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The consequences of climate change may directly or indirectly impact the marine biosphere. Although ocean stratification has been recognized as one of the crucial consequences of ocean warming, its impacts on several critical aspects of marine microbes remain largely unknown in the Indian Ocean. Here, we investigate the effects of water stratification, in both surface and subsurface layers, on hydrogeographic parameters and bacterioplankton diversity within the equatorial eastern Indian Ocean (EIO). Strong stratification in the upper 200 m of equatorial EIO was detected with evidential low primary productivity. The vertical bacterioplankton diversity of the whole water columns displayed noticeable variation, with lower diversity occurring in the surface layer than in the subsurface layers. Horizontal heterogeneity of bacterioplankton communities was also in the well-mixed layer among different stations. SAR11 and Prochlorococcus displayed uncharacteristic low abundance in the surface water. Some amplicon sequence variants (ASVs) were identified as potential biomarkers for their specific depths in strongly-stratified water columns. Thus, barriers resulting from stratification are proposed to function as an ‘ASV filter’ to regulate the vertical bacterioplankton community diversity along the water columns. Overall, our results suggest that the effects of stratification on the structure and diversity of bacterioplankton can extend up to the bathypelagic zone in the strongly-stratified waters of the equatorial EIO. This study provides the first insight into the effect of stratification on the subsurface microbial communities in the equatorial eastern Indian Ocean.

The phytoplankton sinking rate in the eastern Indian Ocean was measured during spring 2017 based on the SETCOL method. The range of phytoplankton sinking rates was −0.291 to 2.188 md−1, with an average of 0.420 ± 0.646 md−1. The phytoplankton sinking rate in the Equator (EQ) and the eastern boundary of the Indian Ocean near Sumatra (EB) was lower than that in the Bay of Bengal (BOB). The sinking rate above 100 m was low and increased rapidly below 100 m in all the three regions. The phytoplankton community composition had an important impact on the phytoplankton sinking rate in the east Indian Ocean. The strong stratification in BOB resulted in Trichodesmium spp. bloom and a lower phytoplankton diversity and evenness in BOB, while the phytoplankton in the deep layer are senescent cells that sink down from the upper layer and cannot actively regulate the state of the cells, resulting in a higher sinking rate. Depth and temperature have a great impact on the physiological state of phytoplankton. The sinking rate of phytoplankton depend on the dominant groups composing the phytoplankton community. For the eastern Indian Ocean, seawater stratification caused by temperature changes the distribution of nutrients in the upper layer, and phytoplankton are affected by temperature and nutrients, resulting in changes in community structure, and finally showing different subsidence characteristics.

Picophytoplankton (pico) in the eastern Indian Ocean (EIO) were investigated during the inter-monsoon periods. They were found to typically comprise Prochlorococcus (Pro), Synechococcus (Syn), and Picoeukaryotes (PEuks). In the survey area, the pico showed two different vertical distribution patterns in different regions, whereby the Syn abundance decreased with depth, whereas those of Pro and PEuks increased and then decreased with depth, with the maximum depths ranging from 50 to 100 m. The cell abundance and community structure of the pico were similar at the equator (EQ) and the eastern boundary of the Indian Ocean near Sumatra (EB), but the pico cell abundance was significantly lower in the Bay of Bengal (BOB). Pro dominated most regions of the entire EIO and were approximately one-to-two orders of magnitude more abundant than Syn and PEuks. The distributions of Syn and PEuks showed little difference across various regions. Influenced by the physicochemistry of circulation and water masses, there were many different environmental factors in the different regions. The abundance of pico domination by Pro showed a strong positive correlation with the nutrients and salinity in the survey area, indicating increasing nutrient availability, particularly in the oligotrophic EIO. Generalized additive models (GAMs) analysis showed the differences in their responses to environmental variability. Pro and PEuks both increased strongly with warming up to below 26 °C, and Pro and PEuks were more responsive to chemical (nutrient) variability. Syn showed a broader tolerance of low-salinity conditions. In a certain range, an increase in nitrite and nitric acid can improve the cell abundance of Pro. As a significant contributor to primary productivity in oligotrophic waters, this study provides essential information for studying pico communities in the EIO and its adjacent marine ecosystems.

