Southwest Indian Ocean Research Articles

Microplastics in the insular marine environment of the Southwest Indian Ocean carry a microbiome including antimicrobial resistant (AMR) bacteria: A case study from Reunion Island

The increasing threats to ecosystems and humans from marine plastic pollution require a comprehensive assessment. We present a plastisphere case study from Reunion Island, a remote oceanic island located in the Southwest Indian Ocean, polluted by plastics. We characterized the plastic pollution on the island's coastal waters, described the associated microbiome, explored viable bacterial flora and the presence of antimicrobial resistant (AMR) bacteria. Reunion Island faces plastic pollution with up to 10,000 items/km2 in coastal water. These plastics host microbiomes dominated by Proteobacteria (80 %), including dominant genera such as Psychrobacter, Photobacterium, Pseudoalteromonas and Vibrio. Culturable microbiomes reach 107 CFU/g of microplastics, with dominance of Exiguobacterium and Pseudomonas. Plastics also carry AMR bacteria including β-lactam resistance. Thus, Southwest Indian Ocean islands are facing serious plastic pollution. This pollution requires vigilant monitoring as it harbors a plastisphere including AMR, that threatens pristine ecosystems and potentially human health through the marine food chain.

Thermococcus thermotolerans sp. nov., a hyperthermophilic archaeon isolated from a chimney in the Southwest Indian Ocean.

An anaerobic hyperthermophilic archaeon was isolated from a black smoker chimney with a snail attachment at a water depth of 2 739 m in the Southwest Indian Ocean. The sample was taken from the chimney exterior wall. The enrichment was conducted under a continuous culture with temperature fluctuation of 80-130 °C over 24 h for 42 days at 28 MPa. The isolation was performed at 90 °C at 0.1 MPa. Cells of the isolated strain 813A4T were irregular cocci. Strain 813A4T grew at 60-94 °C (optimal growth at 85 °C) at 0.1 MPa, and growth was detected at up to 99 °C at 28 MPa. At 85 °C, the strain was able to grow at pressures ranging from 0.1 to 110 MPa (optimal pressure, 0.1-40 MPa). At 85 °C, the cells of 813A4T grew at pH 5.5-9 (optimal, pH 7.0) and a NaCl concentration of 1.0-4.0 % (w/v; optimum concentration, 2.5 % NaCl). Strain 813A4T utilized yeast extract, tryptone and peptone as single carbon sources for growth. Elemental sulphur stimulated its growth. The G+C content of the complete genome was 53.48 mol%. Phylogenetic analysis based on 16S rRNA gene sequences revealed that strain 813A4T belonged to the genus Thermococcus, with the highest sequence similarity to Thermococcus barossii SHCK-94T (99.73 %). The average nucleotide identity between strains 813A4T and SHCK-94T was 82.56 %. All these data indicated that strain 813A4T should be classified as representing a novel species of the genus Thermococcus, for which Thermococcus thermotolerans sp. nov. is proposed. The type strain is 813A4T (=JCM 39367T=MCCC M28628T).

High‐frequency variability dominates potential connectivity between remote coral reefs

AbstractCoral larval dispersal establishes connectivity between reefs, but larval fluxes vary over timescales from daily to multidecadal due to oceanographic variability. Using a 2‐km‐resolution ocean model, we simulate daily spawning events from 1993 to 2019 and assess the potential connectivity between all reefs in the tropical southwest Indian Ocean. Although there is a significant seasonal cycle in potential connectivity, day‐to‐day variability generally dominates. Larval dispersal pathways on any particular day provide limited information about the dispersal pathways a few days later. The magnitude of this high‐frequency variability depends on the local geography and oceanography, with small and isolated reefs generally subject to the most variability. Stochastic oceanographic variability introduces considerable uncertainty to dispersal predictions, imposing fundamental limitations on what simulations can tell us about inter‐reef connectivity. Protracted spawning over only a few days can significantly reduce variability associated with the likelihood of a larva settling. The duration of spawning is therefore a more important parameter in modeling coral connectivity than the exact timing of spawning onset. Finally, we find that a small proportion of spawning events account for the majority of settling larvae, particularly at remote islands, and demonstrate that a time‐mean picture of dispersal may be inappropriate for predicting demographic and genetic connectivity. Given the diversity of coral reef environments in the southwest Indian Ocean, we expect that these results will apply to inter‐reef coral connectivity across the tropics more broadly, as well as other weakly swimming reef taxa with the potential for long‐distance dispersal.

