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.

Interactions among microscopic planktonic organisms underpin the functioning of open ocean ecosystems. With few exceptions, these organisms lack advanced eyes and thus rely largely on chemical sensing to perceive their surroundings. However, few of the signaling molecules involved in interactions among marine plankton have been identified. We report a group of eight small molecules released by copepods, the most abundant zooplankton in the sea, which play a central role in food webs and biogeochemical cycles. The compounds, named copepodamides, are polar lipids connecting taurine via an amide to isoprenoid fatty acid conjugate of varying composition. The bloom-forming dinoflagellate Alexandrium minutum responds to pico- to nanomolar concentrations of copepodamides with up to a 20-fold increase in production of paralytic shellfish toxins. Different copepod species exude distinct copepodamide blends that contribute to the species-specific defensive responses observed in phytoplankton. The signaling system described here has far reaching implications for marine ecosystems by redirecting grazing pressure and facilitating the formation of large scale harmful algal blooms.

Pore water pH distributions are closely coupled to early diagenetic reactions and transport processes in surficial sediments. In this study, an optical plate fluorosensor for rapid two-dimensional detection of H ϩ concentration patterns in sediment pore waters was developed. The dual excitation (405/450 nm) single emission (520 nm) pH fluorophore HPTS (8-hydroxypyrene 1, 3, 6, trisulfonic acid trisodium salt) was immobilized onto transparent, supporting sensor foils. Excitation/emission spectra of immobilized HPTS exhibited pH dependent shifts in seawater standards, and fluorescence excitation ratios were used for two-wavelength, ratiometric detection of pH distributions in sediment and bottom water close to the sediment-water interface (pH ഠ 6.5 to 8.5). Sensor foils were fixed to the inner sides of plastic box corers or glass plate aquaria containing sediment and overlying seawater. Equilibration with pore water occurred in seconds, and fluorescence response (excitation/emission) was scanned nondestructively using a monochrome, integrating CCD camera. The two-dimensional sensor system has a vertical and horizontal resolution of 56 ϫ 54 m (5 ϫ 5 pixels) applied over an image of 34 ϫ 26 mm. Reproducibility and accuracy were best when foils were prepared from solutions of similar ionic strength to samples. Optical sensor pH distributions correlated well with high-resolution, vertical distributions measured simultaneously with a minicombination pH electrode, and two-dimensional pH patterns demonstrated directly the transport-reaction heterogeneity associated with microtopography and biogenic structures in surface deposits and overlying waters close to the sediment-water interface. Sensor foils retained calibration characteristics for at least 50 d.

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.

Chain formation is common among phytoplankton organisms but the underlying reasons and consequences are poorly understood. Here we show that chain formation is strongly impaired by waterborne cues from copepod grazers in the dinoflagellate Alexandrium tamarense. Chains of Alexandrium cells exposed to copepod cues responded by splitting into single cells or shorter chains. Motion analysis revealed significantly lower swimming velocities for single cells compared with chains, with two-to fivefold higher simulated predator encounter rates for two-and four-cell chains, respectively. In addition, the few remaining two-cell chains in grazed treatments were swimming at approximately half the speed of two-cell chains in treatments without grazers, which reduced encounter rates with grazers to values similar to that of single cells. Chain length plasticity and swimming behavior constitute unique mechanisms to reduce encounters with grazers. We argue that dinoflagellates can regulate the balance between motility and predator avoidance by adjusting chain length. The high predator encounter rate for motile chains may have contributed to the low prevalence of chain formation in motile phytoplankton compared with in nonmotile phytoplankton where chain formation is more common.inducible defense | chemical ecology | hydrodynamics | plankton ecology O ceanic primary producers contribute ∼50% of the global carbon dioxide fixation and have profound effects on biogeochemical cycles (1), yet our understanding of their functional morphology is still rudimentary. As an example, chain formation is common mainly among nonmotile groups of marine phytoplankton, e.g., diatoms and cyanobacteria, but less common in motile groups like dinoflagellates. The ultimate reasons for chain formation are poorly understood and several alternative explanations have been proposed (2). Chains and colonies have been suggested to provide lower sinking rates, allowing nonmotile phytoplankters to remain in surface waters, although this result has little theoretical and experimental support (3). Size changes dramatically with chain and colony formation, allowing both motile and nonmotile phytoplankton organisms to enter size-limited grazer refuges. For example, a large Phaeocystis colony has a diameter of >10 2 times the diameter of a single cell. With the exception of parasites and pathogens, pelagic consumers are typically not able to feed on such a large size range of prey (4). Thus, it is likely that size selective grazing contributed to the evolution of size and colony formation in phytoplankton organisms (5). This relationship is further supported by the ability of Phaeocystis globosa and Scenedesmus subspicatus to sense and respond to grazer presence by forming colonies larger than the capture size of the inducing grazer (6, 7) or by breaking up colonies into sizes too small to be retained (8). Moreover, chain length correlates to growth rate in some diatoms and dinoflagellates (9, 10), suggesting that chain length may also depend on growth conditions. ...