ORIGINAL ARTICLE Adaptations of araphid pennate diatoms to a planktonic existence Wiebe H. C. F. Kooistra, Gandi Forlani & Mario De Stefano* Stazione Zoologica Anton Dohrn, Naples, Italy Problem Diatoms are unicellular microalgae whose most conspicu- ous character is their compound silica cell wall, called a frustule. The frustule is composed of two valves and their accompanying cingular bands, which together envelop the protoplasm while permitting growth and mitotic cell divi- sion (Round et al. 1990; see also illustrations in Figs 1 and 2). Rows of poroids called striae (see e.g. Fig. 1h) in the valves and cingular bands permit exchange with the environment. Within the diatoms, the pennates form a clade (Kooistra et al. 2003, 2004, 2007; Medlin & Kaczmarska 2004; Sinninghe-Damste ´ et al. 2004; Sorhannus 2004; Alverson et al. 2006; Sims et al. 2006). Pennate species are generally elongated with valves shaped like a ship’s hull; valvar striae are organized in parallel rows perpendicular on a midrib, called the sternum (Round et al. 1990). Within the pennates, Round et al. (1990) recognize the raphid pennates (Bacillariophyceae Haeckel emend. Mann) and the araphid pennates (Fragilariophyceae Round). The raphid pennates possess a pair of mid-slits or a single one – called a raphe – in or near the sternum, which enables active movement over surfaces (see Round et al. 1990 for Keywords Adaptation; araphid pennate diatoms; benthic; phylogeny; plankton; SSU rDNA. Correspondence Wiebe H. C. F. Kooistra, Lab. of Ecology and Evolution of Plankton, Stazione Zoologica Anton Dohrn, Villa Comunale, 80121 Naples, Italy. E-mail: kooistra@szn.it *Present address: Department of Environmental Science, The 2nd University of Naples, via Vivaldi 43, 81100 Caserta, Italy. Accepted: 4 August 2008 doi:10.1111/j.1439-0485.2008.00262.x Abstract Pennate diatoms are monophyletic. Their principal cell wall elements, called valves, are shaped like a ship’s hull. Within the pennates, the araphids are para- phyletic; they possess rimoportulae and pore fields located at the valve apices. The pore fields exude mucilage pads with which cells attach to one another to form chains. Many taxa use the pads also for attachment to substrata. Only a few genera are truly planktonic. The main question addressed in this study is whether the planktonic lifestyle is ancestral or derived. Phylogenies inferred from nuclear SSU rDNA gene sequences of diatoms indicated that the attached lifestyle is ancestral among the araphids, whereas a typically planktonic lifestyle seems to have developed at least three times and possibly four times indepen- dently. Acquisition of a planktonic lifestyle from benthic ancestry was accom- panied by a reduction in the silicification of cell-wall elements, but changes in morphological characters shared by all four clades were not detected. The reason why only three or four araphid pennate clades have adopted a plank- tonic lifestyle may be related to constraints associated to their sexual reproduc- tion mode. Partner cells of opposite mating type align with one another and produce isogametes. These gametes lack flagella; they move to one another in an amoeboid fashion, which functions well on surfaces, but seems a liability in a turbulent water column. The planktonic lineages must have overcome this constraint, e.g. by sinking to the bottom, or aggregating, to perform sexual reproduction. Members of the four araphid pennate lineages are now common constituents of the plankton, suggesting that they are ecologically successful. Marine Ecology. ISSN 0173-9565 Marine Ecology 30 (2009) 1–15 ª 2008 The Authors. Journal compilation ª 2008 Blackwell Publishing Ltd 1