First report of Fusarium wilt of Cavendish bananas caused by Fusarium oxysporum f. sp. cubense Tropical Race 4 in the Grande Comoros Island.

Fusarium wilt of banana, caused by the soil-borne pathogen Fusarium oxysporum f. sp. cubense (Foc), is a major constraint to banana production worldwide (Viljoen et al., 2020). Currently, Cavendish bananas are severely affected by Foc Tropical Race 4 (TR4) globally. In Africa, Foc TR4 was first detected in northern Mozambique in 2013 (Viljoen et al., 2020), and has since been found on the island of Mayotte in the Mozambique Channel off the coast of southeastern Africa (Aguayo et al., 2021). In early 2023, severe leaf-yellowing and wilting of Cavendish banana plants was observed on Cavendish banana plants at several small holder farmer properties in Grande Comoros (Ngazidja) including in Ntsinimoipanga (-11,790054 S, 43°25'47,04384 E), Batou (-11,499716 S, 43°21'51,71976''E), Madjeweni (-11,8217 S , 43°16'41''E) and Mdé (-11°41'54'' S, 43°15'20''E). When the pseudostems of these plants were split open, a reddish-brown internal discoloration of the vascular tissue became apparent. Discolored strands of diseased plants were collected, and the causal agent identified using DNA-based techniques, vegetative compatibility group (VCG) analysis and pathogenicity testing. The samples were plated onto potato dextrose agar and single-spored and isolated from individual diseased plants identified as F. oxysporum based on cultural and morphological characteristics. These include the production of white fungal colonies with a purple center, infrequent production of macroconidia, but an abundance of microconidia on short monophialides, and terminal or intercalary chlamydospores (Leslie and Summerell, 2006). Foc TR4 was identified from seven isolates by conventional (Dita et al., 2010) and quantitative-PCR (Matthews et al., 2020), and with loop-mediated isothermal amplification (LAMP) (Ordóñez et al., 2021). All seven isolates were confirmed as members of the VCG 01213/16 complex when nit-1 mutants of the unknown Foc isolates were compatible with Nit-M mutants of the Foc VCG 01213 and VCG 01216 tester strains. Two isolates were then selected for pathogenicity testing, and 2-month-old tissue culture-derived Cavendish plants (cv. Williams) inoculated by using the method described by Ndayihanzamaso et al. (2022). After 4 weeks, the Foc TR4-inoculated plants produced wilting symptoms and internal rhizome discoloration typical of Fusarium wilt. Foc TR4 was reisolated from the inoculated plants and identified by qPCR (Matthews et al. 2020), thereby fulfilling Koch's postulates. These results provide scientific proof of the presence of Foc TR4 in a second island in the Comoros archipelago. Comprehensive surveys will be conducted in all three of the Comoros Islands to assess the presence and impact of Foc TR4 to implement containment strategies. Collaborative initiatives and coordinated actions among growers and other stakeholders are needed to prevent the spread of Foc TR4 to more Southwest Indian Ocean islands and countries on the East African coasts. The importance of banana for food security and livelihoods, and the unique genetic diversity of bananas found on the Comoros islands, requires the eradication and isolation of diseased bananas on the short term, and the screening of local banana varieties for Foc TR4 resistance on the longer term.

Multi‐Year Predictability of Global Sea Surface Temperature Using Model‐Analogs

AbstractThe multi‐year predictability of global sea surface temperature (SST) is examined by applying a model‐analog method to four control simulations to make forecasts at leads of 1–36 months over 1961–2015. The forecasts are found to have skill for annual mean SST at Year 2 (i.e., leads of 13–24 months) or even Year 3 (i.e., leads of 25–36 months) in the tropical Pacific, the North and South Pacific, the southwest Indian Ocean, and the northwestern tropical Atlantic. The seasonality in forecast skill suggests that July is the best time to initialize multi‐year forecasts. The evolution of forecast skill for tropical Pacific SSTs indicates that the predictability of some El Niño Southern Oscillation events is nearly the same at leads of 6 and 18 or 24 months. Further, these El Niño and La Niña events can be predicted at leads of up to 24 and 30 months, respectively.

