Among the thousands of unicellular phytoplankton species described in the sea, some frequently occurring and bloom-forming marine dinoflagellates are known to produce the potent neurotoxins causing paralytic shellfish poisoning. The natural function of these toxins is not clear, although they have been hypothesized to act as a chemical defence towards grazers. Here, we show that waterborne cues from the copepod Acartia tonsa induce paralytic shellfish toxin (PST) production in the harmful algal bloom-forming dinoflagellate Alexandrium minutum. Induced A. minutum contained up to 2.5 times more toxins than controls and was more resistant to further copepod grazing. Ingestion of non-toxic alternative prey was not affected by the presence of induced A. minutum. The ability of A. minutum to sense and respond to the presence of grazers by increased PST production and increased resistance to grazing may facilitate the formation of harmful algal blooms in the sea.
Ocean acidification (OA) caused by anthropogenic CO2 emission is projected for thousands of years to come, and significant effects are predicted for many marine organisms. While significant evolutionary responses are expected during such persistent environmental change, most studies consider only short-term effects. Little is known about the transgenerational effects of parental environments or natural selection on the capacity of populations to counter detrimental OA effects. In this study, six laboratory populations of the calanoid copepod Pseudocalanus acuspes were established at three different CO2 partial pressures (pCO2 of 400, 900 and 1550 μatm) and grown for two generations at these conditions. Our results show evidence of alleviation of OA effects as a result of transgenerational effects in P. acuspes. Second generation adults showed a 29% decrease in fecundity at 900 μatm CO2 compared to 400 μatm CO2 . This was accompanied by a 10% increase in metabolic rate indicative of metabolic stress. Reciprocal transplant tests demonstrated that this effect was reversible and the expression of phenotypic plasticity. Furthermore, these tests showed that at a pCO2 exceeding the natural range experienced by P. acuspes (1550 μatm), fecundity would have decreased by as much as 67% compared to at 400 μatm CO2 as a result of this plasticity. However, transgenerational effects partly reduced OA effects so that the loss of fecundity remained at a level comparable to that at 900 μatm CO2 . This also relieved the copepods from metabolic stress, and respiration rates were lower than at 900 μatm CO2 . These results highlight the importance of tests for transgenerational effects to avoid overestimation of the effects of OA.
Using 14 C-labeled phytoplankton as tracer, we investigated 2 mechanisms of immediate dissolved organic carbon (DOC) release during grazing activity of Calanus spp. -sloppy feeding and leakage from newly expelled fecal pellets. Half of the carbon cleared by Calanus spp. was released as DOC through sloppy feeding. Freshly expelled fecal pellets lost more than 20% of their carbon content within the first hour, corresponding to 6% of the carbon cleared. Thus, copepods should not only be considered as an essential link to higher trophic levels, but also as a feedback link to the microbial food web.KEY WORDS: Dissolved organic carbon · DOC · Sloppy feeding · Fecal pellets · Calanus spp. Resale or republication not permitted without written consent of the publisher
We show that Skeletonema marinoi suppresses chain formation in response to copepod cues. The presence of three different copepod species (Acartia tonsa, Centropages hamatus, or Temora longicornis) significantly reduced chain length. Furthermore, chain length was significantly reduced when S. marinoi was exposed to chemical cues from caged A. tonsa without physical contact with the responding cells. The reductions in chain length significantly reduced copepod grazing; grazing rates on chains (four cells or more) were several times higher compared to that of single cells. This suggests that chain length plasticity is a means for S. marinoi to reduce copepod grazing. In contrast, chain length was not suppressed in cultures exposed to the microzooplankton grazer Gyrodinium dominans. Size-selective predation may have played a key role in the evolution of chain formation and chain length plasticity in diatoms.
