects of diatom-derived α,β,γ,δ-unsaturated aldehydes and polyunsaturated fatty acids on Calanus helgolandicus in the field
Introduction
Zooplankton species hold a central position in marine food webs and a combination of predation and resource limitation typically regulates the population demography. Hereby it is well established that not only the food availability, but also the food quality of phytoplankton governs the success of copepods (Kiørboe et al., 1985, Jones and Flynn, 2005). Fatty acid composition, nutrient balances and metabolites such as amino acids, vitamins and sterols are particularly under discussion (Ederington et al., 1995, Jónasdóttir and Kiørboe, 1996, Jónasdóttir et al., 1998). The generally accepted view that diatoms, as the most dominant micro-phytoplankter during spring blooms, can supply such a high quality food for zooplankton and boost secondary production was challenged by several laboratory experiments (Poulet et al., 1994, Ban et al., 1997, Miralto et al., 1999). Attention has been focused on the role of copepod maternal diatom diets, which influence the development and survival of embryonic and early naupliar stages in the offspring (Poulet et al., 1994, Laabir et al., 1995a). It became evident that some diatom diets potentially reduce egg hatchability, but others promote high egg-hatching success (Ianora et al., 1995). Indeed, Ban and co-workers (1997) found varying effects of diatom-rich diets on copepod egg production and hatching rates, whereas the feeding copepod itself is unaffected. In the last decade this work has stimulated ecologists and chemists to search for the underlying mechanism. In laboratory experiments, Miralto and co-workers (1999) found a negative effect of diatom-derived α,β,γ,δ-unsaturated aldehydes (PUAs, e.g. 2,4-octadienal or 2,4,7-decatrienal) on copepod hatching success. Negative influence of PUAs on the hatching success was later confirmed in several laboratory experiments (e.g. d’Ippolito et al., 2002, Pohnert et al., 2002). These studies have stimulated a controversial discussion about the “Role of diatoms in copepod production” (Jónasdóttir et al., 1998) and questioned: “Are diatoms good or toxic for copepods?” (Ianora et al., 1999). Ianora and co-workers (2004) have shown that PUAs also impair the reproductive success of copepods by causing malformation of the hatched embryos. They concluded that food-derived PUAs might be responsible for the delayed peak in copepod biomass following a diatom spring bloom in the Adriatic Sea. Consequently, low grazing pressure during spring–summer blooms results in mass sinking of ungrazed cells. A third reproductive parameter, the egg production rate (EPR), is not affected by diatom-derived PUAs (Poulet et al., 2006).
PUAs are derived from oxidative transformation of polyunsaturated fatty acids (PUFAs) upon cell disruption supporting the concept of activated chemical defense in microalgae (see review by Pohnert, 2005). This wound-activated reaction is under the control of phospholipase A2 and galactolipase and can be initiated by mesozooplankton grazing resulting in the release of polyunsaturated fatty acids (Pohnert, 2002, d’Ippolito et al., 2004, Wichard et al., 2007).
Numerous laboratory studies were set up in order to get a deeper insight into diatom/copepod interactions. While some authors found a negative effect of diatom diets (Miralto et al., 1999, Ban et al., 2000, Ianora et al., 2004), others did not observe any adverse effect or negative relationship between occurrence of diatoms and hatching success (Jónasdóttir, 1994, Jónasdóttir et al., 1998, Irigoien et al., 2002). It was argued that poor egg production and viability results from nutritional inadequacy of mono-algal diets (Jónasdóttir and Kiørboe, 1996, Pond et al., 1996, Jones and Flynn, 2005) rather than from deleterious compounds like PUAs in diatom diets. But even if nutritional inadequacy or detrimental chemical factors in copepod diets negatively influence the reproductive response in laboratory experiments, deleterious effects of diatoms are often not properly evaluated in nature. Recently, a survey of the chemical defense potential of 51 diatom species has demonstrated that ca. 36% of the investigated species release elevated amounts of PUA in a wide concentration range of two orders of magnitude (Wichard et al., 2005a). Some of the controversial discussions on reproductive parameters might be explained by a species- and strain-specific biological activity versus production of PUAs (Pohnert et al., 2002, Wichard et al., 2005a).
