Nitrogen liberated via allelopathy can promote harmful algal blooms.

Allelopathy is a biological mechanism that can promote harmful algal blooms (HAB) via the inhibition of sympatric phytoplankton. While nutrient loading can also promote HABs, the ability of allelopathy to stimulate HABs via the regeneration of nutrients has yet to be explored. To examine the impacts of allelopathically liberated N on HAB species, a series of experiments were performed using multiple allelopathic HAB species including the dinoflagellates Alexandrium catenella and Margalefidinium polykrikoides, and the pelagophyte, Aureoumbra lagunensis. These HAB species were paired with the cosmopolitan dinoflagellate, Akashiwo sanguinea, that was labeled with 15NO3- or 15NH4+, allowing the release and transfer of N to be traced as a time course during allelopathic interactions. During all experiments, the allelopathic inhibition of Akashiwo was accompanied by increases in cell densities, growth rates, and the δ15N content of the HAB species, evidencing the transfer of N liberated from Akashiwo. The cellular transfer of 15N and release of dissolved N was higher when Akashiwo was grown with 15NO3- compared to 15NH4+ suggesting a differential subcellular-compartmentalization of N sources. Regardless of the type of N, HAB species obtained 60 - 100% of their cellular N from lysed Akashiwo cells and there was an enrichment of the δ15N content of the dissolved NH4+ pool post-lysis of Akashiwo. Collectively, the results demonstrate that beyond facilitating species succession, allelopathy can supply HABs with N and, therefore, is likely important for promoting and sustaining HABs. Given that allelopathy is known to be a dose-dependent process, allelopathy may induce a positive feedback loop, whereby competitors are lysed, N is liberated, HABs are intensified and, in turn, become more strongly allelopathic.

Coupling and decoupling of marine stramenopiles and cyanobacteria in eutrophic coastal waters of Korea.

The spatiotemporal distribution of MASTs (MArine STramenopiles), mostly affiliated with heterotrophic protists, and their interactions with Synechococcales were investigated in an anthropogenically polluted bay of the East Sea using 18S rRNA and 16S rRNA gene sequences. The bay was characterized by strong stratification between the surface and bottom layers and cold and nutrient-rich water intrusion in summer, whereas the bay water was well mixed in winter. MAST-3, MAST-6, MAST-7, and MAST-9 were the major MAST clades, whereas the dominance of MAST-9 declined from >80 % in summer to <10 % in winter and the diversity of MAST communities increased in winter. Co-occurrence network analysis via the sparse partial least squares revealed that MAST-3 had a Synechococcales-specific interaction during the study periods but prey-specific interactions with other MAST clades were not detected. Temperature and salinity markedly influenced the relative abundance of major MAST clades. The relative abundance of MAST-3 increased at temperatures above 20 °C and salinities above 33 ‰, however, the abundance of MAST-9 decreased under the same conditions. Analysis of the metabolic functions of cyanobacteria using the FAPROTAX (Functional Annotation of Prokaryotic Taxa) indicated that the response of photosynthetic cyanobacteria to NH4+ and PO43- was significant in summer but these functions were not tightly coupled with the abundance of Synechococcales. Similarly, strong associations of MAST-3 with high temperature/salinity and Synechococcales were indicative of coupled cascading during bottom-up processes. However, other major MAST clades were likely decoupled with Synechococcales and environmental conditions in which cyanobacteria can thrive. Therefore, our results demonstrated that MAST communities can couple or decouple with environmental variables and potential prey depending on MAST clades. Collectively, our findings provide novel insights into the role of MAST communities in microbial food webs in eutrophic coastal waters.

Coastal warming heightens direct impacts of seawater temperature on nutrients near aquaculture farms in Korea.

The potential roles of temperature and phytoplankton in nutrient cycling throughout the water column were investigated nearby aquaculture farms. Using the convergent cross mapping (CCM), we examined the relative strength of phytoplankton and temperature effects on nutrients. High δ15N values of particulate organic matter in the inner bay were detected compared to those in the outer bay. δ15N values >5 ‰ throughout the bay indicate that nitrogen influxes from the aquaculture farms are the critical nitrogen source in the study region. Our CCM models revealed that temperature positively and strongly affected the potential regeneration of nutrients, associated with PO43- while phytoplankton utilized nutrients as soon as available. The temperature-driven nutrient regeneration was higher in the bottom layer than that in the surface layer, indicating that temperature was a more important controlling factor in nutrient fluxes from the surface sediments.