An ocean front dataset for the Mediterranean sea and southwest Indian ocean

Fronts are ubiquitous discrete features of the global ocean often associated with enhanced vertical velocities, in turn boosting primary production. Fronts thus form dynamical and ephemeral ecosystems where numerous species meet across all trophic levels. Fronts are also targeted by fisheries. Capturing ocean fronts and studying their long-term variability in relation with climate change is thus key for marine resource management and spatial planning. The Mediterranean Sea and the Southwest Indian Ocean are natural laboratories to study front-marine life interactions due to their energetic flow at sub-to-mesoscales, high biodiversity (including endemic and endangered species) and numerous conservation initiatives. Based on remotely-sensed Sea Surface Temperature and Height, we compute thermal fronts (2003–2020) and attracting Lagrangian coherent structures (1994–2020), in both regions over several decades. We advocate for the combined use of both thermal fronts and attracting Lagrangian coherent structures to study front-marine life interactions. The resulting front dataset differs from other alternatives by its high spatio-temporal resolution, long time coverage, and relevant thresholds defined for ecological provinces.

First Report of Diseases and Compromised Health Conditions on Hard Corals around Rodrigues Island, Southwest Indian Ocean

Coral diseases represent a prominent menace to coral reefs and to the associated ecological services they provide to the surrounding coastal communities. Studies on diseases and compromised health conditions (CHCs) on hard corals in the Southwest Indian Ocean (SWIO) are scarce, and their consequences are often overlooked. This study aimed to establish the baseline prevalence of diseases and CHC of hard corals around Rodrigues Island. Coral disease and CHC prevalence were visually assessed using 2 m × 50 m belt transects at eight sites around the island. This is the first report of four coral diseases, namely White Plague (WP), White Syndrome (WS), Black Band (BB), and Growth Anomalies (GA), and two CHCs, two forms of Pink Pigmentation Responses (PPR)—Pink Patches (PP) and Pink Line Syndrome (PLS)—observed on six genera of hard corals from the island of Rodrigues. PP on Fungia (15.92 ± 5.65%), followed by the WS on Montipora (4.67 ± 3.72%) and GA on Gardineroseris (4.16 ± 4.16%), so far unreported from the SWIO, were the most prevalent around the island. The least prevalent disease was BB on Montipora (0.13 ± 0.13%). Although the overall disease and CHC prevalence for Rodrigues Island (0.98 ± 0.30%) were much lower than the surrounding islands in the SWIO, the observations of these diseases and CHCs on hard corals and relevant environmental parameters warrant further in-depth characterization to better inform coral reefs management and conservation actions.

Glacial expansion of carbon-rich deep waters into the Southwestern Indian Ocean over the last 630 kyr

Oceanic carbon storage is one of the main sinks for atmospheric CO2, and thought to be the major contributing factor for CO2 drawdown during past glacial periods. Both physical and biogeochemical processes control the capacity of carbon storage in the ocean. During glacial periods of the Pleistocene the larger volume of deep-water masses of Southern Hemisphere origin in the Atlantic has been shown to promote carbon storage in the Southern Ocean. However, the latitudinal extension of this water mass in the Indian Ocean has been scarcely studied. In this study, we combine foraminiferal εNd and benthic δ13C of two sediment cores in the southwest Indian Ocean (MD96–2077, 33°S, 3781 m water depth; MD96–2052, 19°S, 2627 m water depth), to reconstruct the spatial and temporal evolution of glacial carbon-rich deep waters in the SW Indian over the last 630 kyr. The combined use of foraminiferal εNd and benthic δ13C allows to distinguish δ13C changes related to water mass mixing and from respired carbon accumulation within the water masses. Nutrient-rich deep waters, which cannot be explained by the enhanced proportion of southern-sourced waters, were present at core sites deeper than 2700 m during glacial periods and extended at least until 33°S into the SW Indian Ocean. From Marine Isotope Stage (MIS) 14 to MIS 10, glacial carbon storage increased gradually until reaching its highest capacity during the extreme glacial periods MIS 12 and 10. Orbital forcing (100-kyr eccentricity, 41-kyr obliquity), restricted air-sea exchange and enhanced ocean stratification, fostered higher carbon storage during periods of relatively lower eccentricity and obliquity. Furthermore, after MIS 10, a progressive transition was observed from 100-kyr eccentricity to 41-kyr obliquity cycles in benthic δ13C and δ18O records of core MD96–2077 and sea-ice cover changes derived from ice-rafted debris of the Agulhas Plateau composite core site.

