Suppose you were asked to describe Chennai, ‘the Gateway to the South', what would be the first thing that comes to your mind? Its Beaches of course! Being a coastal city, Chennai is blessed with numerous beaches ranging from the Marina Beach in the North to the Kanathur Beach, Muttukadu in the South, spanning a distance of about 27 kilometers. These beaches attract tens of thousands of tourists each weekend. Nearly two years ago, on the night of 18th August, 2019, spectators on the Injambakkam Beach in the East Coast Road, and the Edward Elliot's Beach in Besant Nagar observed a breathtaking visual. It was a pleasant surprise when they spotted the waves sparkling with an almost magical glow of deep blue colour. Numerous excited people took to social media to share the videos and pictures of this phenomenon. Marine Ecologists attributed the glowing beaches to the bioluminescence of "sea sparkle".
What is bioluminescence?
All bodies emit radiation at all temperatures above the ‘absolute zero’ (0 Kelvin), and it appears to ‘glow’ if the wavelength of the emitted light falls in the visible range. This is known as incandescence. Luminescence on the other hand is commonly known as ‘cold heat’, and is the emission from a body that has absorbed energy from a source such as UV, X-Ray, electron beam or chemical reactions. Some of us have been fortunate to have seen fireflies and jelly combs that glow in bright red, yellow or green. These are common examples of bioluminescence, a phenomenon where light emitted by organisms through chemical reactions in their bodies.
Sea sparkle is the common name of Noctiluca scintillans, a type of free-living phytoplankton. These microscopic organisms that live in the oceans illuminate as a result of chemical reactions that occur inside their cells. By engulfing its victims through the process of phagocytosis, the Noctiluca feeds on various marine micro-organisms like diatoms, copepods and bacteria. Although it can ‘hunt’ its victims, Noctiluca’s main source of food in the tropics is via an endosymbiotic relationship with Pedinomonas noctilucae, a photosynthetic microorganism that lives in thousands inside the vacuoles of a single Noctiluca.
Although these bioluminescent waves are claimed not to be a regular phenomenon at the Eastern Coast of Chennai, they have been quite a common phenomenon on the western coast for the past couple of decades. Goa, Mumbai, and the backwaters of Kerala have witnessed these algal blooms every single year sometime between January and March. The Coastal Processes and Shoreline Management Studies at the National Centre for Coastal Research (NCCR) have been monitoring the coast for a few years and they speculate that the bioluminescent waves are developed as a result of high discharge of sewage into the ocean.
Water containing algal blooms was analyzed and showed a significant reduction in nitrate-nitrogen and enrichment in phosphate-phosphorus during N. scintillans blooms. It also showed elevated levels of chlorophyll a, pheophytin and carotenoids. While the release of chemical, nutrient-rich effluents like industrial and domestic sewage effluents, soaps and detergents, fertilizers, etc, pollute the water bodies, it also adds nutrients to the growth of these microscopic algae. Thus a disappointing fact is that, although the bioluminescent waves look spectacular, they indicate poor health of the waterbody and also point to climate change, with a particularly massive impact on deep-sea fishing.
What makes these glowing microscopic organisms dangerous?
Noctiluca can deplete the oxygen in water, causing significant damage to the environment and the aquatic ecosystem. By the process of respiration, they take up the dissolved oxygen near the surface. Under hypoxic conditions, where the levels of dissolved oxygen in the water body are alarmingly reduced, P. noctilucae, the endosymbionts of N. scintillans, can fix carbon much more efficiently as compared to other phytoplanktons. The efficient carbon fixing ability of P. noctilucae added to the release of oxygen-deficient and nutrient enriched water into the system, facilitates a massive outbreak of N. scintillans, into the euphotic zone, the uppermost layer of the ocean which receives enough sunlight for photosynthesis. The rapid growth of these, which is termed as an algal bloom, prevents sunlight from reaching the depths of the ocean, thereby causing the death of many marine photosynthetic organisms. They are the primary producers in the food chain of the marine ecosystem and their death disrupts the entire food chain. As these photosynthetic organisms die, the decomposition process also consumes a large amount of oxygen - Biochemical Oxygen Demand. In some seas, the areas dominated by these algae are even nicknamed 'the death zones' for the reasons described above.
Over the last couple of years, collaborative studies by the Indian National Centre for Ocean Information Services (INCOIS) and the National Oceanic and Atmospheric Administration (NOAA) have revealed some shocking facts. Due to the warming of the oceans, there is a stratification of ocean - water with different salinity, density and temperature form layers, which act as barriers for water mixing. Ocean water is generally stratified with denser water underlying lighter water, which mixes as a result of upwelling and downwelling caused by the flow of the winds above it. This stratification prevents the mixing of nutrients in the different layers that range between the ocean depth and the surface. This would mean the growth of diatoms that demand the nutrients from the bottom of the ocean is prevented, while it encourages intense blooming of Noctiluca that feeds on them. As fishes don't thrive in such waters, we observe high mortality in them. Currently, only the western coast is affected, but if the ocean warming is hastened, fisheries along the entire Indian Coast will be drastically affected.
Following the research and the shocking revelation, INCOIS has started a Marine Observation System Along Indian Coast (MOSAIC) to monitor the Indian coast for its water quality. This initiative would be tremendously helpful in forecasting the effects of climate change in the marine ecosystem.
This article is an extended and updated version of the one written as a part of the Scicatalyst Vol. IX | August 2020. Editors: Ananya Dash, Rachita Dash, and Mandira Choppella.
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