Phytoplankton cell lysis associated with polyunsaturated aldehyde release in the Northern Adriatic Sea

Diatoms are able to react to biotic and abiotic stress, such as competition, predation and unfavorable growth conditions, by producing bioactive compounds including polyunsaturated aldehydes (PUAs). PUAs have been shown to act against grazers and either enhance or inhibit the growth of different phytoplankton and bacteria both in culture and in the field. Presence of nanomolar concentrations of dissolved PUAs in seawater has been reported in the North Adriatic Sea (Mediterranean), suggesting that these compounds are released in seawater following diatom cell lysis. However, the origin of the PUAs and their effects on natural phytoplankton assemblages remain unclear. Here we present data from four oceanographic cruises that took place during diatom blooms in the northern Adriatic Sea where concentrations of particulate and dissolved PUAs were monitored along with phytoplankton cell lysis. Cell lysis was positively correlated with both concentrations of particulate and dissolved PUAs (R = 0.69 and R = 0.77, respectively), supporting the hypothesis that these compounds are released by cell lysis. However, the highest concentration of dissolved PUAs (2.53 nM) was measured when cell lysis was high (0.24 d(-1)) but no known PUA-producing diatoms were detected, suggesting either that other organisms can produce PUAs or that PUA-producing enzymes retain activity extracellularly after diatom cells have lysed. Although in situ concentrations of dissolved PUAs were one to three orders of magnitude lower than those typically used in laboratory culture experiments, we argue that concentrations produced in the field could induce similar effects to those observed in culture and therefore may help shape plankton community composition and function in the oceans.

Effects of grazer presence on genetic structure of a phenotypically diverse diatom population

Studies of predator-prey systems in both aquatic and terrestrial environments have shown that grazers structure the intraspecific diversity of prey species, given that the prey populations are phenotypically variable. Populations of phytoplankton have traditionally considered comprising only low intraspecific variation, hence selective grazing as a potentially structuring factor of both genetic and phenotypic diversity has not been comprehensively studied. In this study, we compared strain specific growth rates, production of polyunsaturated aldehydes, and chain length of the marine diatom Skeletonema marinoi in both grazer and non-grazer conditions by conducting monoclonal experiments. Additionally, a mesocosm experiment was performed with multiclonal experimental S. marinoi populations exposed to grazers at different levels of copepod concentration to test effects of grazer presence on diatom diversity in close to natural conditions. Our results show that distinct genotypes of a geographically restricted population exhibit variable phenotypic traits relevant to grazing interactions such as chain length and growth rates. Grazer presence affected clonal richness and evenness of multiclonal Skeletonema populations in the mesocosms, likely in conjunction with intrinsic interactions among the diatom strains. Only the production of polyunsaturated aldehydes was not affected by grazer presence. Our findings suggest that grazing can be an important factor structuring diatom population diversity in the sea and emphasize the importance of considering clonal differences when characterizing species and their role in nature.

Total synthesis and detection of the bilirubin oxidation product (Z)-2-(3-ethenyl-4-methyl-5-oxo-1,5-dihydro-2H-pyrrol-2-ylidene)ethanamide (Z-BOX A)

The selective total synthesis of the pure Z-isomer of BOX A (8a), a product of oxidative heme degradation with significant physiological impact, was achieved in four to six steps starting from 3-bromo-4-methylfuran-2,5-dione (1). Z-BOX A forms a strong hydrogen bridge framework in the crystalline state. LC-MS techniques allow identification and characterization of isomeric forms of BOX A.

Domoic acid improves the competitive ability of Pseudo-nitzschia delicatissima against the diatom Skeletonema marinoi

Because domoic acid, a neurotoxic secondary metabolite produced by marine diatoms in the genus Pseudo-nitzschia, is hypothesized to be part of a high affinity iron uptake system, we investigated whether domoic acid could improve the competitive ability of Pseudo-nitzschia delicatissima, and whether the availability of iron changed the outcome of competition experiments. We found that domoic acid had a slight negative effect on growth of the diatom Skeletonema marinoi when it was grown in monocultures. However, when S. marinoi was cultured with P. delicatissima the presence of domoic acid resulted in a reduction of S. marinoi cells by up to 38% and an increase in P. delicatissima cell numbers by up to 17% under iron replete conditions. Similar effects were not observed in low iron treatments. Domoic acid was not taken up by P. delicatissima cells. Overall, our results indicate that domoic acid can improve the competitive ability of Pseudo-nitzschia spp. and that iron is likely to be involved. This study provides an unusual example of indirect inhibition of competitor growth mediated by a secondary metabolite.

