Acanthoceras H. Honigmann E Urosolenia (F.E. Round & R. M. Crawford) E. Rott, H. Kling & J. Mcgregor (Coscinodiscophyceae, Bacillariophyta) das regiões centro-oeste e norte do Brasil

Abstract

Acanthoceras H. Honigmann and Urosolenia (F.E. Round & R. M. Crawford) E. Rott, H. Kling & J. Mcgregor (Coscinodiscophyceae, Bacillariophyta) are planktonic genera with cylindrical frustules, and conical valves. The small size and slightly silicified frustules make these genera difficult to study by light microscopy analysis. Diacritical characteristics are base on ultrastructure of the frustules. The ornamentation of valve surface is very important to the taxonomy of the group. In Brazil few taxonomic and morphological studies have been developed based on these genera. This work aimed to study the ultrastructural characterization of these genera and to inventory them. The study area consists of floodplain environments on two Basin (Amazon Basin – Amazonas and Pará States and Tocantins-Araguaia Basin – Goiás and Tocantins States). We analyzed 454 samples (Amazon Basin n=182; Tocantins-Araguaia n= 272) in Zeis Axioscop 40 microscope, equipped with image capture Axiovision system. The ultrastructure was analyzed in Jeol JSM 6610 scanning electron microscope. Five taxa of Urosolenia were found and studied according to the morphology (U. amazonica, U. delicatissima, U. eriensis var. morsa, U. longiseta e Urosolenia sp1). This genus was registered in 156 samples. A. zachariasii (Brun) Simonsen was the only taxon of the genus Acanthoceras. Details of ultrastructure of all taxa were presented in this study. Acanthoceras occurred in 14 samples. Both genera were present in 85% of samples from Tocantins-Araguaia Basin. In general, representatives of Urosolenia were found in oligotrophic environments with low pH, while Acanthoceras occurred in alkaline and highly transparent environments, from mesotrophic to eutrophic conditions. Urosolenia was widely distributed along the study area. U. amazonica and U. delicatissima were first records for Brazil.