Sedimentary DNA reveals the link between microbial community dynamics and climate during the late last glaciation in the offshore region of the Zambezi River, Southwest Indian Ocean

Reconstructing the relationship between microbial communities and past abrupt climate change is of great importance for understanding current biodiversity patterns and predicting changes under future climate scenarios. However, little is currently known about how microbial communities respond to changes in key environmental stages due to a lack of research in this area. Here, we examine the variability in the communities of bacteria, archaea, and fungi from sediments deposited offshore region of the Zambezi River between 21.7 and 9.6 thousand years ago (ka) (covering the last glacial maximum, or LGM, and the early Holocene) using DNA metabarcoding approach via high-throughput sequencing. The results showed that (1) microbial assemblages differed across three key time intervals, with the last deglaciation having the most homogeneous prokaryotic assemblages, while for fungal communities in the LGM, and the early Holocene and LGM differing the most; (2) the warm early Holocene showed the highest diversity, whereas the lowest diversity was found in the LGM; and (3) the selected indicator species better reflected the climatic characteristics of different environmental stages. These results highlight the power of ancient sedimentary DNA to refine our understanding of microbial dynamics in marine sedimentary systems near large rivers, thus providing a basis for better modeling ecological processes in further research.

Impact of intra‐decadal variability of meridional heat transport on the rainfall over Southern Africa during austral summer

AbstractThe present study examines the impact of intra‐decadal variability of Meridional Heat Transport (MHT) associated with Indian Ocean Shallow Meridional Overturning Circulation (SMOC) on the rainfall variability over Southern Africa (e.g., Zambia, Malawi, Mozambique, south‐eastern Congo, Zimbabwe, southern Tanzania and Madagascar) during austral summer (December to February). Century long ocean reanalysis, atmospheric reanalysis and rainfall observations are used for the study. Spectrum analysis of MHT index and rainfall anomaly displays intra‐decadal (5–7 years) variability significant at 95% confidence level. The oceanic and atmospheric conditions during strong and weak phases of intra‐decadal MHT variability are examined. Strong (weak) MHT years displayed reduction (increase) in Sea Surface Temperature (SST) in southwest Indian Ocean, which are out‐of‐phase with net heat flux but in phase with ocean circulation. The negative (positive) SST anomaly favours positive (negative) sea level pressure anomaly, which supports the low level moisture divergence (convergence) and anticyclonic (cyclonic) circulation in the lower troposphere. This circulation leads to anomalous subsidence (convection) and below (above) normal moist static energy over the study region resulting in below (above) normal convection, that causes negative (positive) rainfall anomaly over the Southern Africa. The association between intra‐decadal variability of MHT associated with SMOC and rainfall over Southern Africa during austral summer are reported for the first time. The study may pave the way for enhanced predictability of intra‐decadal Southern African austral summer rainfall.

Multiweek tropical cyclone prediction for the Southern Hemisphere in ACCESS‐S2: Maintaining operational skill and continuity of service

AbstractThe skill of subseasonal (multiweek) forecasts of tropical cyclone (TC) occurrence over the Southern Hemisphere is examined in the Australian Bureau of Meteorology's (BoM) multiweek to seasonal prediction system, ACCESS‐S2. Relative to its predecessor, ACCESS‐S1, ACCESS‐S2 shows improved biases in spatial TC frequency in the South Pacific and Southwest Indian Ocean. However, there is no improvement to the known negative bias in TC frequency off the coast of NW Australia. The ability of ACCESS‐S2 to provide probabilistic forecasts of TC occurrence for the Southern Hemisphere on multiweek timescales is examined using reliability measures and Brier skill scores. For the period November–February 1990–2012, both ACCESS‐S1 and ACCESS‐S2 show positive skill relative to climatology for calibrated forecasts out to week 5. However, the skill of ACCESS‐S2 is slightly reduced compared to ACCESS‐S1 at all lead times, which may be due to the fewer number of ensemble members available. For the full ACCESS‐S2 hindcast period, November–April 1981–2018, ACCESS‐S2 again shows positive skill of calibrated forecasts over climatology out to week 5. For weeks 1–2, skill is reduced compared to the shorter 1990–2012 period; whereas it is marginally improved for longer lead times (weeks 3–5). Use of lagged ensembles, an alternative linear regression calibration, as well as removing weaker model TCs were examined to potentially improve the skill of ACCESS‐S2 forecasts; however, none of these methods were able to significantly increase skill at all lead times. Continued use of the original calibration method is therefore recommended in order to retain skill and continuity of service of the BoM operational and public multiweek TC forecasts.

