Influence of organic compounds and light limitation on the growth rate of estuarine benthic diatoms

Abstract

Ten species of benthic diatoms from the Eems-Dollard estuary were grown in axenic cultures under various combinations of irradiance and supply of organic substrates.Six species were capable of growth in the dark on yeast extract, casamino acids, or glucose.Four of these species grew best in the presence of glucose, whereas the growth of the other two species was supported only by yeast extract and casamino acids.The light limited growth rate of only those species that were also capable of heterotrophic growth in the dark was increased by organic substrates.The rate of this "mixed" growth together with the absence of a lag-phase upon change from autotrophic to heterotrophic conditions indicates the nutritional versatility of these diatom species.A positive relation between the organic matter content of the natural habitat and the heterotrophic capacities of the diatom species is suggested.All species with heterotrophic capacities were isolated from muddy sediments, whereas two species isolated from a sandflat seem to be obligately autotrophic.Also two species from muddy sediments apparently had no heterotrophic capacities.The cells of the six species with heterotrophic capacities differed from those of the four species without such capacities in their higher surface to volume ratio.The present study was carried out as part of a series of experimental investigations on the relations between benthic diatoms from the Eems-Dollard estuary and their physico-chemical environment.Tbie microphytobenthos of the Eems-Dollard estuary, a part of the Dutch-German Wadden Sea, is dominated by diatoms that inhabit the large intertidal sand-and mudflats.The sediments of these flats have a rather high content of organic matter (up to 20 mg/g dry sediment, Schr6der et al., 1976), especially in the more sheltered inner part of the estuary.Large amounts of organic wastewater are discharged at a few places in the estuary, causing organic enrichment of certain areas, in which the diatoms are abundant (Fig. 1).Since the benthic diatoms live in intimate contact with these sediments, the organic carbon in the sediment may be of nutritive value for the diatoms.Moreover the surface of the sediments are flushed with very turbid tidal waters that considerably reduce the effective photoperiod for the benthic diatoms.In a previous paper (Admiraal, 1977a) it was shown that short daylengths strongly reduce the growth rate of benthic diatoms in culture.In the same paper it was suggested that the autotrophic growth rate of the diatoms on the intertidal flats is limited by the low daily irradiances reaching the sediment surface, especially in winter when daily photoperiods of only a few hours can occur.Furthermore a considerable part of the benthic diatom population is found