Dense Blooms Research Articles

Filamentous green algae Spirogyra regulates methane emissions from eutrophic rivers.

Excessive growth of filamentous green algae in rivers has attracted much attention due to their functional importance to primary production and carbon cycling. However, comprehensive knowledge of how filamentous green algae affect carbon cycling, especially the CH4 emissions from river ecosystems, remains limited. In this study, incubation experiments were conducted to examine the factors regulating CH4 emissions from a eutrophic river with dense growth of filamentous green algae Spirogyra through combinations of biogeochemical, molecular biological, and stable carbon isotope analyses. Results showed that although water dissolved oxygen (DO) in the algae+sediment (A+S) incubation groups increased up to 19 mg L-1, average CH4 flux of the groups was 13.09 μmol m-2 day-1, nearly up to two times higher than that from sediments without algae (S groups). The significant increase of sediment CH4 oxidation potential and methanotroph abundances identified the enhancing sediment CH4 oxidation during Spirogyra bloom. However, the increased water CH4 concentration was consistent with depleted water [Formula: see text] and decreased apparent fractionation factor (αapp), suggesting the important contribution of Spirogyra to the oxic water CH4 production. It can thus be concluded that high DO concentration during the algal bloom promoted the CH4 consumption by enhancing sediment CH4 oxidation, while algal-linked oxic water CH4 production as a major component of water CH4 promoted the CH4 emissions from the river. Our study highlights the regulation of Spirogyra in aquatic CH4 fluxes and will help to estimate accurately CH4 emissions from eutrophic rivers with dense blooms of filamentous green algae. Graphical abstract.

Implications of Pyrosoma atlanticum range expansion on phytoplankton standing stocks in the Northern California Current

Coinciding with the marine heat wave in 2014 and a strong El Niño in 2016, blooms of the tropical-temperate pelagic colonial tunicate, Pyrosoma atlanticum, expanded into the Northern California Current (NCC). Given the high densities of P. atlanticum colonies observed in the NCC, and their potential for high grazing rates, we hypothesized that the species could have a substantial negative effect on the phytoplankton standing stock in the Northeastern Pacific Ocean. In the late winter, spring and early autumn of 2018, we measured abundances and length frequencies of P. atlanticum colonies off the coast of Oregon and estimated their spring grazing impact on the local phytoplankton standing stock. Abundances of P. atlanticum peaked in late winter (0.7 colonies m−3), dropped off in spring (0.42 colonies m−3), and declined to near-absent in early autumn (0.07 colonies m−3). We observed a doubling in the median size of P. atlanticum colonies from late winter to spring, followed by a decline in size in early autumn. With their high individual colony gut pigment contents and bloom densities, the overall spring grazing impact of P. atlanticum was substantial, reaching a maximum of 22% of the available phytoplankton standing stock.

A Mediterranean Alexandrium taylorii (Dinophyceae) Strain Produces Goniodomin A and Lytic Compounds but Not Paralytic Shellfish Toxins.

Species of the dinophyte genus Alexandrium are widely distributed and are notorious bloom formers and producers of various potent phycotoxins. The species Alexandrium taylorii is known to form recurrent and dense blooms in the Mediterranean, but its toxin production potential is poorly studied. Here we investigated toxin production potential of a Mediterranean A. taylorii clonal strain by combining state-of-the-art screening for various toxins known to be produced within Alexandrium with a sound morphological and molecular designation of the studied strain. As shown by a detailed thecal plate analysis, morphology of the A. taylorii strain AY7T from the Adriatic Sea conformed with the original species description. Moreover, newly obtained Large Subunit (LSU) and Internal Transcribed Spacers (ITS) rDNA sequences perfectly matched with the majority of other Mediterranean A. taylorii strains from the databases. Based on both ion pair chromatography coupled to post-column derivatization and fluorescence detection (LC-FLD) and liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) analysis it is shown that A. taylorii AY7T does not produce paralytic shellfish toxins (PST) above a detection limit of ca. 1 fg cell−1, and also lacks any traces of spirolides and gymnodimines. The strain caused cell lysis of protistan species due to poorly characterized lytic compounds, with a density of 185 cells mL−1 causing 50% cell lysis of cryptophyte bioassay target cells (EC50). As shown here for the first time A. taylorii AY7T produced goniodomin A (GDA) at a cellular level of 11.7 pg cell−1. This first report of goniodomin (GD) production of A. taylorii supports the close evolutionary relationship of A. taylorii to other identified GD-producing Alexandrium species. As GD have been causatively linked to fish kills, future studies of Mediterranean A. taylorii blooms should include analysis of GD and should draw attention to potential links to fish kills or other environmental damage.