Currently, there is scant information about the biodiversity and functional diversity of microbes in the eastern Indian Ocean (EIO). Here, we used a combination of high-throughput sequencing of 16S rRNA genes and a metagenomic approach to investigate the microbial population structure and its metabolic function in the equatorial EIO. Our results show that Cyanobacterial Prochlorococcus made up the majority of the population. Interestingly, there were fewer contributions from clades SAR11 (Alphaproteobacteria) and SAR86 (Gammaproteobacteria) to microbial communities than contributions from Prochlorococcus. Based on functional gene analysis, functional genes rbcL, narB, and nasA were relatively abundant among the relevant genes. The abundance of Prochlorococcus implies its typically ecological adaptation in the local ecosystem. The microbial metabolic potential shows that in addition to the main carbon fixation pathway Calvin cycle, the rTCA cycle and the 3-HP/4-HB cycle have potential alternative carbon fixation contributions to local ecosystems. For the nitrogen cycle, the assimilatory nitrate and nitrite reduction pathway is potentially the crucial form of nitrogen utilization; unexpectedly, nitrogen fixation activity was relatively weak. This study extends our knowledge of the roles of microbes in energy and resource cycling in the EIO and provides a foundation for revealing profound biogeochemical processes driven by the microbial community in the ocean.

The consequences of climate change may directly or indirectly impact the marine biosphere. Although ocean stratification has been recognized as one of the crucial consequences of ocean warming, its impacts on several critical aspects of marine microbes remain largely unknown in the Indian Ocean. Here, we investigate the effects of water stratification, in both surface and subsurface layers, on hydrogeographic parameters and bacterioplankton diversity within the equatorial eastern Indian Ocean (EIO). Strong stratification in the upper 200 m of equatorial EIO was detected with evidential low primary productivity. The vertical bacterioplankton diversity of the whole water columns displayed noticeable variation, with lower diversity occurring in the surface layer than in the subsurface layers. Horizontal heterogeneity of bacterioplankton communities was also in the well-mixed layer among different stations. SAR11 and Prochlorococcus displayed uncharacteristic low abundance in the surface water. Some amplicon sequence variants (ASVs) were identified as potential biomarkers for their specific depths in strongly-stratified water columns. Thus, barriers resulting from stratification are proposed to function as an 'ASV filter' to regulate the vertical bacterioplankton community diversity along the water columns. Overall, our results suggest that the effects of stratification on the structure and diversity of bacterioplankton can extend up to the bathypelagic zone in the strongly-stratified waters of the equatorial EIO. This study provides the first insight into the effect of stratification on the subsurface microbial communities in the equatorial eastern Indian Ocean.

This study assessed the biodiversity of sponges within the Integrated Marine and Coastal Regionalisation for Australia (IMCRA) bioregions of the Pilbara using datasets amalgamated from the Western Australian Museum and the Atlas of Living Australia. The Pilbara accounts for a total of 1164 Linnean and morphospecies. A high level of “apparent endemism” was recorded with 78% of species found in only one of six bioregions, with less than 10% confirmed as widely distributed. The Ningaloo, Pilbara Nearshore and Pilbara Offshore bioregions are biodiversity hotspots (>250 species) and are recognised as having the highest conservation value, followed by North West Shelf containing 232 species. Species compositions differed between bioregions, with those that are less spatially separated sharing more species. Notably, the North West Province bioregion (110 species) exhibited the most distinct species composition, highlighting it as a unique habitat within the Pilbara. While sponge biodiversity is apparently high, incomplete sampling effort for the region was identified, with only two sampling events recorded for the Central West Transition bioregion. Furthermore, only 15% of records in the dataset are presently described (Linnean) species, highlighting the continuing need for taxonomic expertise for the conservation and management of marine biodiversity resources.

The phytoplankton sinking rate in the eastern Indian Ocean was measured during spring 2017 based on the SETCOL method. The range of phytoplankton sinking rates was -0.291 to 2.188 md -1 , with an average of 0.420 ± 0.646 md -1 . The phytoplankton sinking rate in the Equator (EQ) and the eastern boundary of the Indian Ocean near Sumatra (EB) was lower than that in the Bay of Bengal (BOB). The sinking rate above 100 m was low and increased rapidly below 100 m in all the three regions. The phytoplankton community composition had an important impact on the phytoplankton sinking rate in the east Indian Ocean. The strong stratification in BOB resulted in Trichodesmium spp. bloom and a lower phytoplankton diversity and evenness in BOB, while the phytoplankton in the deep layer are senescent cells that sink down from the upper layer and cannot actively regulate the state of the cells, resulting in a higher sinking rate. Depth and temperature have a great impact on the physiological state of phytoplankton. The sinking rate of phytoplankton depend on the dominant groups composing the phytoplankton community. For the eastern Indian Ocean, seawater stratification caused by temperature changes the distribution of nutrients in the upper layer, and phytoplankton are affected by temperature and nutrients, resulting in changes in community structure, and finally showing different subsidence characteristics.