Harmful algae blooms have become a recurring problem for many places around the US and Canada, from the Great Lakes to the Gulf of Mexico and coast to coast. In North America, some algae blooms produce toxins that can harm humans and other animals, and others can result in hypoxic or anoxic conditions.
Recently, the Arabian Sea has been facing its own algal problems. An organism all but unheard of in the Sea 20 years ago has been spreading at an alarming pace, forming green swirls that are visible even from space. The organism is Noctiluca scintillans—a millimeter-sized planktonic organism with an extraordinary capacity to survive, thrive, and force out diatoms, the photosynthesizing plankton that have traditionally supported the Arabian Sea food web. Noctiluca is not a preferred food for larger organisms, so these large blooms, recurring annually and lasting for several months, are disrupting the base of the region's marine food chain, threatening fisheries that sustain 150 million people, and possibly exacerbating the rise of criminal piracy in the region.
In North America, one of the major causes of harmful algae blooms is nutrients like nitrogen and phosphorus that are carried into water bodies by surface runoff. The Noctiluca blooms in the Arabian Sea are also fueled in part by nutrient runoff and the organisms are particularly good at converting CO2 into energy. But new research published this week in Nature's Scientific Reports describes how the continued loss of snow over the Himalayan-Tibetan Plateau region is also fueling the expansion of this destructive algal bloom. The study uses field data, laboratory experiments, and decades of NASA satellite imagery to link the rise of Noctiluca in the Arabian Sea with melting glaciers and a weakened winter monsoon.
Normally, cold winter monsoon winds blowing from the Himalayas cool the surface of the oceans. These colder waters sink and are replaced with nutrient-rich waters from below. During this time, phytoplankton, the primary producers of the food chain, thrive in the sunlit, nutrient-rich upper layers, and surrounding countries see a bounty of fish that feed directly or indirectly on the phytoplankton. But with the shrinking of glaciers and snow cover in the Himalayas, the monsoon winds blowing offshore from land are warmer and moister, resulting in diminished convective mixing and decreased fertilization of the upper layers.
In this scenario, phytoplankton such as diatoms are at a disadvantage, but not Noctiluca. Unlike diatoms, Noctiluca (also known as sea sparkle) doesn't rely only on sunlight and nutrients; it can also survive by eating other microorganisms. Noctiluca hosts thousands of photosynthesizing endosymbionts within its bulbous, transparent, greenhouse-like cell. The green endosymbionts provide it with energy from photosynthesis, while its tail-like flagella allow it to grab any microscopic plankton from the surrounding water as an additional source of food.
This dual-mode of energy acquisition gives it a tremendous advantage to flourish and disrupt the classic food chain of the Arabian Sea. Noctiluca's second advantage is that its endosymbionts accumulate a lot of ammonia in the cell, making the organism unpalatable to larger grazers. As a third advantage, the accumulated ammonia is also a repository of nitrogenous nutrients for the endosymbionts, making them less vulnerable to diminishing inputs of nutrients from a weakened convective mixing.
Noctiluca blooms first appeared in the late 1990s. The sheer size of their blooms, which occur annually, threaten the Arabian Sea's already vulnerable food chain because its symbionts not only compete with phytoplankton for the annually replenished nutrients but feed on the phytoplankton themselves. However, only jellyfish and salps seem to find Noctiluca palatable. In Oman, desalination plants, oil refineries, and natural gas plants are forced to scale down operations because they are choked by Noctiluca blooms and the jellyfish that swarm to feed on them. The resulting pressure on the marine food supply and economic security may also have fueled the rise in piracy in countries like Yemen and Somalia.