Reduced forms of nitrogen control the spatial distribution of phytoplankton communities: The functional winner, dinoflagellates in an anthropogenically polluted estuary.

The effects of reduced forms of nitrogen (NH4+ and dissolved organic nitrogen (DON)) on the spatial distribution of diatoms and dinoflagellates in an estuarine-coastal water continuum were investigated from 2015 to 2019. The proportion of non-DIN in total nitrogen was utilized as an indicator of DON along with direct measurements of DON. While NO3- originated from Seomjin River, the abundant NH4+ and DON occurred from Gwangyang Bay through Namhae. Diatoms were mostly confined to the upper estuarine system and dinoflagellates dominated in the regions with high levels of NH4+ and DON. Generalized additive models also presented the different responses of diatoms and dinoflagellates to increases in NH4+ and DON. Thus, our results highlight that diatoms dominate in NO3--replete water with full access to the source and dinoflagellates take over the ecologically open niche in an anthropogenically polluted estuary with full access to reduced forms of nitrogen.

Metatranscriptome Library Preparation Influences Analyses of Viral Community Activity During a Brown Tide Bloom.

There is growing interest in the use of metatranscriptomics to study virus community dynamics. We used RNA samples collected from harmful brown tides caused by the eukaryotic alga Aureococcus anophagefferens within New York (United States) estuaries and in the process observed how preprocessing of libraries by either selection for polyadenylation or reduction in ribosomal RNA (rRNA) influenced virus community analyses. As expected, more reads mapped to the A. anophagefferens genome in polyadenylation-selected libraries compared to the rRNA-reduced libraries, with reads mapped in each sample correlating to one another regardless of preprocessing of libraries. Yet, this trend was not seen for reads mapping to the Aureococcus anophagefferens Virus (AaV), where significantly more reads (approximately two orders of magnitude) were mapped to the AaV genome in the rRNA-reduced libraries. In the rRNA-reduced libraries, there was a strong and significant correlation between reads mappings to AaV and A. anophagefferens. Overall, polyadenylation-selected libraries produced fewer viral contigs, fewer reads mapped to viral contigs, and different proportions across viral realms and families, compared to their rRNA-reduced pairs. This study provides evidence that libraries generated by rRNA reduction and not selected for polyadenylation are more appropriate for quantitative characterization of viral communities in aquatic ecosystems by metatranscriptomics.

Year-to-year variation in phytoplankton biomass in an anthropogenically polluted and complex estuary: A novel paradigm for river discharge influence.

We examined the effects of nutrient availability and turbidity on phytoplankton biomass over 9 years in Gwanyang Bay, Korea, which is an anthropogenically polluted and complex estuary. While dredging and reclamation shaped geochemical features, river discharge with low-turbidity water and sewage treatment plants contributed to nutrient loading. The replete levels of nutrients and short water-residence time suggest the inapplicability of the washout theory, whereas the presence of NH4+ suppressed the growth of phytoplankton. A reduction in the river discharge caused a concomitant decline in the loading and dilution of suspended particles. All these features led to an increase in SPM, light limitation, and NH4+ concentration. GLM estimates revealed negative effects of NH4+ and SPM on chlorophyll a over 9 years while SEM verified synergistic effects of NH4+ and SPM compared with positive effects of NO2 + NO3-. Our findings provide new insights into phytoplankton bloom dynamics in Gwangyang Bay.

Ocean warming since 1982 has expanded the niche of toxic algal blooms in the North Atlantic and North Pacific oceans.

Global ocean temperatures are rising, yet the impacts of such changes on harmful algal blooms (HABs) are not fully understood. Here we used high-resolution sea-surface temperature records (1982 to 2016) and temperature-dependent growth rates of two algae that produce potent biotoxins, Alexandrium fundyense and Dinophysis acuminata, to evaluate recent changes in these HABs. For both species, potential mean annual growth rates and duration of bloom seasons significantly increased within many coastal Atlantic regions between 40°N and 60°N, where incidents of these HABs have emerged and expanded in recent decades. Widespread trends were less evident across the North Pacific, although regions were identified across the Salish Sea and along the Alaskan coastline where blooms have recently emerged, and there have been significant increases in the potential growth rates and duration of these HAB events. We conclude that increasing ocean temperature is an important factor facilitating the intensification of these, and likely other, HABs and thus contributes to an expanding human health threat.

