Myeik University

The Bay of Bengal (BoB) and adjacent seas receive substantial freshwater inputs, leading to strong stratification and nutrient limitation, along with frequent mesoscale eddies. These processes create significant physicochemical heterogeneity, which is presumed to regulate the diazotrophic community and nitrogen fixation rates (NFRs), though the underlying mechanisms remain unclear. This study examined diazotrophs and size-fractionated (>10 mu m and <10 mu m) NFRs and primary production (PP) across BoB, Andaman Sea (AS), and equatorial Indian Ocean (EIO) during March-April 2021. Surface abundances of Trichodesmium and Richelia/Calothrix, as well as NFRs, were significantly higher in AS than in BoB and EIO. The southern AS, likely enriched by terrestrial iron and phosphorus, sustained Trichodesmium blooms (>10,000 trichomes L-1) and elevated NFRs (up to 15.33 nmol N L-1 d(-1)). Based on the Redfield C/N ratio, NFRs were estimated to supply 1.3-21.3% of the nitrogen required for surface PP. nifH amplicon sequencing revealed Trichodesmium dominance in the iron-replete AS and BoB, whereas Proteobacteria prevailed in the iron-depleted EIO. Size-fractionated NFRs experiments indicated that the >10 mu m fraction dominated in AS and BoB, whereas the <10 mu m fraction contributed more in EIO. Anti-cyclonic eddies hosted higher Trichodesmium abundances and NFRs compared to a cyclonic eddy and the Wyrtki Jets. Redundancy analysis and generalized additive models demonstrated that diazotrophic composition and NFRs were closely associated with salinity, temperature, and phosphate availability. These findings highlight that spatial variability in diazotrophic communities and NFRs across the BoB and adjacent waters is largely regulated by terrestrial inputs and mesoscale eddy dynamics.

Pyropia is a genus comprising red algae of the Bangiaceae family that is commonly found in intertidal zones worldwide. However, understanding of Pyropia species that are prone to tropical regions remains limited despite recent breakthroughs in genomic research. Within the realm of Pyropia s pecies thriving in tropical regions, P. vietnamensis stands out as a widely recognized species. In this study, we aimed to investigate Pyropia species in the southwest coast of Myanmar using physiological and molecular approaches, culture-based analyses, chloroplast rbcL and nuclear SSU gene sequencing, and whole chloroplast and mitochondrial genome sequencing. Physiological analysis showed that the Myanmar samples were more heat-tolerant than their Japanese counterparts, including those of subtropical origin. Additionally, molecular characterization revealed that the Myanmar samples were closely related to P. vietnamensis from India. This study is the first to sequence the chloroplast and mitochondrial genomes of Pyropia species from tropical regions. A unique deletion event was observed within a ribosomal RNA gene cluster in the chloroplast genome of the studied Pyropia species, which is a deviation from the usual characteristics of most Pyropia species. This study improves current understanding of the physiological and molecular characteristics of this comparatively understudied Pyropia species that grows in tropical regions.

The dissolved organic matter (DOM) in the ocean is a large carbon pool that plays an important role in the global carbon cycle. Investigation of the characteristics and behaviors of DOM in some areas facilitates a better understanding of biochemical processes in the water column. In February 2020, water samples were collected to characterize the distribution and optical properties of DOM from the Northern Andaman Sea and the Northeastern Bay of Bengal. A high dissolved organic carbon (DOC) concentration appeared in the Northern Andaman Sea, which is strongly affected by the DOM contents of the Irrawaddy and Salween Rivers. A barrier layer that resulted from the freshwater input was observed above the thermocline. This layer was believed to have had a consistent effect on the concentration of DOM in the surface water and contributed to the differences in DOC concentration on each side of the Preparis Channel. Based on the fluorescence excitation emission matrix and parallel factor analysis, four fluorescent components were identified, including three humic-like components (C1, C2, and C4 with their maximal Ex/Em at ≤240/418 nm, 315/384 nm, and 270 (360)/442 nm and represented as peaks A, M, and A+C, respectively) and one protein-like component (C3 with maximal Ex/Em at 275/334 nm and represented as peak T). The humic-like components were mainly derived from terrestrial inputs. Low levels of humic-like components were confined in the upper water, with strong photodegradation in the euphotic zone. C3 was a typical tryptophan-like component that represented freshly produced autochthonous DOM. The correlation between C3 and biological index and apparent oxygen utilization suggested that it was highly bioavailable and not easy to be preserved in the deep sea. Overall, our results showed the distributions of DOM in the Northern Andaman Sea and the Northeastern Bay of Bengal and revealed the behaviors and controlling factors for DOM in the upper water.

This study explores the spatial structure and transport characteristics of eddies in the Arabian Sea (AS) using Argo profiles and satellite measurements. The majority of eddies occur in the northern AS, especially along its northeastern boundary. In contrast, the western AS had a relatively higher eddy kinetic energy compared to the eastern part. Particularly, the strongest energetic eddies were present in the Somali Current system. The composite results revealed the evident thermohaline anomalies caused by cyclonic eddies (CEs) and anticyclonic eddies (AEs) in the upper 300 m layers. The anomalous temperature structure within CEs and AEs showed a dominant dipole structure in the near-surface layer and a monopole structure below, with maximum temperature anomalies of approximately −0.8°C and +1.0°C located at depths of 100–150 m, respectively. The composited salinity structures for CEs and AEs exhibited monopole vertical structures and sandwich-like patterns. For AEs, large positive salinity anomalies occurred at subsurface layers of 60–180m with a peak value of about 0.07, and weak negative values were observed above 60m and below 180m. A similar vertical structure but with an opposite sign operates for CEs. The composited CE and AE caused an equatorward salt flux with values of −8.1 × 10 4 and −2.2 × 10 4 kg s −1 , respectively. CEs caused an equatorward heat flux of −7.7 × 10 11 W, and AEs induced a poleward flux of 1.5×10 11 W.

This study investigated the reproductive cycle of the edible oyster, Crassostrea belcheri, harvested originally from creeks near Pedaing village in the southern part of Myanmar in September 2017-October 2018 through histological analysis. Five maturity stages were detected in the C. belcheri gonads: indifferent, developing, mature, spawning, and spent. The overall sex ratio of males to females of 1:0.98 showed no statistically significant difference from the expected sex ratio of 1:1 in any month. Hermaphrodites could not be observed in 250 oysters. Moreover, the quantitative monthly gonad index values varied from 1.5 in November and June to 2.75 in March. Histological analysis of C. belcheri revealed that mature male and female gonads were detected in almost all months, except in November-December 2017 and June 2018. Spawning occurred throughout the study period, except in January-March 2018, peaking at the beginning and end of the dry season in October-November and April-May, respectively.