Distribution, biomass, and species composition of salps and doliolids in the Oyashio–Kuroshio transitional region: potential impact of massive bloom on the pelagic food web

The Oyashio–Kuroshio Transitional Region (OKTR) is a nursery ground for pelagic fishes; thus, community succession of mesozooplankton is pivotal to understand prey availability, and it subsequently affects recruitment success. We investigated the surface distribution, biomass, and species composition of thaliaceans (salps and doliolids) that potentially have a significant impact on the food web and biogeochemical cycle by forming intensive blooms. A total of 11 thaliacean species were recorded from 62 locations along two transects at 36° N and 38.5° N extending from 142° E to 180° in May to June 1993. Salps and doliolids were found at 93% and 84% of the stations, respectively, and their biomass values varied widely in space (0.00011–79.56 and 0.00001–5.11 mg C m−3 for salps and doliolids, respectively). Thalia democratica was the most dominant salp and particularly abundant at stations east of 165°E on the 36°N-line; the abundance varied from 103 to 1379 ind m−3. Dolioletta gegenbauri and Doliolum denticulatum were dominant doliolids, although their biomass values were far lower than that of salps. Multivariate statistical analysis with PRIMER revealed that the distribution of thaliaceans in the OKTR was not uniform in space and was affected by oceanographic conditions; doliolids tended to occur in much warmer (14.10–15.63 °C) and saline water mass (34.54–34.72) than salps (13.16–14.95 °C and 34.40–34.53). In terms of population clearance rates, the most dense salp blooms have the potential to sweep > 200% of their resident water per day, indicating that salp blooms cause deleterious feeding conditions for pelagic fishes through non-selective filter feeding.

Nitrogen fixation estimates for the Baltic Sea indicate high rates for the previously overlooked Bothnian Sea

Dense blooms of diazotrophic filamentous cyanobacteria are formed every summer in the Baltic Sea. We estimated their contribution to nitrogen fixation by combining two decades of cyanobacterial biovolume monitoring data with recently measured genera-specific nitrogen fixation rates. In the Bothnian Sea, estimated nitrogen fixation rates were 80 kt N year−1, which has doubled during recent decades and now exceeds external loading from rivers and atmospheric deposition of 69 kt year−1. The estimated contribution to the Baltic Proper was 399 kt N year−1, which agrees well with previous estimates using other approaches and is greater than the external input of 374 kt N year−1. Our approach can potentially be applied to continuously estimate nitrogen loads via nitrogen fixation. Those estimates are crucial for ecosystem adaptive management since internal nitrogen loading may counteract the positive effects of decreased external nutrient loading.

Warming favors subtropical lake cyanobacterial biomass increasing

Subtropical lakes are increasingly subject to cyanobacterial blooms resulting from climate change and anthropogenic activities, but the lack of long-term historical data limits understanding of how climate changes have affected cyanobacterial growth in deep subtropical lakes. Using high-resolution DNA data derived from a sediment core from a deep lake in southwestern China, together with analysis of other sedimentary hydroclimatic proxies, we investigated cyanobacterial biomass and microbial biodiversity in relation to climate changes during the last millennium. Our results show that both cyanobacterial abundance and microbial biodiversity were higher during warmer periods, including the Medieval Warm Period (930–1350 CE) and the Current Warm Period (1900 CE–present), but lower during cold periods, including the Little Ice Age (1400–1850 CE). The significant increases in cyanobacterial abundance and microbial biodiversity during warmer intervals are probably because warm climate not only favors cyanobacterial growth but also concentrates lake water nutrients through water budgets between evaporation and precipitation. Furthermore, because rising temperatures result in greater vertical stratification in deep lakes, cyanobacteria may have exploited these stratified conditions and accumulated in dense surface blooms. We anticipate that under anthropogenic warming conditions, cyanobacterial biomass may continue to increase in subtropical deep lakes.