Complete genome analysis reveals environmental adaptability of sulfur-oxidizing bacterium Thioclava nitratireducens M1-LQ-LJL-11 and symbiotic relationship with deep-sea hydrothermal vent Chrysomallon squamiferum.

One sulfur-oxidizing bacterium Thioclava sp. M1-LQ-LJL-11 was isolated from the gill of Chrysomallon squamiferum collected from 2700m deep hydrothermal named Longqi on the southwest Indian Ocean ridge. In order to understand its survival mechanism in hydrothermal extreme environment and symbiotic relationship with its host, the complete genome of strain M1-LQ-LJL-11 was sequenced and analyzed. A total of 6117Mb of valid data was obtained, including 4096 coding genes, 61 non coding genes, including 9 rRNAs (among them, there are 3 in 23S rRNA, 3 in 5S rRNA, and 3 in 16S rRNA.), 52 tRNAs and 35 genomic islands. Strain M1-LQ-LJL-11 contains one chromosome and two plasmids. In the genome annotation information of the strain, we found 28 genes including cys sox, sor, sqr, tst related to sulfur metabolism and 17 metal resistance genes. Interestingly, a pair of quorum sensing system which probably regulating biofilm formation located in chromosome was found. These genes are critical for self-adaptation against severe environment as well as host survival. This study provides a basis understanding for the adaptive strategies of deep-sea hydrothermal bacteria and symbiotic relationship with its host in extreme environments through gene level.

A Climatology of Dryline‐Related Convection on the Western Plateau of Subtropical Southern Africa

AbstractDrylines occur regularly during summer over the interior of sub‐tropical southern Africa. The highest frequency of drylines is over the western interior where it is also climatologically much drier than the eastern parts. An existing drylines data set is compared with lightning data between 2010 and 2021, to determine how often drylines were associated with convective storms. A minimum stroke threshold is applied to eliminate instances of weak convection. Seventy‐six percent of drylines during austral summer (October–March) were associated with significant convection with a standard deviation of 8.6. There is a peak frequency of 12 convective dryline days during December. Composite synoptic analyses were conducted for 591 convective and 186 non‐convective dryline days to determine the ideal formation environment for convective drylines. Composites were also created for the days leading up to convective and non‐convective drylines, showing the importance of moisture advection from the southwest Indian Ocean, through the Zambezi and Limpopo River Valleys. Early summers tend to be drier than late summers over the southern African interior and colder in the mid‐levels. These differences affect the formation environment of convective drylines. Composites show that dryline‐related convection during early summer tends to occur on days with a deep surface trough and a trough at 500 hPa further west. Convection during late summer is associated with increased surface moisture, higher instability, and a weaker trough at 500 hPa than during early summer.

Exploring Genetic and Morphological Integrity across Ocean Basins: A Case Study of the Mesopelagic Shrimp Systellaspis debilis (Decapoda: Oplophoridae)

Plankton communities often consist of cosmopolitan species with an extensive gene flow between populations. Nevertheless, populations of some plankton species are genetically structured, owing to various barriers such as ocean currents, hydrological fronts, and continents. Drivers that could explain the genetic structures of most mesopelagic species remain unknown on an ocean-basin scale, and our study aims to analyze the genetic and morphological differences between populations of a cosmopolitan mesopelagic shrimp, Systellaspis debilis, from the Southern and Northern Atlantic Ocean, and the Southwest Indian Ocean. We analyzed the ITS-1 and COI markers of 75 specimens and assessed the genetic integrity and within-species variability of these genes. We also coded 32 morphological characteristics in 73 specimens, analyzed their variability, and assessed the correlation between morphological and genetic characteristics using a Redundancy analysis and Mantel test. Systellaspis debilis was genetically cohesive across the whole Atlantic and Southwest Indian Oceans, which is possibly a result of an intensive gene flow through ecological barriers, the resistance of species to hydrological gradients, a purifying selection of mitochondrial genes, etc. In contrast, we found significant morphological differences between populations from different regions, which mirrors morphological diversification and calls for further genomic approaches in order to understand the basis of these variations and uncover potential local adaptations.

Case series of 12 Bartonella quintana endocarditis from the Southwest Indian Ocean.

