Evolution of the diatoms: insights from fossil, biological and molecular data

Molecular sequence analyses have yielded many important insights into diatom evolution, but there have been few attempts to relate these to the extensive fossil record of diatoms, probably because of unfamiliarity with the data available, which are scattered widely through the geological literature. We review the main features of molecular phylogenies and concentrate on the correspondence between these and the fossil record; we also review the evolution of major morphological, cytological and life cycle characteristics, and possible diatom origins. The first physical remains of diatoms are from the Jurassic, and well-preserved, diverse floras are available from the Lower Cretaceous. Though these are unequivocally identifiable as centric diatoms, none except a possible Stephanopyxis can be unequivocally linked to lineages of extant diatoms, although it is almost certain that members of the Coscinodiscophyceae (radial centrics) and Mediophyceae (polar centrics) were present; some display curious morphological features that hint at an unorthodox cell division mechanism and life cycle. It seems most likely that the earliest diatoms were marine, but recently discovered fossil deposits hint that episodes of terrestrial colonization may have occurred in the Mesozoic, though the main invasion of freshwaters appears to have been delayed until the Cenozoic. By the Upper Cretaceous, many lineages are present that can be convincingly related to extant diatom taxa. Pennate diatoms appear in the late Cretaceous and raphid diatoms in the Palaeocene, though molecular phylogenies imply that raphid diatoms did in fact evolve considerably earlier. Recent evidence shows that diatoms are substantially underclassified at the species level, with many semicryptic or cryptic species to be recognized; however, there is little prospect of being able to discriminate between such taxa in fossil material

Morphology and ecology of the planktonic diatom Palmerina hardmaniana (Greville) Hasle in southern Brazil

The diatom Palmerina hardmaniana (Greville) Hasle presents a wide geographical distribution in neritic tropical and subtropical regions. In the present work we analyzed plankton samples collected monthly between 1995 and 2007 at the surf zone of Cassino Beach, RS (32° 12’ S and 52° 10’ W), and in winter 2005 and summer 2007 at the continental shelf and slope in southern Brazil, Santa Marta Grande Cape, SC and Albardão-Chuí, RS regions (28° 23’-33° 07’ S and 48° 41’-52° 26’ W). We present the detailed morphological description of P. hardmaniana, and the first study including electron scanning microscope observations for material from the southwestern Atlantic Ocean. The morphometric data confirm the identity of the species in all its ultra-structural details. Palmerina hardmaniana was only observed in summer-autumn months with low cell density (< 500 cells.L–1) at both Cassino Beach surf zone and coastal shelf stations. The warm water temperature (18-29 °C) indicates the most probable origin of its inoculum are tropical/subtropical regions. Salinities of 23-36 and the relatively high silicate content indicate the importance of the terrestrial discharge during occasions when P. hardmaniana was observed, probably with influence on the nutrient availability. We emphasize that the species was not cited previously for Argentinean and Uruguayan waters and suggest that the southern Brazilian region is close to the southern geographical distribution limit of Palmerina hardmaniana in the southwestern Atlantic Ocean.A diatomácea Palmerina hardmaniana(Greville) Hasle apresenta ampla distribuição geográfica em águas neríticas tropicais e subtropicais. No presente trabalho foram analisadas amostras de plâncton, coletadas mensalmente entre 1995 e 2007 na zona de rebentação da Praia do Cassino, RS (32° 12’ S e 52° 10’ W), e no inverno de 2005 e verão de 2007 na plataforma continental e talude do sul do Brasil, na região de Cabo de Santa Marta Grande, SC e Albardão-Chuí, RS (28° 23’-33° 07’ S e 48° 41’-52° 26’ W). Apresentamos a descrição detalhada de P. hardmaniana, como primeiro estudo com observações de microscopia eletrônica de varredura para material coletado em águas do Oceano Atlântico Sul Ocidental. Os dados morfológicos analisados confirmam a identificação da espécie em todos os seus detalhes estruturais. Palmerina hardmaniana somente foi observada nos meses de verão/outono, em baixa densidade (< 500 células.L–1) na zona de arrebentação da Praia do Cassino bem como em estações costeiras da plataforma continental. A temperatura quente da água (18-29 °C), indica as águas tropicais/subtropicais como possível origem do inóculo de P. hardmaniana no verão-outono. A salinidade entre 23 e 36 e o teor relativamente alto de sílica também indicam a importância da descarga terrestre nas ocasiões de presença de P. hardmaniana, exercendo importante papel no suprimento de nutrientes. Salienta-se que a espécie não é citada em águas argentinas e uruguaias e assim, sugerimos que o extremo sul do Brasil representa aproximadamente o limite sul da distribuição geográfica de Palmerina hardmaniana no Oceano Atlântico Sul Ocidental