Bioluminescent eddies of the World Ocean.

Mesoscale eddies of the ocean (with a characteristic diameter of about 100 km and a life time-span of about several weeks) are habitats of plankton organisms, many of which are bioluminescent. The spatial heterogeneity of bioluminescence of the upper mixed layer associated with the impact of mesoscale eddies is poorly studied. The 45-year historical data set was retrieved, in order to select the bathy-photometric surveys carried out in the form of station grids and transects across eddies. Data from 71 expeditions deployed in 1966-2022 to the Atlantic Ocean, Indian Ocean and Mediterranean Sea basin were analyzed, in order for the spatial heterogeneity of bioluminescent fields to be elucidated across eddy fields. The stimulated bioluminescence intensity was characterized by the bioluminescent potential, which represented the maximal amount of radiant energy emitted in a given volume of water by bioluminescent organisms. The normalized bioluminescent potential over oceanographic station grids exhibited correlation with the eddy kinetic energy and zooplankton biomass (r = 0.8, at P = 0.001 and r = 0.7, at P = 0.05, respectively), in a broad range of energy and bioluminescence units (0.02-0.2 m2  s-2 ; 0.4-92.0 × 10-8  W cm-2  L-1 , respectively). Overall, estimates of bioluminescent potential variability on the mesoscale contribute to the assessment of the multiple-scale variation of the bioluminescent field of the World Ocean.

Fading bioluminescence of the tropical Atlantic Ocean.

Bioluminescence intensity acts as the indicator of the functional state of a plankton community. Data on bioluminescence intensity, zooplankton biomass, and chlorophyll a from the expeditions to the tropical and subtropical Atlantic Ocean were analyzed. The regression models implied a ~10-fold decline of bioluminescence intensity [bioluminescence potential (BP)] and zooplankton biomass for the 46 year time range from 1970 to 2016. The correlation was low between chlorophyll a (the indicator of phytoplankton biomass) and BP, but it was significant for the zooplankton biomass BP annual time series. The decline of BP was associated with the decreasing abundance of bioluminescent zooplankton, and increasing global warming-driven temperature.

Bioluminescence of the tropical Indian Ocean: a multiple-scale variation.

Large-scale surveys represented by 5800 bathymetric casts in the western Indian Ocean (0-22o N, 54-58o E), elucidated the 10-fold variation of the bioluminescent potential (BP) in the upper mixed layer, during the winter (north-east) monsoon season. The mesoscale survey in February 2017 consisted of 26 drift stations (4o N-3o S, 65-68o E) on which 5-10 bathymetric casts were deployed down to 60 m. The maximal BP was associated with the periphery of a cyclonic eddy. The two-fold to three-fold variation of BP characterized the spatial heterogeneity modulated by a detected eddy. High-frequency casts on drift stations resembled the fine-scale heterogeneity in which the three-fold variation was observed within the BP maximum at a 37 ± 13 m depth. The latter one was located above the deep chlorophyll maximum at a 80 m depth. A general decline of the BP variance from the large scale through mesoscale to fine scale, fits that of the zooplankton biomass.

Living on the edge: biofilms developing in oscillating environmental conditions.

For the first time, the densities and diversity of microorganisms developed on ocean gliders were investigated using flow cytometry and Illumina MiSeq sequencing of 16S and 18S rRNA genes. Ocean gliders are autonomous buoyancy-driven underwater vehicles, equipped with sensors continuously recording physical, chemical, and biological parameters. Microbial biofilms were investigated on unprotected parts of the glider and surfaces coated with base, biocidal and chitosan paints. Biofilms on the glider were exposed to periodical oscillations of salinity, oxygen, temperature, pressure, depth and light, due to periodic ascending and descending of the vehicle. Among the unprotected surfaces, the highest microbial abundance was observed on the bottom of the glider's body, while the lowest density was recorded on the glider's nose. Antifouling paints had the lowest densities of microorganisms. Multidimensional analysis showed that the microbial communities formed on unprotected parts of the glider were significantly different from those on biocidal paint and in seawater.

Understanding how physical-biological coupling influences harmful algal blooms, low oxygen and fish kills in the Sea of Oman and the Western Arabian Sea.

In the last decade, green Noctiluca scintillans with its symbiont and other dinoflagellates such as Cochlodinium polykrikoides, Prorocentrum micans and Scrippsiella trochoidea have become the dominant HABs, partially replacing the previously dominant diatoms and red Noctiluca scintillans, especially during the northeast monsoon. Fish kills in the Sea of Oman are linked to a slow seasonal decline in oxygen concentration from January to November, probably due to the decomposition of a series of algal blooms and the deep, low oxygen waters periodically impinging the Omani shelf. In the western Arabian Sea, cyclonic eddies upwell low oxygen, nutrient-rich water and the subsequent algal bloom decays and lowers the oxygen further and leads to fish kills. Warming of the surface waters by 1.2°C over the last 5 decades has increased stratification and resulted in a shoaling of the oxycline. This has increased the probability and frequency of upwelling low oxygen water and subsequent fish kills.