OCCURRENCE OF RICHELIA INTACELLULARIS (CYANOPHYTA)WITHIN THE DIATOMMMS HEMIAULUS HAUKII ADN H. MEMBRANACEUS OFF HAWAII¹

Abstract

The presence of nitrogen–Fixing cyanobacterial endophytes, such as Richelia intracellularis Schmidit commonly observed within several species of Rhizosolenia, in nitrogen‐limited oceanic waters has obvious implications both for the host organism(s) and for the entire planktonic assemblage. Recently, epi‐fluorescent examination of pllankton samples collected off Oahu, Hawaii, in september 1982 revealed R. intracellularis within the diatoms Hemiaulus membranaceus Cleve and H. hauckii Grunow as well as its expected presence within Rhizosolennia spp. Richelia intracellularis coccurred with within Hemiaulus spp. at frequenccies (ca.80%)comparable to those noted for Rhizosolenia spp. Standard bright field or phaseconrast microsocopy could not reliably distinguish the cyanobacteria within Hemiaulus spp. If this association is common, the occurrence of R.intracellularis (and its significance in the nitrogen dynamics of the ocean)may have been greatly underestimated in previous studies.

Nitrogen-fixation by the cyanobacterial symbiont of the diatom genus Hemiaulus

“…The curved H. hauckii (n = 30) showed no host chlorophyll autofluorescence, contained no symbionts, and appeared dead. These observations are similar to Heinbokel's (1986) observations of ca 8 0 % symbiont-containing Hemiaulus cells north of Hawaii. Although H. hauckii and H. sinensis were not examined individually for acetylene reduction, it is reasonable to assume the symbionts of both species were fixing nitrogen.…”

“…Kimor et al (1978) noted that up to 62 % of the Hemiaulus membranaceus Cleve collected off southern California contained symbionts, although more extensive collections offshore contained only 16.1 to 18.6 % symbiotic Hemiaulus. Heinbokel (1986) noted that the Hemiaulus symbionts were frequently visible only under epifluorescent illumination, and that ca 80 % (n = 668) of the Hemiaulus hauckii and H. m e mbranaceus in samples north of Hawaii contained symbionts. He suggested that symbiont abundance, and their role in oceanic nitrogen fixation, may have been underestimated due to the need to use epifluorescence to reliably identify the symbiont.…”

“…Cyanobacterial symbionts usually identified as Richelia intracellularis Schmidt are also known in Hemiaulus; however, the identity of the symbiont is uncertain (Sundstrom 1984). The presence of a heterocyst suggests N2-fixation is likely in this symbiosis (Kimor et al 1978, Heinbokel 1986), but NZ-fixation by Hemiaulus symbionts has never been examined.…”

Seasonal dynamics of the endosymbiotic, nitrogen-fixing cyanobacterium Richelia intracellularis in the eastern Mediterranean Sea

“…The distinguishing spindles connecting the H. hauckii cells and their length (40 mm), enabled identification of the Hemiaulus as H. hauckii (Heinbokel, 1986;Kimor et al, 1987Kimor et al, , 1992Villareal, 1994). Under chlorophyll excitation only H. hauckii chloroplasts were visible (Figure 3a).…”

Light driven seasonal patterns of chlorophyll and nitrate in the lower euphotic zone of the North Pacific Subtropical Gyre

“…The dominant species detected during these summer bloom events have in common their capacity to regulate buoyancy (Walsby 1978;Villareal et al 1993Villareal et al , 1999. Furthermore, while Trichodesmium fixes dinitrogen (Letelier and Karl 1998 and references therein), the dominant diatom species responsible for summer blooms can carry the endosymbiotic dinitrogen-fixing cyanobacterium Richelia intracellularis (Heinbokel 1986). The nitrogen isotope signature of particulate material collected in the sediment traps at HOT indicates that these summer blooms are largely supported by dinitrogen fixation (Karl et al 1997;Dore et al 2002).…”