Predicting cyanobacterial biovolume from water temperature and conductivity using a Bayesian compound Poisson-Gamma model

Eutrophication and climate change scenarios engender the need to develop good predictive models for harmful cyanobacterial blooms (CyanoHABs). Nevertheless, modeling cyanobacterial biomass is a challenging task due to strongly skewed distributions that include many absences as well as extreme values (dense blooms). Most modeling approaches alter the natural distribution of the data by splitting them into zeros (absences) and positive values, assuming that different processes underlie these two components. Our objectives were (1) to develop a probabilistic model relating cyanobacterial biovolume to environmental variables in the Río de la Plata Estuary (35°S, 56°W, n = 205 observations) considering all biovolume values (zeros and positive biomass) as part of the same process; and (2) to use the model to predict cyanobacterial biovolume under different risk level scenarios using water temperature and conductivity as explanatory variables. We developed a compound Poisson-Gamma (CPG) regression model, an approach that has not previously been used for modeling phytoplankton biovolume, within a Bayesian hierarchical framework. Posterior predictive checks showed that the fitted model had a good overall fit to the observed cyanobacterial biovolume and to more specific features of the data, such as the proportion of samples crossing three threshold risk levels (0.2, 1 and 2 mm³ L−1) at different water temperatures and conductivities. The CPG model highlights the strong control of cyanobacterial biovolume by nonlinear and interactive effects of water temperature and conductivity. The highest probability of crossing the three biovolume levels occurred at 22.2 °C and at the lowest observed conductivity (∼0.1 mS cm−1). Cross-validation of the fitted model using out-of-sample observations (n = 72) showed the model’s potential to be used in situ, as it enabled prediction of cyanobacterial biomass based on two readily measured variables (temperature and conductivity), making it an interesting tool for early alert systems and management strategies. Furthermore, this novel application demonstrates the potential of the Bayesian CPG approach for predicting cyanobacterial dynamics in response to environmental change.

Antioxidant responses of triangle sail mussel Hyriopsis cumingii exposed to harmful algae Microcystis aeruginosa and high pH.

In lakes and reservoirs, harmful algal blooms and high pH have been deemed to be two important stressors related to eutrophication, especially in the case of CO2 depletion caused by dense blooms. However, the effects of these stressors on the economically important shellfish that inhabit these waters are still not well-understood. This study evaluated the combined effects of the harmful algae Microcystis aeruginosa (0%, 50%, and 100% of total dietary dry weight) and high pH (8.0, 8.5 and 9.0) on the antioxidant responses of the triangle sail mussel H.cumingii. The mussels were exposed to algae and high pH for 14 d, followed by a 7-day depuration period. Reactive oxygen species (ROS) in the mussel hemolymph, antioxidant and detoxifying enzymes, such as glutathione-S-transferase (GST), glutathione (GSH), superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), and malondialdehyde (MDA) in the digestive glands were analyzed during the experimental period. GST, SOD and GPx activity levels and the content of GSH increased following exposure to toxic M.aeruginosa, whereas CAT activity was inhibited. pH showed no significant effects on the immune defense mechanisms and detoxification processes. However, a high pH could cause increased ROS and MDA levels, resulting in oxidative injury. After a 7-day depuration period, exposure to toxic M.aeruginosa or high pH resulted in latent effects for most of the examined parameters. The treatment group exposed to the highest pH (9.0) displayed an increased oxidation state compared with the other pH treatments (8.0 and 8.5) for the same concentrations of toxic M.aeruginosa. The trends observed for ROS, MDA, GPx, GST, SOD and GSH levels indicated that a high density of toxic algae could result in severe and continuous effects on mussel health.

