Blooms of diatom and dinoflagellate associated with nutrient imbalance driven by cycling of nitrogen and phosphorus in anaerobic sediments in Johor Strait (Malaysia)

Analysis of the causes of N/P imbalance in mangrove water caused by high elevation shrimp ponds

Due to its unique estuarine location at the junction of land and sea, mangrove wetlands are surrounded by numerous high-elevation shrimp ponds. The high-elevation shrimp ponds around the mangrove forest undergo 2.3 clearances by quicklime (CaO) disinfectant per year in China, but the impact of the quicklime disinfectant used and emitted on the mangrove wetland ecosystem is seriously underestimated. Due to the relatively limited data provided by high-elevation shrimp pond aquaculture in estuarine areas for the mangrove ecosystem, this study established an algorithm for calculating the reaction rate of quicklime disinfectants used in high-elevation shrimp pond aquaculture, which is the fundamental reason for the imbalance of N/P ratio in mangrove wetlands. Results showed that the amount of Ca(OH)2 produced by quicklime during the initial cleaning of the shrimp pond was 1303.4 t/a. The annual consumption of Ca(OH)2 by organic acids, strong chlorine disinfectants, and TP in the marine system was 154.6-171.5 t, 1.7 t, and < 284.5 t, respectively. The lack of phosphorus and the imbalance of N/P ratio caused by quicklime disinfectants may be a factor in the changes of mangrove wetlands and surrounding nearshore waters, the growth and decline of marine species, and even global changes.

Temporal bloom dynamics of the marine dinoflagellate Tripos furca in the Penang Strait

The dinoflagellate Tripos furca, known for its frequent and massive blooms in coastal waters, has been associated with significant fish mortality in aquaculture areas. In mid-May 2022, a notable bloom event, characterized by intense red discoloration, was observed along the Penang Strait in the northern Malacca Strait. Our field survey identified a high-density bloom of T. furca. To investigate the mechanisms driving the bloom dynamics of this species, monthly sampling was undertaken until the bloom subsided, covering 19 stations across the Penang Strait. Our results showed that the abundances of T. furca changed over time and space, a bloom peak of 8.2 × 105 cells l-1 was observed in late June, triggered by elevated sea surface temperatures and phosphate availability, while nitrogen was consistently abundant. The bloom's persistence was associated with the influence of the 2020-2022 La Niña and Indian Ocean Dipole, which caused warmer sea temperatures. Metabarcoding of the V7-V9 18S rDNA region revealed high intraspecific genetic diversity within the T. furca bloom subpopulations, suggesting both clonal reproduction and possible sexual processes. The bloom termination was linked to a seasonal shift in temperatures and changes in nutrient regimes that caused a transition of phytoplankton compositions to Noctiluca- and diatom-dominated populations contributed to the bloom's decline. Early detection of the bloom has successfully prevented severe losses to the aquaculture farms in the area, emphasizing the importance of early intervention. This study also enhances our understanding of T. furca bloom dynamics and provides insights into managing harmful algal blooms in tropical coastal regions.

Phytoplankton-environment dynamics in a tropical estuary of the northeastern Arabian Sea: a Generalized Additive Model (GAM) approach

Phytoplankton are diverse photosynthetic organisms in estuarine ecosystems and sensitive indicators of environmental changes. This study employed Generalized Additive Model (GAM) to explore the impact of environmental variables on the abundance of six dominant phytoplankton species in the tropical Karanja estuary, India. Data were collected from five sampling stations between January 2022 and March 2023. The GAM model explained ≥ 55% of the variability in species distribution, predicting that the Karanja estuary provides a suitable habitat for these phytoplankton. Spatiotemporal predictions revealed higher abundances of Asterionellopsis glacialis, Coscinodiscus sp., Pseudo-nitzschia pungens, Skeletonema costatum and Thalassionema frauenfeldii during post-monsoon and lower in pre-monsoon. Conversely, Odontella sinensis thrived during pre-monsoon. Optimal growth conditions included a water temperature range of 24-32 °C for O. sinensis, P. pungens and S. costatum, while chlorophyll-b concentrations between 1 and 20 mg/m3 favored A. glacialis, O. sinensis, P. pungens, S. costatum and T. frauenfeldii. P. pungens, S. costatum and T. frauenfeldii exhibited broad salinity tolerance ranging from 10-40‰. Additionally, Coscinodiscus sp., P. pungens and T. frauenfeldii were found in environments with nitrate content of 0.1-1 mg/L, while O. sinensis preferred silicate levels of 1-10 mg/L. Notably, these species serve as indicators of eutrophication, providing insights into ecosystem health. This study represents the first application of GAM for exploring phytoplankton-environment interactions in India, offering critical data for water quality management, pollution control and food web preservation. Policymakers can use these findings to develop evidence-based regulations that support proactive coastal management, enhance estuarine resilience to climate stressors and ensure sustainable resource conservation.

