A study of Si deposition synchrony in Rhizosolenia (Bacillariophyceae) mats using a novel Si-32 autoradiographic method

A Si-32 autoradiographic technique using a liquid photographic emulsion was developed for the study of diatom silica deposition in culture or in natur al water samples, The method was used in the Central North Pacific to study silica deposition by diatoms of the genus Rhizosolenia. The species examin ed form centimeter-sized aggregates commonly referred to as mats. The Rhizo solenia mats examined were composed of a matrix of R. fallax Sundstrom chai ns, embedded with chains of larger cells, either R debyana H. Peragallo or R. acuminata H. Peragallo, The autoradiographs revealed distinct rings of l abeled intercalary bands and/or labeled valves. A greater proportion of the frustule of the larger species was labeled during the incubations with Si- 32, implying higher rates of silicification by R. debyana and R. accuminata compared to R. fallax. A quantitative consideration of these differences i n species-specific Si production combined with abundance and surface area e stimates for each species indicates that cells of the larger species carry out the majority of silica production in Rhizosolenia mats, The large cell size (pervalvar axis 240 to 3000 mu m) and elongate frustule morphology of Rhizosolenia cells enabled us to localize the deposition of silica along th e pervalvar axis. Positions of labeled bands along this axis indicate progr ess through the Si deposition cycle, and the results suggest that cell divi sion is phased, with either a bimodal or unimodal age distribution of cells within the cell cycle for all species in a mat. Species-specific doubling times from 25 to 60 h were implied by the mean fractions of frustule that w ere labeled. Si-32 autoradiography revealed unique species-specific differe nces in diel patterns of cell division and silica deposition and has potent ial for studies of Si deposition by other diatom species and assemblages.