Synchronized regulation of different zwitterionic metabolites in the osmoadaption of phytoplankton

The ability to adapt to different seawater salinities is essential for cosmopolitan marine phytoplankton living in very diverse habitats. In this study, we examined the role of small zwitterionic metabolites in the osmoadaption of two common microalgae species Emiliania huxleyi and Prorocentrum minimum. By cultivation of the algae under salinities between 16‰ and 38‰ and subsequent analysis of dimethylsulfoniopropionate (DMSP), glycine betaine (GBT), gonyol, homarine, trigonelline, dimethylsulfonioacetate, trimethylammonium propionate, and trimethylammonium butyrate using HPLC-MS, we could reveal two fundamentally different osmoadaption mechanisms. While E. huxleyi responded with cell size reduction and a nearly constant ratio between the major metabolites DMSP, GBT and homarine to increasing salinity, osmolyte composition of P. minimum changed dramatically. In this alga DMSP concentration remained nearly constant at 18.6 mM between 20‰ and 32‰ but the amount of GBT and dimethylsulfonioacetate increased from 4% to 30% of total investigated osmolytes. Direct quantification of zwitterionic metabolites via LC-MS is a powerful tool to unravel the complex osmoadaption and regulation mechanisms of marine phytoplankton.

Induction of protease release of the resistant diatom Chaetoceros didymus in response to lytic enzymes from an algicidal bacterium

Marine lytic bacteria can have a substantial effect on phytoplankton and are even capable to terminate blooms of microalgae. The bacterium Kordia algicida was reported to lyse cells of the diatom Skeletonema costatum and several other diatoms by a quorum sensing controlled excretion of proteases. However the diatom Chaetoceros didymus is fully resistant against the bacterial enzymes. We show that the growth curve of this diatom is essentially unaffected by addition of bacterial filtrates that are active against other diatoms. By monitoring proteases from the medium using zymography and fluorescence based activity assays we demonstrate that C. didymus responds to the presence of the lytic bacteria with the induced production of algal proteases. These proteases exhibit a substantially increased activity compared to the bacterial counterparts. The induction is also triggered by signals in the supernatant of a K. algicida culture. Size fractionation shows that only the >30 kD fraction of the bacterial exudates acts as an inducing cue. Implications for a potential induced defense of the diatom C. didymus are discussed.

Laser desorption/ionization mediated by bionanostructures from microalgae

RATIONALE

Organic matrices are the state-of-the-art ionization mediators in Laser Desorption/Ionization Mass Spectrometry (LDI-MS). Despite improvements in understanding matrix chemistry, interfering matrix-related signals complicate the analysis. Surface-assisted LDI techniques like desorption/ionization on silicon (DIOS) or nanostructure initiator mass spectrometry (NIMS) provide promising alternatives but rely often on elaborate materials.

METHODS

We introduce nanopatterned biomineralized cell walls of microalgae as easily accessible biological surfaces that support the ionization of embedded molecules in LDI-MS. Microalgae cell walls were cleaned through oxidation and washing before pipetting on a stainless-steel matrix-assisted laser desorption/ionization (MALDI) target. Added molecules were efficiently ionized in positive and negative ionization mode in common MALDI sources. The method was rigorously validated by comparison with established MALDI experiments.

RESULTS

Ionization of PEG600, D-sphingosine and raffinose was successfully mediated by nanostructured cell wall preparations from two different microalgae. Without any change in protocol, steric acid could be detected in the negative ionization mode. Ionization is also supported by commercially available celite, a material containing mineralized diatom cell walls. Characteristic ingredients of fresh coffee were detected in LDI-MS after pipetting it on celite without further sample preparation. Caffeine and saccharose were detected in positive and characteristic fatty acids in negative ionization mode. Detection limits were comparable to established MALDI experiments.

CONCLUSIONS

Bionanostructure-enhanced ionization allows the analysis of a diverse selection of analytes including polymers, sugars, amino alcohols, and organic acids without interfering matrix signals. We also show that celite, a commercially available porous material containing mineralized algal bionanostructures, supports LDI-MS.