Impacts of a shallow shipwreck on a coral reef: A case study from St. Brandon's Atoll, Mauritius, Indian Ocean

Shallow shipwrecks, can have severe ecological and toxicological impacts on coral atolls. In 2012, a tuna longliner ran aground on the reef crest of St Brandon's Atoll, Mauritius, broke up into three pieces which was moved by currents and storms into the lagoon. In the months following the grounding, the coral around the wreck became dead and black. Down-current from the wreck, a dense bloom of filamentous algae (Ulva sp.) attached to coral occurred. To determine the ecological effects of the wreck on the system, the marine biota around the wreck, in the algal bloom, and fish reference zones were counted in 2014. Metal concentrations in reference and affected coral was determined using inductively coupled plasma mass spectrometry (ICP/MS). A pronounced difference was seen in the metal concentration pattern between coral from the wreck- and algal zones, and the coral reference zone. While the wreck zone contained the highest abundance of fish, the fish reference zone had the highest species diversity but with fewer fish. We also counted eleven Critically Endangered hawksbill sea turtles Eretmochelys imbricata and significantly more sea cucumbers in the algal zone than the reference zones. The effects of shipwrecks on coral reefs must be considered a threat over periods of years and should be studied further.

Evidence for Production of Sexual Resting Cysts by the Toxic Dinoflagellate Karenia mikimotoi in Clonal Cultures and Marine Sediments.

The toxic dinoflagellate Karenia mikimotoi has been well-known for causing large-scale and dense harmful algal blooms (HABs) in coastal waters worldwide and serious economic loss in aquaculture and fisheries and other adverse effects on marine ecosystems. Whether K.mikimotoi forms resting cysts has been a puzzling issue regarding to the mechanisms of bloom initiation and geographic expansion of this species. We provide morphological and molecular confirmation of sexually produced thin-walled resting cysts by K.mikimotoi based on observations of laboratory cultures and their direct detection in marine sediments. Light and scanning electron microscopy evidences for sexual reproduction include attraction and pairing of gametes, gamete fusion, formation of planozygote and thin-walled cyst, and the documentation of the thin-walled cyst germination processes. Evidence for cysts in marine sediments was in three aspects: positive PCR detection of cysts using species-specific primers in the DNA extracted from whole sediments; fluorescence insitu hybridization detection of cysts using FISH probes; and single-cell PCR sequencing for cysts positively labeled with FISH probes. The existence of sexually produced, thin-walled resting cysts by K.mikimotoi provides a possible mechanism accounting for the initiation of annually recurring blooms at certain regions and global expansion of the species during the past decades.

Microalgal blooms in a eutrophic estuary (Golden Horn, Sea of Marmara) following a remediation effort

Abstract The present work describes microalgal blooms that occurred in a eutrophic estuary (Golden Horn, Sea of Marmara, Turkey) between October 2013 and September 2014 following a remediation effort. The relationships between bloom-forming microalgal species and environmental factors were investigated during the study period. The changing environmental conditions (e.g. increasing water transparency and salinity) after seawater transfer to the Golden Horn Estuary stimulated phytoplankton growth with dense algal blooms. Annual average values of Secchi depth, salinity and dissolved oxygen increased in comparison with those in an earlier study in 2009–2010. Nine microalgal species, which consisted of four diatoms, two dinoflagellates, one cryptophycean, one raphidophycean and one euglenophycean, formed the blooms with water discolorations during spring and summer. The species that reached the highest bloom density were Plagioselmis prolonga (62.4 × 106 cells l−1) among crytophyceans, Heterocapsa triquetra (21.8 × 106 cells l−1) among dinoflagellates and Skeletonema marinoi (39 × 106 cells l−1) among diatoms. The abundance of dinoflagellates and phytoflagellates increased particularly in the upper estuary when compared to diatoms and their rapid growth and bloom formation revealed that they have a wide range of tolerance to changing environmental conditions and a strong ability to compete with other species in this study area.

