An extensive bloom of the N2-fixing cyanobacterium Trichodesmium erythraeum in the central Arabian Sea

Abstract

ABSTRACT-LVe encountered an extensive surface bloom of the N, fixing cyanobactenum Trichodesrniurn erythraeum in the central basin of the Arabian Sea during the spring ~nter-n~onsoon of 1995. The bloom, which occurred dunng a penod of calm winds and relatively high atmospher~c iron content, was metabollcally active. Carbon fixation by the bloom represented about one-quarter of water column primary productivity while input by h:: flxation could account for a major fraction of the estimated 'new' N demand of pnmary production. Isotopic measurements of the N in surface suspended material confirmed a direct contribution of N, fixation to the organic nltrogen pools of the upper water column. Retrospective analysis of NOAA-12 AVHRR imagery indicated that blooms covered up to 2 X 106 km2, or 20% of the Arabian Sea surface, during the period from 22 to 27 May 1995. In addition to their biogeochemical impact, surface blooms of this extent may have secondary effects on sea surface albedo and light penetration as well as heat and gas exchange across the air-sea interface. A preliminary extrapolation based on our observed, non-bloom rates of N, fixation from our limited sampling in the spring intermonsoon, including a conservative estimate of the input by blooms, suggest N2 fixation may account for an input of about 1 Tg N yr-I This is substantial, but relatively minor compared to current estimates of the removal of N through denitrification in the basin. However, N2 fixation may also occur in the central basin through the mild winter monsoon, be considerably greater during the fall intermonsoon than we observed during the spring intermonsoon, and may also occur at higher levels in the chronically oligotrophic southern basin. Ongoing satellite observations will help to determine more accurately the distribution and density of Trichodesmium in this and other tropical oceanic basins, as well as resolving the actual frequency and duration of bloom occurrence.

Bioavailability of iron to Trichodesmium colonies in the western subtropical Atlantic Ocean

“…There are many organisms associated with natural colonies of Trichodesmium, especially bacteria (Capone et al 1997). These heterotrophic bacteria associated with the colonies are capable of siderophore production (K. Barbeau et al and E. Mann et al unpubl.).…”

“…Although Trichodesmium can occur as free trichomes (or filaments) in the water column, natural populations of trichomes typically aggregate to form macroscopic colonies (Letelier and Karl 1996). Several hundred trichomes aggregate to form either spherical colonies (puffs) or fusiform colonies (tufts) (Capone et al 1997). When environmental conditions are ideal, these Trichodesmium colonies are capable of forming massive surface blooms (Capone et al 1998).…”

“…Several hundred trichomes aggregate to form either spherical colonies (puffs) or fusiform colonies (tufts) (Capone et al 1997). When environmental conditions are ideal, these Trichodesmium colonies are capable of forming massive surface blooms (Capone et al 1998). It has been estimated that these Trichodesmium blooms are responsible for approximately one fourth of the total N, fixation in the ocean (Rueter et al 1990) and that this organism is the largest source of new N in the surface waters of the north Atlantic Ocean (Lipschultz et al 2002).…”

“…Cyanobacteria are ubiquitous in marine environments and play a significant role in the global carbon (C) and N cycles (Capone et al 1997). The bioavailability of Fe is an important factor affecting the productivity of cyanobacteria, as Fe is necessary for essential metabolic processes such as photosynthesis and N 2 fixation (Wilhelm 1995).…”

Population dynamics and environmental conditions affecting Trichodesmium spp. (filamentous cyanobacteria) blooms in the south–west lagoon of New Caledonia

“…Actually, each of the two species is able 340 to prevail in the open ocean. It is T. erythraeum in the Coral Sea and T. thiebautii in the Caribbean 341 Sea (O'Neil et al, 1996 ), T. thiebautii in the Kuroshio area (Saino and Hattori, 1980) and the 342 Atlantic (Carpenter et al, 2004), but T. erythaeum in the north Indian Ocean (Capone et al, 1998). 343…”