Conserved and species-specific oxylipin pathways in the wound-activated chemical defense of the noninvasive red alga Gracilaria chilensis and the invasive Gracilaria vermiculophylla

Chemical defense of the invasive red alga Gracilaria vermiculophylla has been studied and compared to that of the noninvasive but related Gracilaria chilensis. Both species rely on a wound-activated chemical defense that makes them less attractive to the herbivorous sea snail Echinolittorina peruviana. The chemical stress response of both species was monitored by LC–ESIMS-based metabolic profiling and revealed commonalities and differences. Both algae rely on a rapid lipoxygenase mediated transformation of arachidonic acid to known and novel oxylipins. Common products are 7,8-dihydroxyeicosatetraenoic acid and a novel eicosanoid with an unusual γ-lactone moiety. Several prostaglandins were predominantly formed by the invasive species. The role of some of these metabolites was investigated by surveying the attachment of E. peruviana on artificial food containing the respective oxylipins. Both algae species are defended against this general herbivore by 7,8-dihydroxyeicosatetraenoic acid, whereas the prostaglandins and the novel oxylipins were inactive at naturally occurring concentrations. The role of different oxylipins in the invasive potential of Gracilaria spp. is discussed.

Seaweeds early development: detrimental effects of desiccation and attenuation by algal extracts

The effects of desiccation on the early development stages of Mazzaella laminarioides, Scytosiphon lomentaria and Lessonia nigrescens, algal species with different patterns of distribution across the intertidal zone, were examined in the laboratory. In addition, the protective effect against desiccation was evaluated using algal extracts, including those from Porphyra columbina, a macroalga tolerant to desiccation that lives in the uppermost part of the intertidal zone. Our results showed that M. laminarioides displayed the highest resistance to daily desiccation, followed by S. lomentaria, whereas L. nigrescens was the most susceptible. Spores from L. nigrescens exposed to desiccation, although being able to germinate, ceased further post-germination development. In addition, our results showed that all species exposed to extracts from desiccated P. columbina successfully completed their development and strongly suggest the occurrence of compounds with protective properties that help in attenuating the stress caused by desiccation. Finally, our results indicate that the magnitude of the effects generated by desiccation on the early algal development is related to the position of the species in the intertidal zone, and that the protective effects of P. columbina extracts reveal an exceptional metabolism of this species under desiccation stress.

Influence of temperature and elevated carbon dioxide on the production of dimethylsulfoniopropionate and glycine betaine by marine phytoplankton

The sulfur metabolite dimethylsulfoniopropionate (DMSP) is the most important precursor of the climate relevant metabolite dimethylsulfide (DMS). It has thus gained interest in the context of climate change and several studies investigated the influence of elevated temperature and/or CO(2) on DMSP in complex plankton communities. However, only little information about changes in response to these factors in single species is available. Therefore, we analyzed DMSP in different phytoplankton cultures (Thalassiosira pseudonana, Phaeodactylum tricornutum, Emiliania huxleyi) under the influence of increased temperature by 6 °C and elevated CO(2) to 790 ppmv. In addition, we addressed glycine betaine (GBT) that fulfills a similar function as osmolyte like DMSP. In all cultures GBT concentrations increased at higher temperature and decreased at elevated CO(2). In contrast, diatoms and prymnesiophytes revealed opposite trends for DMSP. In diatoms increased CO(2) and temperature led to decreased DMSP concentrations, while rather elevated levels of this metabolite under the influence of these parameters were observed for E. huxleyi.

Dimethylsulphopropionate (DMSP) and proline from the surface of the brown alga Fucus vesiculosus inhibit bacterial attachment