Basin-specific changes in filamentous cyanobacteria community composition across four decades in the Baltic Sea

Almost every summer, dense blooms of filamentous cyanobacteria are formed in the Baltic Sea. These blooms may cause problems for tourism and ecosystem services, where surface accumulations and beach fouling are commonly occurring. Future changes in environmental drivers, including climate change and other anthropogenic disturbances, may further enhance these problems. By compiling monitoring data from countries adjacent to the Baltic Sea, we present spatial and temporal genus-specific distribution of diazotrophic filamentous cyanobacteria (Nostocales) during four decades (1979–2017). While the summer surface salinity decreased with a half up to one unit, the surface temperature in July-August increased with 2–3 °C in most sub-basins of the Baltic Sea, during the time period. The biovolumes of the toxic Nodularia spumigena did not change in any of the sub-basins during the period. On the other hand, the biovolume of the non-toxic Aphanizomenon sp. and the potentially toxic Dolichospermum spp. increased in the northern parts of the Baltic Sea, along with the decreased salinity and elevated temperatures, but Aphanizomenon sp. decreased in the southern parts despite decreased salinity and increased temperatures. These contradictory changes in biovolume of Aphanizomenon sp. between the northern and southern parts of the Baltic Sea may be due to basin-specific effects of the changed environmental conditions, or can be related to local adaptation by sub-populations of the genera. Overall, this comprehensive dataset presents insights to genus-specific bloom dynamics by potentially harmful diazotrophic filamentous cyanobacteria in the Baltic Sea.

Vertical and horizontal distributions of ellipsoidal <i>Alexandrium</i> (Dinophyceae) cysts in coastal sediment with special reference to paralytic shellfish poisoning caused by tsunamis - a case study of Osaka Bay (Japan) and the southern coast of the Korean Peninsula

Severe damages will result in human society, when several different critical natural phenomena coincide. One example relates to the resting cysts of Alexandrium species (dinoflagellates that cause paralytic shellfish poisoning), which are preserved in surface sediments throughout Osaka Bay, Japan. These cysts have been found to accumulate particularly densely in shallow areas in the inner parts of Osaka Bay, where a tsunami caused by an earthquake could occur any time. Damage by a tsunami could cause a change of the coastal ecosystems at Osaka Bay including the resuspension of surface sediments containing resting Alexandrium tamarense cysts and the subsequent redistribution of the cysts in newly deposited sediment. Under certain environmental conditions, these cysts could germinate and form dense blooms, leading to paralytic shellfish poisoning. Such a scenario could also affect other coastal areas, including the southern coast of the Korean Peninsula.

Upwelling Bays: How Coastal Upwelling Controls Circulation, Habitat, and Productivity in Bays

Bays in coastal upwelling regions are physically driven and biochemically fueled by their interaction with open coastal waters. Wind-driven flow over the shelf imposes a circulation in the bay, which is also influenced by local wind stress and thermal bay-ocean density differences. Three types of bays are recognized based on the degree of exposure to coastal currents and winds (wide-open bays, square bays, and elongated bays), and the characteristic circulation and stratification patterns of each type are described. Retention of upwelled waters in bays allows for dense phytoplankton blooms that support productive bay ecosystems. Retention is also important for the accumulation of larvae, which accounts for high recruitment in bays. In addition, bays are coupled to the shelf ecosystem through export of plankton-rich waters during relaxation events. Ocean acidification and deoxygenation are a concern in bays because local extrema can develop beneath strong stratification.

Monitoring of the toxic dinoflagellate Alexandrium catenella in Osaka Bay, Japan using a massively parallel sequencing (MPS)-based technique

Since 2002, blooms of Alexandrium catenella sensu Fraga et al. (2015) and paralytic shellfish toxicity events have occurred almost yearly in Osaka Bay, Japan. To better understand the triggers for reoccurring A. catenella blooms in Osaka Bay, phytoplankton community was monitored during the spring seasons of 2012–2015. Monitoring was performed using massively parallel sequencing (MPS)-based technique on amplicon sequences of the 18S rRNA gene. Dense blooms of A. catenella occurred every year except in 2012, however, there was no significant correlation with the environmental parameters investigated. Plankton community diversity decreased before and middle of the A. catenella blooms, suggesting that the decline in diversity could be an indicator for the bloom occurrence. The yearly abundance pattern of A. catenella cells obtained by morphology-based counting coincided with the relative sequence abundances, which supports the effectiveness of MPS-based phytoplankton monitoring.

Molecular and biochemical basis for the loss of bioluminescence in the dinoflagellate Noctiluca scintillans along the west coast of the USA.

