Persistent, vertical-migration rhythms in benthic microflora: I. The effect of light and temperature on the rhythmic behaviour of Euglena obtusa

Abstract

During daytime low tides on the River Avon at Bristol, England, the exposed river banks become a deep green colour owing to the presence of enormous numbers of Euglena obtusa which emerge out of the black mud. Cell densities on the surface surpass 105 cells/cm2. Before the tide returns to cover the area, the cells re-burrow back into the mud and remain there during high tide and throughout the night.The cells can be prevented from emerging on to the surface mud at low tide by artificially darkening the area with an opaque covering just as the tide recedes. Cells which are already on the surface can be made to re-burrow by similarly placing them in darkness.The vertical-migration rhythm will persist in the laboratory in constant illumi-nation, constant temperature, and away from the influence of the tide, for nearly one month. In these conditions the rhythm is diurnal, rather than tidal. The rhythm will not persist in constant darkness.Because of the excessive turbidity of Avon water, each high tide imposes a period of darkness on the surface mud. It is thought that these dark periods transform the fundamental diurnal rhythm into one of tidal frequency.Various intensities of constant illumination alter the form and amplitude of the rhythm, but not the period. The stable nature of the period, under different light intensities, is thought to be due to a unique, self-stabilizing feature inherent in this type of rhythm.The rhythm is inhibited at 2° C. Between 5° and 15° C, the period of the rhythm is virtually unaltered; it remains approximately 24 h in length.

Discovery of microphytobenthos migration in the subtidal zone

“…As tidal waters recede, microalgae quickly migrate to the sediment surface where they receive the maximum incident light energy available. This endogenous migratory rhythm is maintained when external physical synchronisers are removed (Palmer & Round 1965, Serôdio et al 1997. The migration results in the formation of a biofilm, which consists of a dense layer of cells at the mud surface (Paterson & Crawford 1986).…”

Dissolved inorganic nitrogen uptake by intertidal microphytobenthos: nutrient concentrations, light availability and migration

“…Certain aspects of MPB ecology have been comprehensively characterized, e.g. primary production (Underwood & Kromkamp 1999, Serôdio & Catarino 2000, vertical migration in intertidal (Palmer & Round 1965, Underwood et al 2005 and subtidal regions (Ní Longphuirt et al 2006), and their resuspension during tidal flooding (Lucas et al 2000). However, despite the extensive literature available, very little experimentation has been carried out on the direct uptake of dissolved inorganic nutrients by in situ populations (Thornton et al 1999, Leynaert et al 2009), a process which is routinely represented in modelling and nutrient budget studies.…”

“…Added to this vertical change in substrate availability, concentrations of ammonium and nitrate in intertidal sediments can oscillate greatly between submersion and emersion periods (Kuwae et al 2003, Sakamaki et al 2006. The MPB can therefore be subjected to large nutrient concentration fluctuations, firstly through their vertical migration into the sediment (Palmer & Round 1965) and secondly as a result of the influences of tidal cycles. The uptake of DIN by the MPB will in turn oscillate over these migratory and tidal cycles due to the direct influence of substrate concentrations on uptake processes.…”

Does the size of the microphytobenthic biofilm on intertidal mudflats depend on the available photosynthetic biomass?

“…This basic pattern of vertical migration, which periodically transforms part of the PCB into PAB, has been qualitatively described many times by looking at the presence of microalgal cells or biomass at the surface of the sediment (Palmer & Round 1965, 1967, Paterson et al 1986, Serôdio et al 1997, De Brouwer & Stal 2001, Perkins et al 2001, Tolhurst et al 2003, Herlory et al 2004) and has been reviewed recently by Consalvey et al (2004b). However, the quantitative and dynamic aspects of the relationship between PCB and PAB have so far been overlooked.…”