It was demonstrated previously that polar and non-polar surface extracts of the brown alga Fucus vesiculosus collected during winter from the Kiel Bight (Germany) inhibited bacterial attachment at natural concentrations. The present study describes the bioassay-guided identification of the active metabolites from the polar fraction. Chromatographic separation on a size-exclusion liquid chromatography column and bioassays identified an active fraction that was further investigated using nuclear magnetic resonance spectroscopy and mass spectrometry. This fraction contained the metabolites dimethylsulphopropionate (DMSP), proline and alanine. DMSP and proline caused the anti-attachment activity. The metabolites were further quantified on the algal surface together with its associated boundary layer. DMSP and proline were detected in the range 0.12-1.08 ng cm(-2) and 0.09-0.59 ng cm(-2), respectively. These metabolites were tested in the concentration range from 0.1 to 1000 ng cm(-2) against the attachment of five bacterial strains isolated from algae and sediment co-occurring with F. vesiculosus. The surface concentrations for 50% inhibition of attachment of these strains were always <0.38 ng cm(-2) for DMSP and in four cases <0.1 ng cm(-2) for proline, while one strain required 1.66 ng cm(-2) of proline for 50% inhibition. Two further bacterial strains that had been directly isolated from F. vesiculosus were also tested, but proved to be the least sensitive. This study shows that DMSP and proline have an ecologically relevant role as surface inhibitors against bacterial attachment on F. vesiculosus.

Disruption-free imaging by Raman spectroscopy reveals a chemical sphere with antifouling metabolites around macroalgae

Investigations of the surface chemistry of marine organisms are essential to understand their chemically mediated interactions with fouling organisms. In this context, the concentration of natural products in the immediate vicinity of algal surfaces, as well as their biological activity, are of particular importance. However, due to lack of appropriate methods, the distribution of compounds within the chemical sphere around marine algae is unknown. This study demonstrates the suitability of confocal resonance Raman microspectroscopy for the determination of metabolites around algal surfaces with a micrometer resolution. The spatial distribution of carotenoids in the diffusion boundary layer of the brown alga Fucus vesiculosus and the green alga Ulva sp. was determined using the disruption-free optical method. A gradient of carotenoids was determined within 0 to 150 μm from the surface of the algae, thereby demonstrating the release of the non-polar metabolites involved in antifouling processes. The differences in the carotenoid composition of the brown and green algae were reflected in the spectra. Resonance Raman microspectroscopy also allowed visualization of the lateral distribution of fucoxanthin on the algal surface and localization of concentration maxima within a 50 × 50 μm(2) area. The results from this work show clearly that established dipping techniques suitable for the extraction of the diffusion boundary layer of macroalgae only provide an average of the local strongly variable concentrations of metabolites on algal surfaces.

Metabolomic assessment of induced and activated chemical defence in the invasive red alga Gracilaria vermiculophylla

In comparison with terrestrial plants the mechanistic knowledge of chemical defences is poor for marine macroalgae. This restricts our understanding in the chemically mediated interactions that take place between algae and other organisms. Technical advances such as metabolomics, however, enable new approaches towards the characterisation of the chemically mediated interactions of organisms with their environment. We address defence responses in the red alga Gracilaria vermiculophylla using mass spectrometry based metabolomics in combination with bioassays. Being invasive in the north Atlantic this alga is likely to possess chemical defences according to the prediction that well-defended exotics are most likely to become successful invaders in systems dominated by generalist grazers, such as marine macroalgal communities. We investigated the effect of intense herbivore feeding and simulated herbivory by mechanical wounding of the algae. Both processes led to similar changes in the metabolic profile. Feeding experiments with the generalist isopod grazer Idotea baltica showed that mechanical wounding caused a significant increase in grazer resistance. Structure elucidation of the metabolites of which some were up-regulated more than 100 times in the wounded tissue, revealed known and novel eicosanoids as major components. Among these were prostaglandins, hydroxylated fatty acids and arachidonic acid derived conjugated lactones. Bioassays with pure metabolites showed that these eicosanoids are part of the innate defence system of macroalgae, similarly to animal systems. In accordance with an induced defence mechanism application of extracts from wounded tissue caused a significant increase in grazer resistance and the up-regulation of other pathways than in the activated defence. Thus, this study suggests that G. vermiculophylla chemically deters herbivory by two lines of defence, a rapid wound-activated process followed by a slower inducible defence. By unravelling involved pathways using metabolomics this work contributes significantly to the understanding of activated and inducible defences for marine macroalgae.

Investigations of the uptake of dimethylsulfoniopropionate by phytoplankton

No change here: Analysis with doubly labeled [(13)C(2)D(6)]DMSP and LC/MS revealed that dissolved DMSP is taken up and stored intracellularly by diverse phytoplankton species without transformation. This is even true for species that produce no quantifiable amounts of DMSP themselves.