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Many studies have demonstrated that some macroalgae (including the widely cultivated red alga Gracilaria lemaneiformis) can inhibit the growth of microalgae via allelopathy and resource competition, with the former as the major contributor. However, little is known currently about whether or not and how microalgae react as a feedback to the inhibitory effects or, more generally, the interactions between seaweed and microalgae in their co-culturing system. Here, we report a laboratory-based study on the interactions between the seaweed Gracilaria lemaneiformis and the common harmful algal blooms (HABs)-forming dinoflagellate Akashiwo sanguinea. We found that while both the fresh thalli of G. lemaneiformis and the extracts of fresh and dried G. lemaneiformis could significantly inhibit the growth of A. sanguinea, the dead cells of A. sanguinea “revenged” G. lemaneiformis via promoting the growth of bacteria and consequently slowed the growth of G. lemaneiformis, which was evidenced by the lowered pH, slowed nutrients consumption in the cultures, the elevated counts of bacteria, and the reduced biomass of G. lemaneiformis. Collectively, our results showed that while G. lemaneiformis could inhibit the growth of microalgae with allelopathy as a major contributor, the death of allelopathy-affected microalgae could promote bacterial growth, which sequentially inhibits the growth of G. lemaneiformis as a feedback.

A study was conducted in the marine finfish cage culture area in Sungai Udang, Penang, at the northern part of the Malacca Straits to examine the phytoplankton composition and abundance especially for potentially harmful phytoplankton. Monthly sampling were taken from March 2016 to January 2017 at nine sampling stations. Physio-chemical parameters of surface seawater such as pH, dissolved oxygen, temperature, salinity, total suspended solids, and nutrients (nitrate, nitrite, ammonium, phosphate and silicate) were also measured. A total of 54 phytoplankton taxa were recorded, with 37 genera belonged to diatoms, 15 of dinoflagellates, and 2 of cyanobacteria. The composition of phytoplankton was dominated by diatoms (>85%) at all sampling stations throughout the sampling period. The phytoplankton abundance ranged between 2.6×103 cells L-1 and 5.8×106 cells L-1. The potentially harmful toxic phytoplankton observed throughout the sampling period are dinoflagellates Alexandrium spp., Prorocentrum micans and Dinophysis caudata and diatoms, Pseudo-nitzchia spp but in low cell density. A total of six bloom-forming phytoplankton that can potentially trigger mass mortality of cultured fish such as Akashiwo sanguinea, Chaetoceros spp., Ceratium furca, Ceratium fusus, Margalefidinium spp. and Karlodinium spp. recorded at this area were relatively low in cell densities. Furthermore, no fish kill incident was reported in the area from blooms of phytoplankton during the period of study. Even though potentially harmful phytoplankton present were in low densities, they may pose significant risks to aquaculture activity if there is a sudden bloom. Hence, a monitoring program should be implemented to provide early warning of harmful algae blooms and safeguard the aquaculture industry in Sungai Udang, Penang.

Background Phytoplankton blooms are frequent events in coastal areas and increase the production of organic matter that initially shapes the growth of opportunistic heterotrophic bacteria. However, it is unclear how these opportunists are involved in the transformation of dissolved organic matter (DOM) when blooms occur and the subsequent impacts on biogeochemical cycles. Results We used a combination of genomic, proteomic, and metabolomic approaches to study bacterial diversity, genome traits, and metabolic responses to assess the source and lability of DOM in a spring coastal bloom of Akashiwo sanguinea. We identified molecules that significantly increased during bloom development, predominantly belonging to amino acids, dipeptides, lipids, nucleotides, and nucleosides. The opportunistic members of the bacterial genera Polaribacter, Lentibacter, and Litoricola represented a significant proportion of the free-living and particle-associated bacterial assemblages during the stationary phase of the bloom. Polaribacter marinivivus, Lentibacter algarum, and Litoricola marina were isolated and their genomes exhibited streamlining characterized by small genome size and low GC content and non-coding densities, as well as a smaller number of transporters and peptidases compared to closely related species. However, the core proteomes identified house-keeping functions, such as various substrate transporters, peptidases, motility, chemotaxis, and antioxidants, in response to bloom-derived DOM. We observed a unique metabolic signature for the three species in the utilization of multiple dissolved organic nitrogen compounds. The metabolomic data showed that amino acids and dipeptides (such as isoleucine and proline) were preferentially taken up by P. marinivivus and L. algarum, whereas nucleotides and nucleosides (such as adenosine and purine) were preferentially selected by L. marina. Conclusions The results suggest that the enriched DOM in stationary phase of phytoplankton bloom is a result of ammonium depletion. This environment drives genomic streamlining of opportunistic bacteria to exploit their preferred nitrogen-containing compounds and maintain nutrient cycling.