Up to now the knowledge about the ecological relevance of PUAs is scarce, because field studies focusing on the relationship between diatom blooms and copepod reproductive success were not monitoring deleterious PUAs in phytoplankton diets. Moreover, most field investigations did not provide data for all aspects of copepod reproductive response (i.e. egg production, hatching and larval development). Ianora et al., 2004, Halsband-Lenk et al., 2005 have correlated the occurrence of diatoms, which are known as aldehyde producers, with the deformities of copepod nauplii without measuring the in situ PUA production by phytoplankton. Given the high variability of strain/species and even culture condition in relation to PUA production, these studies could only give indirect clues (Wichard et al., 2005a, Ribalet et al., 2007). It is evident that interdisciplinary efforts are therefore necessary to test the hypothesis of whether PUAs are the chemical causes that affect negatively hatchability and larval development rather than other parameters such as nutritional limitation (Paffenhöfer et al., 2005).
This study presents an integrative approach that covers both toxic and nutritional aspects of food biochemistry along with the reproductive response of Calanus helgolandicus during the phytoplankton succession in the well-mixed coastal waters off Roscoff (Western English Channel, France). We have monitored PUA content of the diets, nutritional parameters/indicators specified as polyunsaturated fatty acids, the DHA/EPA [22:6 ω3/20:5 ω3] and C/N ratios as well as environmental parameters (chlorophyll a, water-temperature). The work belongs to a series of extended field experiments from spring to fall in 2003 and 2004 (Poulet et al., 2006, Poulet et al., 2007a). We used a sensitive in situ derivatisation approach for determination and quantification of PUAs in phytoplankton (Wichard et al., 2005b). In parallel we studied the seasonal succession of phytoplankton and the impact of various naturally-occurring diatom diets on the hatching success and egg production rate in C. helgolandicus (Poulet et al., 2006).
Section snippets
Materials and methods
Three parallel sets of experiments form the basis of this research conducted from March/April to October in 2003/2004 (see schematic overview: Table 1). (1) Estimation of in situ hatching success (HS) and production of abnormal larvae (AL) in C. helgolandicus on 32 occasions (n = 32). (2) Investigation of the influence of NDAs and single algal diets of freshly isolated diatoms on the hatching success of C. helgolandicus. (3) Determination of the composition of the natural diatom assemblages
Diatom succession in 2003 and 2004
About 25 diatom species (with an abundance of >0.5% of total phytoplankton >11 μm) were observed in 2003 and 2004. The major bloom species include Thalassiosira rotula and Rhizosolenia setigera in spring and Guinardia sp. as well as Chaetoceros on a few occasions (Table 3A, Table 3B). Whereas the genera Chaetoceros, Thalassiosira and Rhizosolenia are well documented in the French coastal waters, Guinardia appears particularly in the coastal waters off Roscoff during the summer. We isolated the
Discussion
This study provides novel insights into the seasonal reproductive dynamics of C. helgolandicus along with in situ determination of diatom-derived α,β,γ,δ-unsaturated aldehydes (PUAs) and indicators of phytoplankton diet quantity and quality. In agreement with other studies in the English Channel (e.g. Devreker et al., 2005) no correlation of hatching success with temperature, Chl a, PON and POC was observed (Table 4). Therefore we set out an integrative approach of in situ chemical analyses and
Conclusion
Neither the survey of natural diatom assemblages during the phytoplankton succession nor laboratory experiments with isolated diatoms support the idea that PUA or any of the measured fatty acids affect the EPR, HS, or AL of C. helgolandicus in the coastal waters off Roscoff. In particular, laboratory experiments revealed that the strong PUA producer T. rotula, usable as a biomarker of PUA production, did not impair the reproductive response of C. helgolandicus. This spring blooming diatom does
Acknowledgements
We thank the sailors of the Roscoff Marine Station, in particular J.M. Roualec and R. Michel, for collecting samples at sea. This work was partly funded by CNRS, the French program ‘Biodiversité et Changement Global’, the Max Planck Society and EGIDE-PROCOPE exchange program. The authors (T.W., G.P.) acknowledge the financial funding of the Deutsche Forschungsgemeinschaft (PO628/3). Prof. W. Boland is acknowledged for helpful support and discussion during the preparation of the manuscript. We
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