AUXOSPORE FORMATION BY THE SILICA‐SINKING, OCEANIC DIATOM FRAGILARIOPSIS KERGUELENSIS (BACILLARIOPHYCEAE)¹

Abstract

Size restoration by the auxospore that develops from the zygote is a crucial stage in diatom life cycles. However, information on sexual events in pelagic diatom species is very limited. We report for the first time auxospore formation by the pennate diatom Fragilariopsis kerguelensis (O'Hara) Hustedt during an iron‐induced bloom in the Southern Ocean (EIFEX, European Iron Fertilization EXperiment). Auxospores of F. kerguelensis resembled those described for Pseudo‐nitzschia species. The auxospore was characterized by an outer coating, the perizonium; two caps, one at each distal end; and four chloroplasts, one at each end and two in the central part. Different stages of auxospore elongation were recorded, with a length of 24–91 μm, but only the largest auxospores contained the initial cell, whose apical axis ranged between 76 and 90 μm. Gametangial cell walls were often attached to the auxospores and ranged from 10 to 31 μm in length. Auxospore abundances were consistently higher in the fertilized patch, where an increase in the F. kerguelensis population was observed, as compared with surrounding waters.

Genetic characterization and life cycle of the diatomFragilariopsis kerguelensis

“…This strain underwent sexual reproduction when crossed with compatible strains, showing that at least some F. kerguelensis cells can be sexualized up to 36 µm. Our current observations provide additional information on the size range of initial cells, whichin culture -were 78.4-100.8 µm in length; this is a slightly wider range than was recorded in natural samples by Assmy et al (2006), who reported 76-90 µm. The fact that initial cells are often attached to the theca of the gametangia in F. kerguelensis makes it easy to link the sizes of these two cardinal points or stages (sensu Geitler, 1932) in the life cycles; our data suggest that there is no clear relationship between their size in F. kerguelensis (see Fig.…”

“…Furthermore, we never observed the formation of auxospores nor detected a sudden increase in cell size in any of the clonal cultures (which were monitored for a long time), indicating the absence of homothallic or uniparental (automictic or apomictic) auxosporulation (Kaczmarska et al, 2013). Fragilariopsis kerguelensis is the only species in the genus Fragilariopsis for which information on the life cycle is available (Assmy et al, 2006; this study) and its general pattern conforms to what has been described for other raphid pennate diatoms (Chepurnov et al, 2004). Nevertheless, differences occur between genera and in the following we discuss the main features of the life cycle in the frame of the available literature information.…”

“…The first evidence for sexual reproduction in F. kerguelensis came from a record of auxospores in phytoplankton samples collected in the Southern Ocean (Assmy et al, 2006). In the natural samples, auxospores of various sizes (i.e.…”

“…These results led the authors to propose F. kerguelensis as a biological proxy for palaeo-productivity and temperature conditions in the Southern Ocean. Productivity in this area is strongly linked to iron availability and an increased abundance of F. kerguelensis in response to artificial iron addition has been recorded in three Southern Ocean iron-fertilization experiments (Gall et al, 2001;Assmy et al, 2006Assmy et al, , 2007.…”

“…Despite the ubiquity of F. kerguelensis in the Southern Ocean, evidence of sexual reproduction -e.g. the presence of gametangia, auxospores and initial cells -in field samples was reported for the first time only recently (Assmy et al, 2006).…”