Characterizing ecological relationships between viruses, bacteria and phytoplankton in the ocean is critical to understanding the ecosystem; however, these relationships are infrequently investigated together. To understand the dynamics of microbial communities and environmental factors in harmful algal blooms (HABs), we examined the environmental factors and microbial communities during Akashiwo sanguinea HABs in the Jangmok coastal waters of South Korea by metagenomics. Specific bacterial species showed complex synergistic and antagonistic relationships with the A. sanguinea bloom. The endoparasitic dinoflagellate Amoebophrya sp. 1 controlled the bloom dynamics and correlated with HAB decline. Among nucleocytoplasmic large DNA viruses (NCLDVs), two Pandoraviruses and six Phycodnaviruses were strongly and positively correlated with the HABs. Operational taxonomic units of microbial communities and environmental factors associated with A. sanguinea were visualized by network analysis: A. sanguinea–Amoebophrya sp. 1 (r = .59, time lag: 2 days) and A. sanguinea–Ectocarpus siliculosus virus 1 in Phycodnaviridae (0.50, 4 days) relationships showed close associations. The relationship between A. sanguinea and dissolved inorganic phosphorus relationship also showed a very close correlation (0.74, 0 day). Microbial communities and the environment changed dynamically during the A. sanguinea bloom, and the rapid turnover of microorganisms responded to ecological interactions. A. sanguinea bloom dramatically changes the environments by exuding dissolved carbohydrates via autotrophic processes, followed by changes in microbial communities involving host‐specific viruses, bacteria and parasitoids. Thus, the microbial communities in HAB are composed of various organisms that interact in a complex manner.

No presente trabalho apresentamos a caracterização da assembleia microfitoplanctônica na Baía de Todos os Santos, considerando as condições abióticas locais, em uma curta série temporal, com o objetivo de ampliar o conhecimento sobre os padrões de composição, diversidade e distribuição do fitoplâncton na baía. Foram realizadas amostragens em três dias consecutivos, no mês de setembro 2016, em cinco pontos distribuídos no setor nordeste da baía. Foram medidos in situ a temperatura, salinidade, oxigênio dissolvido e transparência da água. Amostras foram coletadas utilizando-se garrafa de Van Dorn (1L), em subsuperfície (~0,5m), para análises quantitativas do microfitoplâncton. Amostras de 20mL foram coletadas por meio de arrastos horizontais de rede de plâncton de 20μm de abertura de malha, para análises de composição da comunidade. Os taxa foram identificados com base em literatura especializada e as análises quantitativas foram realizadas segundo o método de Utermöhl. As variáveis abióticas evidenciaram um ambiente homogêneo, com uma massa d’água quente e salina, tipicamente marinha. Foram identificados 104 táxons e as diatomáceas apresentaram a maior riqueza e abundância de espécies, demonstrando um padrão típico encontrado em sistemas estuarinos tropicais. Considerando a importância da composição e estrutura da comunidade fitoplanctônica para o equilíbrio ecológico da baía e manutenção dos serviços ecossistêmicos, destacamos a ocorrência de espécies potencialmente nocivas, capazes de provocar danos ao sistema em caso de florações, a exemplo do que já ocorreu na ocasião da floração de Akashiwo sanguinea no ano de 2007.

Oxygen minimum zones (OMZs) are characterized by enhanced carbon dioxide (CO2) levels and low pH and are being further acidified by uptake of anthropogenic atmospheric CO2. With ongoing intensification and expansion of OMZs due to global warming, carbonate chemistry conditions may become more variable and extreme, particularly in the eastern boundary upwelling systems. In austral summer (February–April) 2017, a large-scale mesocosm experiment was conducted in the coastal upwelling area off Callao (Peru) to investigate the impacts of ongoing ocean deoxygenation on biogeochemical processes, coinciding with a rare coastal El Niño event. Here we report on the temporal dynamics of carbonate chemistry in the mesocosms and surrounding Pacific waters over a continuous period of 50 d with high-temporal-resolution observations (every second day). The mesocosm experiment simulated an upwelling event in the mesocosms by addition of nitrogen (N)-deficient and CO2-enriched OMZ water. Surface water in the mesocosms was acidified by the OMZ water addition, with pHT lowered by 0.1–0.2 and pCO2 elevated to above 900 µatm. Thereafter, surface pCO2 quickly dropped to near or below the atmospheric level (405.22 µatm in 2017; Dlugokencky and Tans, 2021; NOAA/Global Monitoring Laboratory (GML)) mainly due to enhanced phytoplankton production with rapid CO2 consumption. Further observations revealed that the dominance of the dinoflagellate Akashiwo sanguinea and contamination of bird excrements played important roles in the dynamics of carbonate chemistry in the mesocosms. Compared to the simulated upwelling, natural upwelling events in the surrounding Pacific waters occurred more frequently with sea-to-air CO2 fluxes of 4.2–14.0 mmol C m−2 d−1. The positive CO2 fluxes indicated our site was a local CO2 source during our study, which may have been impacted by the coastal El Niño. However, our observations of dissolved inorganic carbon (DIC) drawdown in the mesocosms suggest that CO2 fluxes to the atmosphere can be largely dampened by biological processes. Overall, our study characterized carbonate chemistry in nearshore Pacific waters that are rarely sampled in such a temporal resolution and hence provided unique insights into the CO2 dynamics during a rare coastal El Niño event.