The globally distributed heterotrophic dinoflagellate Noctiluca scintillans (Macartney) Kofoid & Swezy is well known for its dense blooms and prominent displays of bioluminescence. Intriguingly, along the west coast of the USA its blooms are not bioluminescent. We investigated the basis for the regional loss of bioluminescence using molecular, cellular and biochemical analyses of isolates from different geographic regions. Prominent differences of the non-bioluminescent strains were: (1) the fused luciferase and luciferin binding protein gene (lcf/lbp) was present but its transcripts were undetectable; (2) lcf/lbp contained multiple potentially deleterious mutations; (3) the substrate luciferin was absent, based on the lack of luciferin blue autofluorescence and the absence of luciferin derived metabolites; (4) although the cells possessed scintillons, the vesicles that contain the luminescent chemistry, electron microscopy revealed additional scintillon-like vesicles with an atypical internal structure; (5) cells isolated from the California coast were 43% smaller in size than bioluminescent cells from the Gulf of Mexico. Phylogenetic analyses based on the large subunit of rDNA did not show divergence of the non-bioluminescent population in relation to other bioluminescent N. scintillans from the Pacific Ocean and Arabian Sea. Our study demonstrates that gene silencing and the lack of the luciferin substrate have resulted in the loss of a significant dinoflagellate functional trait over large spatial scales in the ocean. As the bioluminescence system of dinoflagellates is well characterized, non-bioluminescent N. scintillans is an ideal model to explore the evolutionary and ecological mechanisms that lead to intraspecific functional divergence in natural dinoflagellate populations.

Efficient elimination and re-growth inhibition of harmful bloom-forming cyanobacteria using surface-functionalized microbubbles

The elimination of cyanobacteria is frequently required for treating and controlling the waters with harmful algal blooms. In this study, an improved flotation technology was developed using colloidal gas aphrons (CGAs) surface-modified with the inorganic coagulant of polyaluminum chloride (PACl); the Microcystis aeruginosa (M. aeruginosa) cells were efficiently removed and their re-growth was effectively inhibited. The so-created coagulative CGAs (CCGAs) exhibited the attractive characteristics of both CGAs and PACl for the cell removal. The experimental results clearly showed that 94.2–99.2% of cells were removed within 3 min at the optimum dosage of cetyltrimethyl ammonium bromide (CTAB) and PACl at three different initial cell densities (OD680 = 0.05, 0.26 and 0.76); and the re-growth of M. aeruginosa did not occur in 10 days. The flocs derived from the CCGA-flotation were of smaller size and looser configuration in contrast with those obtained from coagulation-flotation. The CCGAs were robust in charge neutralization, cell capture, cell attack and destruction. Even at low CTAB dosages, those bubbles could provide large surface area for capturing the M. aeruginosa cells in both unicellular and colonial form compared with the unmodified CTAB-CGAs. The CCGAs reduced 59.5–87.9% of CTAB dosage with the assistance of PACl and the required flotation retention time was largely shortened in comparison with the sedimentation and flotation-based treatment options. This would lead to low treatment cost and sludge production. The present work provides a novel insight into the development of flotation technologies for treating and controlling dense harmful algal blooms.

Inherent optical properties and particle characteristics of the sea-surface microlayer