Bartonella spp. are fastidious bacteria frequently identified as the cause of blood culture-negative (BCN) endocarditis. However, Bartonella infections are difficult to diagnose in routine laboratory testing and their incidence is probably underestimated. We investigated the epidemiological and clinical features of Bartonella endocarditis cases diagnosed between 2009 and 2021 on Reunion Island (Southwest Indian Ocean). We retrospectively included all patients diagnosed with Bartonella endocarditis at Reunion Island University Hospital during this period. Endocarditis was diagnosed on the basis of microbiological findings, including serological tests (IFA) and PCR on cardiac valves, and the modified Duke criteria. We used then the multispacer typing (MST) method to genotype the available Bartonella strains. We report 12 cases of B. quintana endocarditis on Reunion Island (83.3% in men, median patient age: 32 years). All the patients originated from the Comoros archipelago. The traditional risk factors for B. quintana infection (homelessness, alcoholism, exposure to body lice) were absent in all but two of the patients, who reported head louse infestations in childhood. Previous heart disease leading to valve dysfunction was recorded in 50% of patients. All patients underwent cardiac valve surgery and antimicrobial therapy with a regimen including doxycycline. All patients presented high C-reactive protein concentrations, anemia and negative blood cultures. The titer of IgG antibodies against Bartonella sp. exceeded 1:800 in 42% of patients. Specific PCR on cardiac valves confirmed the diagnosis of B. quintana endocarditis in all patients. Genotyping by the MST method was performed on four strains detected in preserved excised valves and was contributive for three, which displayed the MST6 genotype. Bartonella quintana is an important cause of infective endocarditis in the Comoros archipelago and should be suspected in patients with mitral valve dysfunction and BCN from this area.

Seafloor and Ocean Crust Structure of the Kerguelen Plateau from Marine Geophysical and Satellite Altimetry Datasets

The volcanic Kerguelen Islands are formed on one of the world’s largest submarine plateaus. Located in the remote segment of the southern Indian Ocean close to Antarctica, the Kerguelen Plateau is notable for a complex tectonic origin and geologic formation related to the Cretaceous history of the continents. This is reflected in the varying age of the oceanic crust adjacent to the plateau and the highly heterogeneous bathymetry of the Kerguelen Plateau, with seafloor structure differing for the southern and northern segments. Remote sensing data derived from marine gravity and satellite radar altimetry surveys serve as an important source of information for mapping complex seafloor features. This study incorporates geospatial information from NOAA, EMAG2, WDMAM, ETOPO1, and EGM96 datasets to refine the extent and distribution of the extracted seafloor features. The cartographic joint analysis of topography, magnetic anomalies, tectonic and gravity grids is based on the integrated mapping performed using the Generic Mapping Tools (GMT) programming suite. Mapping of the submerged features (Broken Ridge, Crozet Islands, seafloor fabric, orientation, and frequency of magnetic anomalies) enables analysis of their correspondence with free-air gravity and magnetic anomalies, geodynamic setting, and seabed structure in the southwest Indian Ocean. The results show that integrating the datasets using advanced cartographic scripting language improves identification and visualization of the seabed objects. The results include 11 new maps of the region covering the Kerguelen Plateau and southwest Indian Ocean. This study contributes to increasing the knowledge of the seafloor structure in the French Southern and Antarctic Lands.

Intraseasonal and synoptic modulation of diurnal surface solar radiation over Reunion island in the South-West Indian Ocean

Understanding the space-time variability of Surface Solar Radiation (SSR) is mandatory for the prediction and, eventually, the skillful forecasting of photovoltaic energy production. This paper addresses the modulation of local-scale SSR over Reunion, a tropical island in the South-West Indian Ocean, by the leading modes of climate variability influencing both regional-scale and local-scale atmospheric convection and its associated cloud cover. Analyses focus on synoptic (tropical cyclones [TCs], synoptic convective regimes, including Tropical-Temperate Troughs [TTTs]) and intraseasonal (Madden-Julian Oscillation [MJO]) timescales. The SSR intra-daily variability is first assessed by a diurnal classification of SARAH-E satellite SSR data, and it is then related to the climate conditions mentioned above. SSR anomalies are found larger (smaller) on the windward (leeward) side of Reunion and in the summer (winter) season. The island-scale “cloudy” conditions can typically last 1 or 2 days. Nearby TCs can strongly reduce SSR by up to 50% on average, depending on their distances from Reunion, their sizes, and particularly, their longitudinal positions, which is observed for the first time. Nearby TCs are associated with significant negative SSR anomaly when located west of Reunion but with less significant or even positive anomaly when located east of the island. Synoptic convective regimes (the intraseasonal MJO) have a relatively weaker impact on SSR, with a value up to 13% (5%) of the mean value. Potential interactions between these SSR modulators are also investigated to understand better and eventually predict the mechanisms likely to modulate SSR (and thus photovoltaic electricity production) at sub-seasonal timescales.