Algicidal bacteria offer an eco-friendly and promising approach for controlling harmful algae blooms (HABs). In this study, repeated batch fermentation of immobilized algicidal bacterium Vibrio brasiliensis H115 was preformed to enhance the productivity of the algicidal compounds. The highest algicidal efficiency of the fermentation products against Akashiwo sanguinea (100%) was achieved when the fermentation time was decreased from 24 to 14 h. The cell-free fermentation broth was then spray-dried and floating microcapsules were prepared from the dried powder. The optimum preparation conditions for floating microcapsules were: sodium alginate (SA), 3%; CaCO3: SA (mass ratio), 3:4; CaCl2, 3%; citric acid, 4%; ethylcellulose, 2%; crosslinking time, 30 min. Under the optimal conditions, the floating microcapsules displayed efficient A. sanguinea cell lysis ability and the algicidal efficiency increased from 10.62% (4 h) to 100% (24 h). These results suggest that the floating microcapsules could potentially be practically used for controlling the outbreaks of A. sanguinea.

Akashiwo sanguinea is closely linked to the healthy growth of Labrenzia sp. PO1. Herein, PO1 and SY1 strains of Labrenzia sp. were isolated from A. sanguinea to investigate their growth. A total of 6067 genes in the Labrenzia sp SY1 and 6067 genes in Labrenzia sp PO1 were identified by RNA sequencing. Totally, 2402 genes out of 6067 were found as differentially expressed genes (DEGs). PO1 strain, 595 with decreased transcript abundance, and 1807 increase transcript abundance. Total 2320 DEGs, 766 with decreased transcript abundance, and 1554 increase transcript abundance were found in the SY1 strain. Transcriptome analysis divulged that DEGs were enriched with regulating metabolic processes, biological processes, and binding activities and may play critical roles in Labrenzia sp. PO1 growth. The ABC transporter pathway was also enriched in PO1 with 23 genes encoding ABC transporters with increased transcript abundance only in this strain. ABC transporter transcript levels and Labrenzia sp. PO1 growth was highest at 32 h, indicating that transporter-encoding genes may be involved in PO1 growth through nutrient transport. Moreover, their low expression in SY1 may be related to its slower growth. This study of Labrenzia sp. PO1 and SY1 transcriptomes may be used as a foundation for future research on the biofilm formation phenomenon of Labrenzia sp. PO1, as the formation was observed only in PO1.

Dinoflagellate blooms currently caused serious environmental problems in different areas of the world. Recent studies revealed close relationship between dinoflagellate blooms and microbial community dynamics, while less attention has been paid on the bacterial culturability change associated with the bloom. Here, we investigated the temporal variation of microbial community composition and culturability during the decline stage of an Akashiwo sanguinea bloom occurred in Shenzhen, China. The daily microbial community phylogenetic structures in water samples collected during a four-day period after the bloom peak were assessed through 16S rRNA gene amplicons sequencing on the MiSeq (Illumina) platform. The environmental parameters, Chlorophyll a concentrations, and total viable and culturable bacterial densities were also measured. Our results showed that Gamma-proteobacteria comprising mostly of Pseudoalteromonadaceae and Vibrionaceae was the predominant microbial class in the post-bloom samples, except for the second day. During that day, the represented groups switched to Alpha-proteobacteria (Rhizobiales) and Beta-proteobacteria (Comamonadaceae), with the microbial culturability decreased. Total viable bacterial densities reached the maximum value on the third day, with Gamma-proteobacteria regained the dominance till the fourth day. The dramatic microbial community succession and culturability variation observed in this study indicated the complication of algae-bacteria interactions during dinoflagellate bloom.

We investigated the dynamics of the bacterial composition and metabolic function within Akashiwo sanguinea bloom using a 100-L indoor microcosm and metagenomic next-generation sequencing. We found that the bacterial community was classified into three groups at 54% similarity. Group I was associated with “during the A. sanguinea bloom stage” and mainly consisted of Alphaproteobacteria, Flavobacteriia and Gammaproteobacteria. Meanwhile, groups II and III were associated with the “late bloom/decline stage to post-bloom stage” with decreased Flavobacteriia and Gammaproteobacteria in these stages. Upon the termination of the A. sanguinea bloom, the concentrations of inorganic nutrients (particularly PO43−, NH4+ and dissolved organic carbon) increased rapidly and then decreased. From the network analysis, we found that the A. sanguinea node is associated with certain bacteria. After the bloom, the specific increases in NH4+ and PO43− nodes are associated with other bacterial taxa. The changes in the functional groups of the bacterial community from chemoheterotrophy to nitrogen association metabolisms were consistent with the environmental impacts during and after A. sanguinea bloom. Consequently, certain bacterial communities and the environments dynamically changed during and after harmful algal blooms and a rapid turnover within the bacterial community and their function can respond to ecological interactions.

