An Ecosystem in Transition: The Emergence of Mixotrophy in the Arabian Sea

Abstract

In the last decade and half, the northern Arabian Sea has witnessed a radical shift in the composition of winter phytoplankton blooms. Diatoms typical of the winter monsoon and favored by nutrient-enriched waters from convective mixing have been replaced by thick and widespread blooms of a large, green dinoflagellate Noctiluca scintillans (Noctiluca). Unlike the exclusively heterotrophic red Noctiluca found in temperate waters, the green species of Noctiluca from the Arabian Sea is a mixotroph. It harbors hundreds of green, free-swimming cells of the symbiont Pedinomonas noctiluca, recently renamed as Protoeuglena noctilucae, Wang et al. (2016), within the central vacuole of its cytoplasm, and can sustain itself either through carbon fixation by its endosymbionts or via ingestion of exogenous prey. Data collected by us aboard Indian research vessels in the Arabian Sea suggest that these recent outbreaks of green Noctiluca blooms are being caused by the spread of hypoxic waters into the euphotic zone and possibly exacerbated by land runoff and enhanced stratification of the water column. Noctiluca is not a preferred food for micro- and mesozooplankton. Instead, its major consumers are mostly salps and jellyfish. It uses inorganic nutrients and grazes on other phytoplankton. Thus, it competes with both its prey and predators for resources, posing special challenges for ecosystem modeling studies. The emergence of this mixotroph as the major plankton player in the ecosystem will require a revision of our earlier understanding of the Arabian Sea food web dynamics and allied biogeochemistry gained from the Joint Global Flux Studies (JGOFS) expeditions of the 1990s.