Metagenome-based analysis of carbon-fixing microorganisms and their carbon-fixing pathways in deep-sea sediments of the southwestern Indian Ocean.

Carbon fixation by chemoautotrophic microorganisms in the dark ocean makes a large contribution to oceanic primary production and the global carbon cycle. In contrast to the Calvin cycle-dominated carbon-fixing pathway in the marine euphotic zone, carbon-fixing pathways and their hosts in deep-sea areas are diverse. In this study, four deep-sea sediment samples close to hydrothermal vents in the southwestern Indian Ocean were collected and processed using metagenomic analysis to investigate carbon fixation potential. Functional annotations revealed that all six carbon-fixing pathways had genes to varied degrees present in the samples. The reductive tricarboxylic acid cycle and Calvin cycle genes occurred in all samples, in contrast to the Wood-Ljungdahl pathway, which previous studies found mainly in the hydrothermal area. The annotations also elucidated the chemoautotrophic microbial members associated with the six carbon-fixing pathways, and the majority of them containing key carbon fixation genes belonged to the phyla Pseudomonadota and Desulfobacterota. The binned metagenome-assembled genomes revealed that key genes for the Calvin cycle and the 3-hydroxypropionate/4-hydroxybutyrate cycle were also found in the order Rhodothermales and the family Hyphomicrobiaceae. By identifying the carbon metabolic pathways and microbial populations in the hydrothermal fields of the southwest Indian Ocean, our study sheds light on complex biogeochemical processes in deep-sea environments and lays the foundation for further in-depth investigations of carbon fixation processes in deep-sea ecosystems.

Do loggerhead sea turtle (Caretta caretta) gut contents reflect the types, colors and sources of plastic pollution in the Southwest Indian Ocean?

We analyzed plastic debris ingested by loggerheads from bycatch between 2007 and 2021 in the Southwest Indian Ocean (SWIO). We also analyzed plastic debris accumulated on beaches of the east coast of Madagascar as a proxy for ocean plastics to compare the characteristics of beached plastics and plastic ingested by turtles. We conducted a “brand audit” of the plastics to determine their country of origin. An oceanic circulation model was used to identify the most likely sources of plastics in the SWIO. In total, 202 of the 266 loggerheads analyzed had ingested plastics. Plastics categorized as “hard” and “white” were equally dominant in loggerheads and on beaches, suggesting no diet selectivity. Both the brand audit and circulation modeling demonstrated that Southeast Asia is the main source of plastic pollution in the region. This study demonstrates that loggerheads can be used as bioindicators of plastic pollution in the SWIO.

Decline or shifting distribution? A first regional trend assessment for white sharks (Carcharodon carcharias) in South Africa

Unprecedented levels of change in ocean ecosystems bring an ever-increasing need for re-analyses of existing data to explore pressing conservation questions. Substantial declines in white shark (Carcharodon carcharias) presence at two primary aggregation sites have raised concerns about the species’ status throughout South Africa. Using the most comprehensive suite of abundance indices compiled to date, we evaluated temporal trends and the strength of evidence for regional redistribution. Individual indices from all primary aggregation sites in South Africa were highly variable. The overall temporal trend from a log-linear Generalized Additive Model was relatively flat, indicating largely unchanged abundance throughout South Africa since protection in 1991. However, reports of human-shark incidents showed a general shift from the Western to the Eastern Cape. Correlations among individual abundance indices demonstrated that movements were not as simple as animals leaving one site to inhabit another. Further research is needed to explore the effect of movement on monitoring data. Our results reaffirm the need for better standardization in data collection methods to generate abundance indices and to develop long-term monitoring programs on the Eastern Cape. Ideally, environmental or operational factors affecting abundance indices should also be explored in future status assessments at a regional level. Our results provide a baseline for future work, directing research to understand drivers of localized and regional changes and focusing management on reducing anthropogenic sources of mortality within their Southwest Indian Ocean range.