The studies on the species diversity, distribution, environmental implications, and molecular basis of resting cysts (stages) of dinoflagellates and a few species of other groups conducted in China during the last three decades are reviewed. The major achievements are summarized as the following five aspects: 1) The continual efforts in detecting the species diversity of resting cysts (spores) in dinoflagellates and other classes using either morphological or molecular approaches, or both, in the four seas of China, which led to identifications of 106 species of dinoflagellate resting cysts and 4 species of resting stages from other groups of microalgae, with a total of 64 species of dinoflagellate cysts and the resting stage of the brown tide-causing Aureococcus anophagefferens being unequivocally identified via molecular approaches from the sediments of Chinese coastal waters; 2) The well-known toxic and HABs-causing dinoflagellates Karenia mikimotoi, Karlodinium veneficum, Akashiwo sanguinea and the pelagophyte A. anophagefferens were proven to be resting cyst (stage) producers via laboratory studies on their life cycles and field detections of resting cysts (resting stage cells). And, via germination experiment and subsequent characterization of vegetative cells, numerous dinoflagellate species that had never been described or found to form cysts were discovered and characterized; 3) The distributions of the resting cysts of Alexandrium catenella, A. pacificum, Gymnodinium catenatum, K. mikimotoi, K. veneficum and Azadinium poporum and the resting stage cells of A. anophagefferens were morphologically and molecularly mapped in all four seas of China, with A. anophagefferens proven to have been present in the Bohai Sea for at least 1,500 years; 4) Obtaining important insights into the ‘indicator’ values of the dinoflagellate cyst assemblages in sediment cores for tracking eutrophication, environmental pollution and other anthropological influences in coastal waters; 5) Studies on the cyst-pertinent processes and genetic basis (transcriptomics together with physiological and chemical measurements) of resting cyst dormancy not only revealed the regulating patterns of some environmental factors in cyst formation and germination, but also identified many characteristically active or inactive metabolic pathways, differentially expressed genes, and the possibly vital regulating function of the phytohormone abscisic acid and a group of molecular chaperones in resting cysts. We also identified seven issues and three themes that should be addressed and explored by Chinese scientists working in the area in the future.

The objective of this study was to evaluate variations in the composition and abundance of the phytoplankton community after an exceptional harmful bloom of Akashiwo sanguinea that occurred in Todos os Santos Bay (BTS) in early March, 2007. Samples were collected every ten days, between April, 2007 and March, 2008, from the estuarine gradient of the Paraguacu River to BTS. The physical and chemical variables were measured in situ using a multiparameter sensor. Water samples were collected for analysis of the dissolved inorganic nutrient concentrations and for the study of composition and abundance of the microphytoplankton. Overall, 135 taxa were identified, with a higher richness of diatoms. The total cell density ranged from 2.92 × 103 to 1.16 × 107 (5.47 × 105 ± 1.69 × 106) cells L−1, with higher values in the freshwater zone than in the marine area. Five species showed peaks of abundance throughout the study, forming small blooms. Four of these blooms occurred in the rainy season, formed by the species Guinardia striata (April, 2007), Scrippsiella cf. acuminata (August, 2007), Euglena gracilis (August, 2007), and Skeletonema cf. costatum (September, 2007), while a new bloom of the species Akashiwo sanguinea occurred during the dry season (December, 2007). The environment was typically oligotrophic, with low spatiotemporal variation in the concentrations of dissolved nutrients. Even so, we observed short-term variations in the structure and composition of the phytoplankton community, demonstrated by rapid bloom events, followed by an increase in the total abundance of microphytoplankton, especially during the rainy season. The blooms did not cause any notable changes in the water column and did not present any harmful effects on the system.

Understanding aspects of the biology of early life stages of marine fish is critical if one hopes to reveal the factors and processes that impact the survival and recruitment (year class) strength. The Peruvian anchovy (Engraulis ringens) is a key species in the Humboldt current system, and the present study provides the first description of the embryonic and larval development of this species reared in captivity. Embryonic and early exogenous feeding stages of larvae were illustrated in detail at 18.5°C. Hatching was completed within 42 and 48 h post-fertilization at 18.5 and 14.5°C, respectively. Mean ± 95% C.I. standard length (LS) at hatch (3.40 ± 0.10 mm at 18.5°C and 2.76 ± 0.34 mm at 14.5°C) was significantly different between the two temperatures. Larval behaviour was assessed at 18.5°C; at the onset of exogenous feeding [3 days post-hatch (dph)], larvae were fed small, motile dinoflagellates, Akashiwo sanguinea. At 7 dph, larvae started to feed almost exclusively on zooplankton (rotifers and Artemia nauplii). Larval activity increased with age, and the first sign of schooling was noted at 31 dph (18.56 mm LS) at 18.5°C. Temperature had a significant effect on size-at-age, but not on body shape (depth to LS ratio). The size-at-age data for larvae (this study) was used to parameterize a temperature-corrected von Bertalanffy growth function for Peruvian anchovy, the accuracy of which was assessed for juveniles and adults (literature values).