Phycospheric bacteria limits the effect of nitrogen and phosphorus imbalance on diatom bloom

Human activities have caused an imbalance in the input nitrogen and phosphorus (N/P) in the biosphere. The imbalance of N/P is one of the characteristics of water eutrophication, which is the fundamental factor responsible for the blooms. The effects of the N/P imbalance on diatom and phycospheric bacteria in blooms are poorly understood. In this study, the N/P molar ratio in real water (14:1) and the predicted N/P molar ratio in future water (65:1) were simulated to analyze the response of Cyclotella sp. and phycospheric bacteria to the N/P imbalance. The results showed that the N/P imbalance inhibited the growth of Cyclotella sp., but prolonged diatom bloom duration. The resistance of Cyclotella sp. to the N/P imbalance is related to phycospheric bacteria, and there are dynamic regulatory mechanisms within the phycospheric bacteria community to resist the N/P imbalance: (1) the increase of HNA bacterial density, the decrease of LNA bacterial density, (2) the increase of phycospheric bacterial diversity and eutrophic bacteria abundance, and the change of denitrifying bacteria abundance, (3) the activity of nitrogen and phosphorus metabolism of HNA bacteria enhanced, while that of LNA bacteria decreased. And the gene hosts of nitrogen and phosphorus metabolism were most enriched in Proteobacteria, indicating that Proteobacteria played an important role in maintaining the stability of phycospheric bacteria and was the dominant phylum resistant to the N/P imbalance. This study clarified that the algal-bacteria system was resistant to the N/P imbalance and implied that the N/P imbalance had little effect on the occurrence of diatom bloom events due to the presence of phycospheric bacteria.

Environmental DNA metabarcoding reveals diverse phytoplankton assemblages and potentially harmful algal distribution along the urban coasts of Türkiye

Marine phytoplankton are widely used to monitor the state of the water column due to their rapid changes in response to environmental conditions. In this study, we aimed to investigate the coastal phytoplankton assemblages, including bloom-forming species using high-throughput sequencing of 18S rRNA genes targeting the V4 region and their relationship with environmental variables along the Istanbul coasts of the Sea of Marmara. A total of 118 genera belonging to six phyla were detected. Among them, Dinoflagellata (36) and Bacillariophyta (26) were represented with the highest number of genera. According to the relative abundance of DNA reads, the most abundant taxa were Dinoflagellata_phylum (18.1%), Emiliania (8.4%), Biecheleria (8.4), and Noctiluca (8.1%). The ANOSIM test showed that there was a significant temporal difference in the assemblages, while the driving environmental factors were pH, water temperature, and salinity. According to the TRIX index, the trophic state of the coasts was highly mesotrophic and eutrophic. In addition, 45 bloom-forming and HAB taxa were detected and two species of Noctiluca and Emiliania, which frequently cause blooms in the area, were recorded in high abundance. Our results provide insight into the phytoplankton assemblages along the urbanized coastlines by analysing the V4 region of 18S rRNA. This data can support future studies that use both traditional methods and metabarcoding, employing various primers and targeting different genes and regions.

Nutrient Deficiencies Impact on the Cellular and Metabolic Responses of Saxitoxin Producing Alexandrium minutum: A Transcriptomic Perspective

Dinoflagellate Alexandrium minutum Halim is commonly associated with harmful algal blooms (HABs) in tropical marine waters due to its saxitoxin production. However, limited information is available regarding the cellular and metabolic changes of A. minutum in nutrient-deficient environments. To fill this gap, our study aimed to investigate the transcriptomic responses of A. minutum under nitrogen and phosphorus deficiency. The induction of nitrogen and phosphorus deficiency resulted in the identification of 1049 and 763 differently expressed genes (DEGs), respectively. Further analysis using gene set enrichment analysis (GSEA) revealed 702 and 1251 enriched gene ontology (GO) terms associated with nitrogen and phosphorus deficiency, respectively. Our results indicate that in laboratory cultures, nitrogen deficiency primarily affects meiosis, carbohydrate catabolism, ammonium assimilation, ion homeostasis, and protein kinase activity. On the other hand, phosphorus deficiency primarily affects the carbon metabolic response, cellular ion transfer, actin-dependent cell movement, signalling pathways, and protein recycling. Our study provides valuable insights into biological processes and genes regulating A. minutum's response to nutrient deficiencies, furthering our understanding of the ecophysiological response of HABs to environmental change.

Frequent pulse disturbances shape resistance and resilience in tropical marine microbial communities

The Johor Strait separates the island of Singapore from Peninsular Malaysia. A 1-kilometer causeway built in the early 1920s in the middle of the strait effectively blocks water flowing to/from either side, resulting in low water turnover rates and build-up of nutrients in the inner Strait. We have previously shown that short-term rather than seasonal environmental changes influence microbial community composition in the Johor Strait. Here, we present a temporally-intensive study that uncovers the factors keeping the microbial populations in check. We sampled the surface water at four sites in the inner Eastern Johor Strait every other day for two months, while measuring various water quality parameters, and analysed 16S amplicon sequences and flow-cytometric counts. We discovered that microbial community succession revolves around a common stable state resulting from frequent pulse disturbances. Among these, sporadic riverine freshwater input and regular tidal currents influence bottom-up controls including the availability of the limiting nutrient nitrogen and its biological release in readily available forms. From the top-down, marine viruses and predatory bacteria limit the proliferation of microbes in the water. Harmful algal blooms, which have been observed historically in these waters, may occur only when there are simultaneous gaps in the top-down and bottom-up controls. This study gains insight into complex interactions between multiple factors contributing to a low-resistance but high-resilience microbial community and speculate about rare events that could lead to the occurrence of an algal bloom.