Silicon availability and cell-cycle progression in marine diatoms

Abstract

The role of silicon availability on cell-cycle progression in marine diatoms was examined using flow cytometric methods. Silicon deprivation halted the progression of cells through the cell cycle with cells arresting in G1. G2 and M in 6 of 7 species examined (5 centric and 1 pennate species). The exception, Phaeodactylurn tricornutum, did not require silicon for growth and did not have a silicondependent segment within its cell cycle. T h s species was also the only one lacking a light-dependent arrest point late in its cell cycle suggesting that the arrest of diatom cells in G2 and M in the dark is related to silicon metabolism. Chaetoceros spp. were unique in that they had 2 silicon-dependent segments In G1: one at the GI/S boundary apparently associated with a silicon requirement for DNA synthesis and a second earher In G1 associated with the d e p o s ~ t ~ o n of siliceous setae. Silicon limitation of Thalaseoslra weissflogii led to an increase in the duratlon of G2 with the duration of G1, S and M remaining as observed under nutrient-replete conditions. In severely llmited cells, G2 comprised 82 % of the cell cycle and lasted for over 2 d , hlore complicated responses were observed for Cylindrotheca fusiformis and Chaetocel-OS simplex. Modest silicon limitation of C. fuslf01-1111s led to increases in the duration of G2 and posslbly M. More severe silicon stress did not lengthen M further, but both G2 and G1 increased in duratlon. For C. simplex, modest silicon l~mitation led to the expansion of G1 alone, while more severe limitation lengthened G1, G2 and M. Changes in cell cycle durations in this species appeared related to a decline in the silicon content of siliceous setae deposited during G1 These results corroborate past observations that silicon metabolism is linked to specific segments of the cell cycle, but indicate that these regions can lengthen dramatically in response to silicon limitation.