Arenibacter sp. strain 6A1 was isolated from seawater during an Akashiwo sanguinea bloom in Shenzhen, China. Here, we present its 4,666,208-bp genome sequence, consisting of 3,623 coding sequences with a G+C content of 38.95%.

Eastern boundary upwelling systems (EBUS) are among the most productive marine ecosystems on Earth. The production of organic material is fueled by upwelling of nutrient-rich deep waters and high incident light at the sea surface. However, biotic and abiotic factors can modify surface production and related biogeochemical processes. Determining these factors is important because EBUS are considered hotspots of climate change, and reliable predictions of their future functioning requires understanding of the mechanisms driving the biogeochemical cycles therein. In this field experiment, we used in situ mesocosms as tools to improve our mechanistic understanding of processes controlling organic matter cycling in the coastal Peruvian upwelling system. Eight mesocosms, each with a volume of ∼55 m3, were deployed for 50 d ∼6 km off Callao (12∘ S) during austral summer 2017, coinciding with a coastal El Niño phase. After mesocosm deployment, we collected subsurface waters at two different locations in the regional oxygen minimum zone (OMZ) and injected these into four mesocosms (mixing ratio ≈1.5 : 1 mesocosm: OMZ water). The focus of this paper is on temporal developments of organic matter production, export, and stoichiometry in the individual mesocosms. The mesocosm phytoplankton communities were initially dominated by diatoms but shifted towards a pronounced dominance of the mixotrophic dinoflagellate (Akashiwo sanguinea) when inorganic nitrogen was exhausted in surface layers. The community shift coincided with a short-term increase in production during the A. sanguinea bloom, which left a pronounced imprint on organic matter C : N : P stoichiometry. However, C, N, and P export fluxes did not increase because A. sanguinea persisted in the water column and did not sink out during the experiment. Accordingly, export fluxes during the study were decoupled from surface production and sustained by the remaining plankton community. Overall, biogeochemical pools and fluxes were surprisingly constant for most of the experiment. We explain this constancy by light limitation through self-shading by phytoplankton and by inorganic nitrogen limitation which constrained phytoplankton growth. Thus, gain and loss processes remained balanced and there were few opportunities for blooms, which represents an event where the system becomes unbalanced. Overall, our mesocosm study revealed some key links between ecological and biogeochemical processes for one of the most economically important regions in the oceans.

Resting cysts represent the dormant life stage produced by sexual reproduction of dinoflagellates; it generally maintain a limited level of metabolic activities. In the present study, changes in the photosynthetic apparatus of Akashiwo sanguinea during the encystment process in different nitrogen (N) conditions were characterized, and the transcriptional responses of nine photosynthesis genes were further evaluated. The photochemical efficiency decreased with elevation of N deprivation and growth of algae, and the resting cysts maintained limited photosynthetic activity. Meanwhile, the transcription of photosystem II genes (psbA and psbD) were down-regulated, especially the levels of the psbA gene, suggesting the inactivation of photosystem II under N limitation. Moreover, genes involved in cytochrome f (petA) and ATP synthase (atpA, atpB) were also transcribed at low levels in N-deprivation cultures. In contrast, the transcription of the RubisCO large subunit gene (rbcL) was up-regulated, indicating the maintenance of carbon fixation in N-limited conditions. The transcription levels of photosystem I genes (psaA and psaB) showed no significant correlation with the N condition. Further, most of the photosynthesis genes were down-regulated in the resting cysts, which was coincided with the inactive physiological status of the dormant stage. The results demonstrated that photosynthesis genes are differentially regulated in response to varying N conditions and life cycles. The findings will provide fundamental knowledge for better understanding the physiological characteristics and molecular regulation mechanisms in the developmental life cycle of marine dinoflagellates.

The dinoflagellate Cochlodinium geminatum can form near-monospecific blooms which have occurred at the same location in the South China Sea repeatedly in the last decade. Here, we demonstrate the allelopathic effects of C. geminatum isolated from the bloom waters of the Pearl River Estuary, South China Sea, in August 2011, which inhibited four of six co-occurring phytoplankton species tested. Inhibition increased with rising cell density and reached a maximum in the declining growth stage. Four different C. geminatum culture components negatively affected growth of Akashiwo sanguinea in order of decreasing percent inhibition: sonicated culture, filtrate of sonicated culture, whole-cell culture and filtrate of whole-cell culture. The highest inhibition was at 25 °С and salinities of 30 PSU and light intensities of 100 μmol photons m−2 s−1. Overall, our study showed that C. geminatum could inhibit growth of phytoplankton competitors during succession of phytoplankton community, achieving nearly monospecific blooms, and the allelopathic effects were regulated by environmental factors.