Discovery of a resting stage in the harmful, brown-tide-causing pelagophyte, Aureoumbra lagunensis: a mechanism potentially facilitating recurrent blooms and geographic expansion.

To date, the life stages of pelagophytes have been poorly described. This study describes the ability of Aureoumbra lagunensis to enter a resting stage in response to environmental stressors including high temperature, nutrient depletion, and darkness as well as their ability to revert from resting cells back to vegetative cells after exposure to optimal light, temperature, and nutrient conditions. Resting cells became round in shape and larger in size, filled with red accumulation bodies, had smaller and fewer plastids, more vacuolar space, contained lower concentrations of chl a and RNA, displayed reduced photosynthetic efficiency, and lower respiration rates relative to vegetative cells. Analysis of vegetative and resting cells using Raman microspectrometry indicated resting cells were enriched in sterols within red accumulation bodies and were depleted in pigments relative to vegetative cells. Upon reverting to vegetative cells, cells increased their chl a content, photosynthetic efficiency, respiration rate, and growth rate and lost accumulation bodies as they became smaller. The time required for resting cells to resume vegetative growth was proportional to both the duration and temperature of dark storage, possibly due to higher metabolic demands on stored energy (sterols) reserves during longer period of storage and/or storage at higher temperature (20°C vs. 10°C). Resting cells kept in the dark at 10°C for 7 months readily reverted back to vegetative cells when transferred to optimal conditions. Thus, the ability of Aureoumbra to form a resting stage likely enables them to form annual blooms within subtropic ecosystems, resist temperature extremes, and may facilitate geographic expansion via anthropogenic transport.

Diversity and dynamics of algal Megaviridae members during a harmful brown tide caused by the pelagophyte, Aureococcus anophagefferens.

Many giant dsDNA algal viruses share a common ancestor with Mimivirus--one of the largest viruses, in terms of genetic content. Together, these viruses form the proposed 'Megaviridae' clade of nucleocytoplasmic large DNA viruses. To gauge Megaviridae diversity, we designed degenerate primers targeting the major capsid protein genes of algae-infecting viruses within this group and probed the clade's diversity during the course of a brown tide bloom caused by the harmful pelagophyte,Aureococcus anophagefferens We amplified target sequences in water samples from two distinct locations (Weesuck Creek and Quantuck Bay, NY) covering 12 weeks concurrent with the proliferation and demise of a bloom. In total, 475 amplicons clustered into 145 operational taxonomic units (OTUs) at 97% identity. One OTU contained 19 sequences with ≥97% identity to AaV, a member of the Megaviridae clade that infects A. anophagefferens, suggesting AaV was present during the bloom. Unifrac analysis showed clear temporal patterns in algal Megaviridae dynamics, with a shift in the virus community structure that corresponded to the Aureococcus bloom decline in both locations. Our data provide insights regarding the environmental relevance of algal Megaviridae members and raise important questions regarding their phylodynamics across different environmental gradients.

Resolving DOM fluorescence fractions during a Karenia brevis bloom patch on the Southwest Florida Shelf.

Organic matter can be supplied naturally from land through rivers or produced in-situ in the marine environment. Current methods of examining natural bulk dissolved organic matter (DOM) are not able to discriminate multiple sources of DOM. A diagnostic tool to identify DOM sources is critical to determine possible sources of organic nutrients that influence K. brevis harmful algal bloom (HAB) development. This study applied multi-wavelength fluorescence coupled with a supervised pattern recognition technique (e.g., parallel factorial analysis (PARAFAC) using samples collected from river, estuary and shelf waters where the toxic dinoflagellate Karenia brevis off of Sanibel Island, Florida was observed. The PARAFAC model distinguished four different fractions of DOM components containing humic-like and protein-like components. The derived terrestrial humic-like material was indicative of land-based sources while the tryptophan-like component was likely produced from in-situ biological production. The study developed and tested the hypothesis that the direct relationship of the protein-like DOM fluorescence with K. brevis cell density indicated that the bloom patch was most likely supported by organic nutrients produced in-situ. The results demonstrated that multi-wavelength fluorescence analysis coupled with PARAFAC modeling technique simultaneously resolved DOM fluorescence fractions and their possible sources-information that are critical in explaining harmful algal bloom formation.