The sea-surface microlayer (SML) is known to have physical, chemical, and biological properties that are distinctly different from the underlying subsurface water (USW). However, only a few studies in the past reported on measurements of the optical properties of the SML and were limited to light absorption. In this study we present results for the main inherent optical properties (IOPs), the spectral absorption coefficients and the volume scattering function, as well as particle size distribution (PSD), from measurements of the SML and USW in contrasting ocean environments with near-surface chlorophyll-a concentration ranging from 0.06 mg m−3 in waters off Hawaiian Islands to 1 mg m−3 in the Santa Barbara Channel. Our observations also included prominent surface slick conditions associated with a dense bloom of Trichodesmium. Significant and highly variable enhancements of the optical properties and particle concentration, including significant changes in the shape of PSD, were observed in the SML compared with USW at all investigated sites. In clear oligotrophic waters the total concentration of particles larger than 0.7 μm in size was enriched in the SML more than 8-fold. In all examined waters the enrichment was consistently higher for larger particles (>10 μm) than smaller particles. The highest enhancement of light absorption coefficients, >100-fold for particulate absorption and >200-fold for phytoplankton absorption in the near-UV and red spectral regions, was observed during the Trichodesmium bloom. In clear oligotrophic waters the particulate absorption coefficient was enhanced by as much as 45-fold in the green spectral region and the non-algal component exhibited consistently higher enhancement than phytoplankton component across the examined spectrum. In contrast to absorption, the volume scattering function was enhanced more in clear oligotrophic waters (>15-fold at scattering angles ψ between about 65° and 80°) than in the situation of Trichodesmium bloom. With the exception of Trichodesmium bloom, we consistently observed significantly lower values of the degree of linear polarization of light scattered by suspended particles and whole seawater samples (by as much as 30% in the vicinity of ψ = 90°) in the SML compared with USW. This result indicates that the SML can have important effect on the state of polarization of downwelling light entering the ocean and upwelling light leaving the ocean across the air-sea interface. The determinations of IOPs in the SML can extend the capabilities for characterizing constituents of microlayer and provide useful information for radiative transfer and remote-sensing related studies.

Quantitative Proteomic Analysis Reveals Novel Insights into Intracellular Silicate Stress-Responsive Mechanisms in the Diatom Skeletonema dohrnii.

Diatoms are a successful group of marine phytoplankton that often thrives under adverse environmental stress conditions. Members of the Skeletonema genus are ecologically important which may subsist during silicate stress and form a dense bloom following higher silicate concentration. However, our understanding of diatoms’ underlying molecular mechanism involved in these intracellular silicate stress-responses are limited. Here an iTRAQ-based proteomic method was coupled with multiple physiological techniques to explore distinct cellular responses associated with oxidative stress in the diatom Skeletonema dohrnii to the silicate limitation. In total, 1768 proteins were detected; 594 proteins were identified as differentially expressed (greater than a two-fold change; p < 0.05). In Si-limited cells, downregulated proteins were mainly related to photosynthesis metabolism, light-harvesting complex, and oxidative phosphorylation, corresponding to inducing oxidative stress, and ROS accumulation. None of these responses were identified in Si-limited cells; in comparing with other literature, Si-stress cells showed that ATP-limited diatoms are unable to rely on photosynthesis, which will break down and reshuffle carbon metabolism to compensate for photosynthetic carbon fixation losses. Our findings have a good correlation with earlier reports and provides a new molecular level insight into the systematic intracellular responses employed by diatoms in response to silicate stress in the marine environment.

Urea Is Both a Carbon and Nitrogen Source for Microcystis aeruginosa: Tracking 13C Incorporation at Bloom pH Conditions.

The use of urea as a nitrogenous fertilizer has increased over the past two decades, with urea itself being readily detected at high concentrations in many lakes. Urea has been linked to cyanobacterial blooms as it is a readily assimilated nitrogen (N) - source for cyanobacteria that possess the enzyme urease. We tested the hypothesis that urea may also act as a carbon (C) source to supplemental growth requirements during the alkaline conditions created by dense cyanobacterial blooms, when concentrations of dissolved CO2 are vanishingly low. High rates of photosynthesis markedly reduce dissolved CO2 concentrations and drive up pH. This was observed in Lake Erie during the largest bloom on record (2015) over long periods (months) and short periods (days) of time, suggesting blooms experience periods of CO2-limitation on a seasonal and daily basis. We used 13C-urea to demonstrate that axenic cultures of the model toxic cyanobacterium, Microcystis aeruginosa NIES843, assimilated C at varying environmentally relevant pH conditions directly into a spectrum of metabolic pools during urea hydrolysis. Primarily, 13C from urea was assimilated into central C metabolism and amino acid biosynthesis pathways, including those important for the production of the hepatotoxin, microcystin, and incorporation into these pathways was at a higher percentage during growth at higher pH. This corresponded to increased growth rates on urea as the sole N source with increasing pH. We propose this ability to incorporate C from urea represents yet another competitive advantage for this cyanobacterium during dense algal blooms.