Carbonyl compounds, especially polyunsaturated aldehydes (PUAs), are increasingly reported as teratogenic to grazers and deleterious to phytoplankton. While PUAs have been considered to be mainly produced by diatoms after cell wounding, little is known about whether microalgae other than diatoms, particularly harmful algal bloom (HAB)-forming species, produce carbonyl compounds. In this study, we analyzed the carbonyl compounds from eight common HAB-forming species (Akashiwo sanguinea, Karenia mikimotoi, Karlodinium veneficum, Margalefidinium polykrikoides, Prorocentrum donghaiense, P. minimum, Scrippsiella trochoidea, and Heterosigma akashiwo) using gas chromatography-mass spectrometry in full scan and selected ion monitoring (SIM) modes. Our results show a ubiquitous presence of carbonyl compounds in both dissolved and particulate forms in all species we examined. In the full scan mode, 133 chromatographic peaks were detected from 51 samples altogether. Both the varieties of carbonyl compounds and their quantities were algal species dependent, although most of the carbonyl compounds could not be fully identified according to the mass spectral database only due to the unavailability of enough standards for all analytes. Aided with nine standards in SIM mode, we further identified and quantified all nine aldehydes (2-methyl-2-pentenal, trans-2-nonenal, cis-6-nonenal, 2,6-dimethyl-5-heptenal, trans-2-hexenal, trans-2-decenal, 2,4-heptadienal, trans-trans-2,4-octadienal, and trans-trans-2,4-decadienal). Four of these nine aldehydes were detected in particulate form, which confirmed that trans-2-nonenal could be produced by K. mikimotoi, Karl. veneficum, P. donghaiense, P. minimum, S. trochoidea, and H. akashiwo, 2,4-heptadienal and trans-trans-2,4-decadienal by A. sanguinea, M. polykrikoides, and S. trochoidea, and trans, trans-2,4-octadienal by S. trochoidea, respectively. We proved that some dinoflagellate and raphidophyte species could contribute to the pool of carbonyl compounds including PUAs in a marine ecosystem. Some carbonyl compounds, particularly those in high cell quota and/or dissolved concentration, may be related to fish-killing or allelopathy which needs further identification and quantification.

The dinoflagellate Karlodinium veneficum blooms kill fish by the production of karlotoxins. We investigated the allelopathy (Akashiwo sanguinea as the target species) and toxicity (Oryzias melastigma, Artemia salina and Brachionus plicatilis as test animals) of four strains of K. veneficum isolated from coastal waters of China and the USA to explore the ecological implications of allelopathy and toxicity. In general, all four strains of K. veneficum at high cell densities (>100 000 cellsmL(-1)) significantly inhibited the growth of A. sanguinea to different extents, but they exhibited either positive or negative allelopathy at lower densities. The toxicity of the four strains showed a decreasing order coinciding with their culture histories in the laboratory, suggesting K. veneficum may have gradually lost toxicity during laboratory culturing. The allelopathic and toxic potencies among K. veneficum strains were not perfectly parallel, indicating that either the allelochemicals and toxins of K. veneficum may be different chemicals, or the same chemicals functioned in different modes. Moreover, we found A. salina avoided feeding on more toxic strains. These results together suggest that toxins provide an advantage for K. veneficum via avoiding predation, but allelopathy may not play an important role in initiating blooms of K. veneficum.

A multi-year study was conducted in the eutrophic Lafayette River, a sub-tributary of the lower Chesapeake Bay during which uptake of inorganic and organic nitrogen (N) and C compounds was measured during multiple seasons and years when different dinoflagellate species were dominant. Seasonal dinoflagellate blooms included a variety of mixotrophic dinoflagellates including Heterocapsa triquetra in the late winter, Prorocentrum minimum in the spring, Akashiwo sanguinea in the early summer, and Scrippsiella trochoidea and Cochlodinium polykrikoides in late summer and fall. Results showed that no single N source fueled algal growth, rather rates of N and C uptake varied on seasonal and diurnal timescales, and within blooms as they initiated and developed. Rates of photosynthetic C uptake were low yielding low assimilation numbers during much of the study period and the ability to assimilate dissolved organic carbon augmented photosynthetic C uptake during bloom and non-bloom periods. The ability to use dissolved organic C during the day and night may allow mixotrophic bloom organisms a competitive advantage over co-occurring phytoplankton that are restricted to photoautotrophic growth, obtaining N and C during the day and in well-lit surface waters.

