Response of coastal phytoplankton to pollution from various sources in the coastal Bay of Bengal

The coastal ocean receives nutrient pollutants from various sources, such as aerosols, municipal sewage, industrial effluents and groundwater discharge, with variable concentrations and stoichiometric ratios. The objective of this study is to examine the response of phytoplankton to these pollutants in the coastal water under silicate-rich and silicate-poor coastal waters. In order to achieve this, a microcosm experiment was conducted by adding the pollutants from various sources to the coastal waters during November and January, when the water column physicochemical characteristics are different. Low salinity and high silicate concentration were observed during November due to the influence of river discharge contrasting to that observed during January. Among the various sources of pollutants used, aerosols and industrial effluents did not contribute silicate whereas groundwater and municipal sewage contained high concentrations of silicate along with nitrate and phosphate during both the study periods. During November, an increase in phytoplankton biomass was noticed in all pollutant-added samples, except municipal sewage, due to the limitation of growth by nitrate. On the other hand, an increase in biomass and abundance of phytoplankton was observed in all pollutant-added samples, except for aerosol, during January. Increase in phytoplankton abundance associated with decrease in biomass was observed in aerosol-added sample due to co-limitation of silicate and phosphate during January. A significant response of Thalassiothrix sp. was observed for industrial effluent-added sample during November, whereas Chaetoceros sp. and Skeletonema sp. increased significantly during January. Higher increase in phytoplankton biomass was observed during November associated with higher availability of silicate in the coastal waters in January. Interestingly, an increase in the contribution of dinoflagellates was observed during January associated with low silicate in the coastal waters, suggesting that the concentration of silicate in the coastal waters determines the response of the phytoplankton group to pollutant inputs. This study suggested that silicate concentration in the coastal waters must be considered, in addition to the coastal currents, while computing dilution factors for the release of pollutants to the coastal ocean to avoid occurrence of unwanted phytoplankton blooms.

Recurrence of Gonyaulax polygramma bloom in the southeastern Arabian Sea

Gonyaulax polygramma, a bloom-forming dinoflagellate, has been repeatedly observed along the southeastern Arabian Sea in recent years. During our study in October 2021, a patch of reddish-brown water was observed in the nearshore waters off Kannur (southwest coast of India) and later identified as Gonyaulax polygramma using scanning electron microscopy (SEM) and HPLC-based phytoplankton marker pigments. Gonyaulax polygramma accounted for 99.4% of the phytoplankton abundance at the bloom location, with high concentrations of peridinin and chlorophyll-a at the study site. High concentration of SiO₄^2- was observed at the bloom site, while other nutrients were lower than the previously reported values. The bloom of Gonyaulax polygramma also resulted in high concentrations of dimethylsulfide, an anti-greenhouse gas, at the bloom site. In addition to onsite observation, Sentinel-3 satellite data was also used in the detection and validation of the observed bloom using the NDCI index. From the satellite image, it was evident that the bloom persisted at the mouth of the rivers during the study period. Since the red tide of Gonyaulax polygramma has been observed recurrently in the southeastern Arabian Sea, it is proposed to use satellites to detect and monitor the bloom on a routine basis.

Exploration of fish gut-associated actinobacteria for its antifouling activity

An attempt has been made to screen the fish gut-associated actinobacterial cultures for antifouling compounds. Fifteen morphologically distinct bacterial cultures were recovered from the biofouling samples scraped from the boat surfaces and other maritime structures in the Kovalam beach (Lat. 12.7870°N; Long. 80.2504°E) coastal areas in Tamil Nadu, India. All the bacterial isolates were identified at generic level from which two isolates namely KB6 and KB7 as Staphylococcus sp. were found the exhibit strong biofilm formation. Four actinobacterial strains viz., IM20, PYA9, F8, and SQA4 were evaluated for inhibitory properties against biofouling bacteria using the agar plug method. Strain IM20 which strongly inhibited the biofouling bacteria was chosen as prospective strain for further studies. When compared to submerged fermentation, IM20 produced a high amount of antifouling chemicals on the agar surface fermentation. Among the solvents tested, better extraction of antifouling compounds was seen in ethyl acetate extract. Antifouling compound production by the strain IM20 was found to be influenced by a number of variables such as glucose, fructose, glutamine, malt extract, pH 7 and 9, temperature 30 and 40^° C and NaCl at 2.5% and 5% concentrations. Gas chromatography-mass spectrometry (GC-MS) analysis of the strain IM20 extract revealed the presence of pyrrolo (1,2-a]pyrazine-1,4-dione, hexahydro) in significant amount. In the present study, the fish gut-associated Streptomyces sp. IM20 was identified as an unusual and newly added source for the isolation of antifouling compounds.