Ocean acidification is expected to have dramatic impacts on oceanic ecosystems, yet surprisingly few studies currently examine long‐term adaptive and plastic responses of marine invertebrates to pCO 2 stress. Here, we exposed populations of the common copepod Pseudocalanus acuspes to three pCO 2 regimes (400, 900, and 1550 μatm) for two generations, after which we conducted a reciprocal transplant experiment. A de novo transcriptome was assembled, annotated, and gene expression data revealed that genes involved in RNA transcription were strongly down‐regulated in populations with long‐term exposure to a high pCO 2 environment, even after transplantation back to control levels. In addition, 747 000 SNPs were identified, out of which 1513 showed consistent changes in nucleotide frequency between replicates of control and high pCO 2 populations. Functions involving RNA transcription and ribosomal function, as well as ion transport and oxidative phosphorylation, were highly overrepresented. We thus conclude that pCO 2 stress appears to impose selection in copepods on RNA synthesis and translation, possibly modulated by helicase expression. Using a physiological hypothesis‐testing strategy to mine gene expression data, we herein increase the power to detect cellular targets of ocean acidification. This novel approach seems promising for future studies of effects of environmental changes in ecologically important nonmodel organisms.
We assessed metabolic balance, RNA content, and egg hatching success (EHS) in Acartia tonsa and A. clausi over a wide salinity range (2 to 33 and 16 to 33, respectively). For A. tonsa, the energy partitioning between ingestion, production and respiration was relatively constant with small differences in gross growth efficiency (GGE) and cost of growth (CG). In contrast, A. clausi exhibited significantly reduced ingestion and GGE, and highly elevated CG at salinities ≤20. In both species, RNA levels mirrored egg production. EHS was generally high in both species, but decreased by 80% for A. clausi at 16. These results contribute to the understanding of distribution patterns of both species along salinity gradients. The observed responses would allow the dominance of A. tonsa at low salinities, although its higher energetic requirement and feeding activity subject it to stronger predation pressure than competing A. clausi.
Fecal pellets were produced by Acartia tonsa fed 14 C-labeled diatom, cryptophyte, and dinoflagellate diets, and were incubated in 1.2 µm-filtered Long Island Sound seawater. Based on the 14 C label, the decrease in fpOC (fecal pellet organic carbon), the release and fate of dissolved organic carbon (DOC) and particulate organic carbon (POC), as well as bacterial production and enzymatic activity, were followed over a 96 h period. fpOC decreased by 9, 14, and 19% d -1 in diatom, cryptophyte, and dinoflagellate pellets, respectively. There was a fast, possibly passive, leakage of DOC from pellets from all 3 diets within a few hours after egestion, which may not have been utilized by attached bacteria. Bacterial production rates were 17, 12, and 31 pg C pellet -1 h -1, on diatom, cryptophyte, and dinoflagellate pellets, respectively. These were 5 orders of magnitude higher than production rates of free-living bacteria, indicating that copepod fecal pellets are hot spots of pelagic microbial production. The high production was caused primarily by high initial bacterial abundances. Accordingly, production and growth were entirely uncoupled in diatom pellets. There were no increases in abundance of attached bacteria on any of the 3 diets, indicating that the produced bacterial cells were released from the fecal pellets. Attached bacteria had a higher ectoenzymatic activity than free-living bacteria, but their production and ectoenzymatic activity were uncoupled and they only assimilated a minor fraction of the released DOC. DOC was therefore released favoring free-living microbes. The chitinase activity, which increased several-fold, was coupled to the production of attached bacteria; thus, chitin may play an important role in bacterial production on copepod fecal pellets.KEY WORDS: Copepod fecal pellets · Fecal pellet decomposition · Pelagic DOC flux · Pelagic POC flux · Attached bacterial production · Ectoenzymatic activity Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 33: [279][280][281][282][283][284][285][286][287][288] 2003 cycled (Wotton & Malmqvist 2001, Turner 2002 and that most carbon originating from these fecal pellets remains part of the long-lived organic carbon in the epipelagic (Legendre & Michaud 1998). Hence, the role of fecal pellets from small copepods is that of supplying nutrients to the epipelagic planktonic microbial community rather than maintaining the vertical flux of organic matter.Copepod fecal pellets host an extensive flora of attached bacteria (Gowing & Silver 1983, Bianchi et al. 1992, Delille & Razouls 1994, Hansen & Bech 1996. Breakdown of the fecal pellets is in part governed by microbial decomposition driven by the hydrolytic activity of these bacteria. The potential hydrolytic activity is generally high in pelagic aggregates of organic matter (Karner & Herndl 1992, Smith et al. 1992, and if this potential is fully exploited in fecal pellets, then decomposition and hence recycling of organic matter may occur rapidly....
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