We performed different consensus methods by combining binary classifiers, mostly machine learning classifiers, with the aim to test their capability as predictive tools for the presence–absence of marine phytoplankton species. The consensus methods were constructed by considering a combination of four methods (i.e., generalized linear models, random forests, boosting and support vector machines). Six different consensus methods were analyzed by taking into account six different ways of combining single-model predictions. Some of these methods are presented here for the first time. To evaluate the performance of the models, we considered eight phytoplankton species presence–absence data sets and data related to environmental variables. Some of the analyzed species are toxic, whereas others provoke water discoloration, which can cause alarm in the population. Besides the phytoplankton data sets, we tested the models on 10 well-known open access data sets. We evaluated the models' performances over a test sample. For most (72%) of the data sets, a consensus method was the method with the lowest classification error. In particular, a consensus method that weighted single-model predictions in accordance with single-model performances (weighted average prediction error — WA-PE model) was the one that presented the lowest classification error most of the time. For the phytoplankton species, the errors of the WA-PE model were between 10% for the species Akashiwo sanguinea and 38% for Dinophysis acuminata . This study provides novel approaches to improve the prediction accuracy in species distribution studies and, in particular, in those concerning marine phytoplankton species.

The unarmored dinoflagellate Akashiwo sanguinea is a well known cosmopolitan harmful alga; however, the toxic nature of this alga has yet to be examined using multiple clonal isolates. Here we examined the toxicity of 3 clonal cultures of A. sanguinea, including JX13 and JX14, isolated from Daya Bay, South China Sea, and AS2, isolated from Chesapeake Bay, USA, to multiple aquatic animals including species of finfish, shellfish, shrimp, and zooplankton. The whole-cell cultures of A. sanguinea exhibited acute lethal effects on the shrimp Litopenaeus vannamei, the bivalve Meretrix meretrix, and 2 species of fish (Mugil cephalus and Mugilidae sp.) with 72 h mortalities ranging from 20 to 100%. The sonicated and filtrated cultures were lethal to brine shrimp Artemia salina, while the filtrates of whole-cell cultures were not, suggesting that the toxins are intracellular. Boiling and freezing led to significant reductions in toxicity. A. sanguinea toxicity differed among the Chinese strains, and the hemolytic activity of 1 Chinese strain was 3-fold greater than that of the US strain. Cultures in exponential phase displayed stronger toxicity, and the greatest toxicity of A. sanguinea was observed at 20 degrees C and a salinity of 35, conditions optimal for growth of the alga. Toxicity was enhanced by increased nutrient supply, suggesting that this species could both directly (via increased growth) and indirectly (e.g. via enhanced toxin production) become more toxic in response to eutrophication. Collectively, our findings suggest that the ability to produce and release toxin(s) may promote A. sanguinea blooms by suppressing predators and competitors.

International audience; This study investigated the genetic diversity of phytoplankton communities in six shallow lagoons located on the French coast of the northwestern Mediterranean Sea that represented a trophic gradient ranging from oligotrophic to hypereutrophic. The phytoplankton communities were sampled once a month from spring (May) to the beginning of autumn (September/early October) in 2012 and fractionated by size. Metabarcodes were generated from cDNAs by targeting the D1-D2 region of the 28S rRNA gene and pyrosequenced using Roche 454 technology. Examination of the annotated barcodes revealed harmful algal species not previously documented in these lagoons. Three ichthyotoxic species belonging to Pfiesteriaceae were detected: Luciella masanensis was relatively widespread and abundant in many samples, whereas Pfiesteria piscicida and Stoeckeria changwonensis were found as single barcode sequences. Furthermore, a phylogenetic analysis of barcodes annotated as belonging to Pfiesteriaceae suggested the existence of two previously undescribed clades. The other toxic or potentially harmful dinoflagellates detected through rare barcodes were Dinophysis acuminata, Vulcanodinium rugosum, Alexandrium andersonii and A. ostenfeldii. The two most abundant dinoflagellate taxa were Gymnodinium litoralis and Akashiwo sanguinea with respect to sequence numbers. Four diatom species from the genus Pseudo-nitzschia that potentially produce domoic acid were identified (P. galaxiae, P. delicatissima, P. brasiliana and P. calliantha). These observations are discussed in terms of the literature and monitoring records related to the identified taxa in this Mediterranean area.

Blooms of the harmful dinoflagellate Akashiwo sanguinea are responsible for the mass mortality of fish and invertebrates in coastal waters. This cosmopolitan species includes several genetically differentiated clades. Four clonal cultures were established by isolating single cells from Xiamen Harbour (the East China Sea) for morphological and genetic analyses. The cultures displayed identical morphology but were genetically different, thus revealing presence of cryptic diversity in the study area. New details of the apical structure complex of Akashiwo sanguinea were also found. To investigate whether the observed cryptic diversity was related to environmental differentiation, 634 cells were obtained from seasonal water samples collected from 2008 to 2012. These cells were sequenced by single-cell PCR. For comparison with Chinese material, additional large subunit ribosomal DNA sequences were obtained for three established strains from Malaysian and French waters. To examine potential ecological differentiation of the distinct genotypes, growth responses of the studied strains were tested under laboratory conditions at temperatures of 12 °C to 33 °C. These experiments showed four distinct ribotypes of A. sanguinea globally, with the ribotypes A and B co-occuring in Xiamen Harbour. Ribotype A of A. sanguinea was present year-round in Xiamen Harbour, but it only bloomed in the winter and spring, thus corresponding to the winter type. In contrast, A. sanguinea ribotype B bloomed only in the summer, corresponding to the summer type. This differentiation supports the temperature optimum conditions that were established for these two ribotypes in the laboratory. Ribotype A grew better at lower temperatures compared to ribotype B which preferred higher temperatures. These findings support the idea that various ribotypes of A. sanguinea correspond to distinct ecotypes and allopatric speciation occurred in different climatic regions followed by dispersal.

Resting cysts are an important part of the life cycle for many harmful algal bloom-forming dinoflagellates, and play vital roles in the recurrence and geographical spread of harmful algal blooms. Numerous factors have been suggested to regulate the formation of resting cysts, although only a few have been proven to be significant. Cyst formation can be induced by adverse environmental conditions such as drastic changes in temperature, light, salinity, and nutrient levels, and by biological interactions. In this study, we evaluated the ability of an artificial factor (fine sand particles) to enhance the formation of resting cysts. Fine sand particles were added to cultures of dinoflagellates that are known to produce cysts. The addition of fine sand particles significantly increased both the production rate and final yield of cysts in cultures of Scrippsiella trochoidea, Biecheleria brevisulcata, and Levanderina fissa (=Gymnodinium fissum, Gyrodinium instriatum, Gyrodinium uncatenum). The largest increase in the final yield (107-fold) of cysts as a result of sand addition was in S. trochoidea. However, addition of fine sand particles did not induce cyst formation, or barely affected cyst formation, in Akashiwo sanguinea, Cochlodinium polykrikoides and Pheopolykrikos hartmannii, which are also known to be cyst-producing species. We speculated that addition of sand significantly increased the chances of cell collision, which triggered cyst formation. However, further research is required to test this idea. Importantly, our findings indicate that the addition of fine sand particles is a useful method to obtain a large quantity of cysts in a short time for laboratory studies or tests; for example, if a cyst viability test is being used to assess the effectiveness of ships' ballast water treatment.

Eastern boundary upwelling systems (EBUS) are among the most productive marine ecosystems on Earth. The high productivity in surface waters is facilitated by upwelling of nutrient-rich deep waters, with high light availability enabling fast phytoplankton growth and nutrient utilization. However, there are numerous biotic and abiotic factors modifying productivity and biogeochemical processes. Determining these factors is important because EBUS are considered hotspots of climate change, and reliable predictions on their future functioning requires understanding of the mechanisms driving biogeochemical cycles therein. In this study, we used in situ mesocosms to obtain mechanistic understanding of processes controlling productivity, organic matter export, and particulate matter stoichiometry in the coastal Peruvian upwelling system. Therefore, eight mesocosm units with a volume of ~50 m3 were deployed for 50 days ~6 km off Callao during austral summer 2017, coinciding with a coastal El Niño event. To compare how upwelling of different water bodies influences plankton succession patterns, we collected two subsurface waters at different locations in the regional oxygen minimum zone (OMZ) and injected these into four replicate mesocosms, respectively (mixing ratio ≈ 1.5:1 mesocosm: OMZ water). The differences in nutrient concentrations between the collected water bodies were relatively small, and therefore we do not consider treatment differences in the present paper. The phytoplankton communities were initially dominated by diatoms but shifted towards a pronounced dominance of the mixotrophic harmful dinoflagellate (Akashiwo sanguinea) when inorganic nitrogen was exhausted in surface layers. The community shift resulted in a major short-term increase in productivity during A. sanguinea growth which left a pronounced imprint on organic matter C:N:P stoichiometry. However, C, N, and P export fluxes were not affected by this ecological regime shift because A. sanguinea persisted in the water column and did not sink out during the experiment. Accordingly, ongoing export fluxes during the study were maintained mainly by a remaining “background” plankton community. Overall, biogeochemical pools and fluxes were surprisingly constant in between the ecological regime shifts. We explain this constancy by light limitation through self-shading by phytoplankton and inorganic nitrogen limitation which constrained phytoplankton growth. Thus, gain and loss processes seemed to be relatively well balanced and there was little opportunity for blooms, which represents an event where the system becomes unbalanced. The mesocosm study revealed key links between ecological and biogeochemical processes for one of the economically most important regions in the oceans.