Microbial Synthesis of Detritus-Like Particulates from Dissolved Organic Carbon Released by Tropical Seagrasses

Abstract

Dried leaves of Thalassia testudinum and Syringodiurn filiforrne released 12.6 % and 19.4 %, respectively, of their organic carbon as dissolved organic carbon (DOC) dunng 3 d of axenic leaching. When inoculated with microbes, the DOC was rapidly converted to bacterial aggregates of a size that could be ingested by macroconsumers. Large populations of ciliates and flagellates also developed, presumably feeding on the unaggregated bacteria. In treatments containing the residual macroparticulate organic carbon (MPOC), 75-95 % of the bacteria present were attached to the leaves, and suspended aggregates were not observed. The protozoan community was dominated by freeswimming flagellates that grazed on the suspended bacteria and were thus responsible for the absence of those forms. Total microbial populations in the DOC treatments were 10-12 times greater per unit of seagrass carbon originally added than in the MPOC containing flasks. These results show that seagrass DOC may rapidly be made available to higher consumers through processes that produce suitable sized food particles-viz., growth of protozoans on bacteria and formation of bacterial aggregates.

Production, decomposition, and heterotrophic utilization of the seagrass Halophila decipiens in a submarine canyon

“…Bacteria have responded similarly to exudation of organic substrate during plant metabolism or the initial stages of decay when dissolved organic matter was leached (Harrison & Mann 1975, Moriarty & Pollard 1982, Robertson et al 1982, Kenworthy & Thayer 1984, Kirchman et al 1984, Moriarty et al 1985b, Moriarty et al 1986. The large bacterial cell sizes we observed, especially for the buried H. decipiens are indicative of elevated concentrati.ons of dissolved organics, nutrient-enriched environments, high metabolic activity, and rapid growth (Donachie et al 1976, Pierucci 1978, Hagstrom et al 1984, Palumbo et al 1984.…”

“…The maximum densities of cells attained in each litterbag treatment, i.e. 1.65 X 10" (g AFDW)-' in the surface bags and 2.12 X l o l ' (g AFDW)-I in the buried bags, exceeded the abundance of bacteria on the standing stock of H. decipiens (5.29 to 7.74 X 10" by about 2 orders of magnitude and are greater than or equal to bacterial cell densi.ties reported for detritus of other seagrass (Fenchel 1970, Newel1 1981, Robertson et al 1982, Rublee & Roman 1982. Cell volume in both surface and buried bags also was larger than cell volumes reported for bacteria attached to Zostera marina leaves, the roots and rhizomes of temperate and tropical seagrasses, suspended particles, and sediments (Newell 1981, Rublee 1982, Kirchman 1983.…”

“…One of several proposed explanations for the relatively large faunal biomass and high secondary produc-O Inter-Research/Printed in F. R. Germany tivity of seagrass systems is that the plant material becomes nutritionally available to consumers after undergoing decomposition to either morphous particulate organic detritus or amorphous detrital aggregates (Odum & de la Cruz 1967;Odurn et al 1971, Robertson et al 1982. Detritus is defined as dead organic matter in various stages of decay with its associated rnicroorganisms, and is formed from complex physical and chemical interactions mediated by the environmental conditions of the sediment or water column a s well a s the metabolism of micro-and macroorganisms closely associated with detritus (Odum et al 1971).…”

“…Since detntivores reportedly assimilate the plant fraction of detritus with efficiencies less than 5.0 % , but may assimilate microflora at efficiencies exceeding 50.0 O h (Hargrave 1976, Yingst 1976, Lopez et al 1977, Cammen 1980, Bianchi & Levinton 1984, it has been argued that a portion of a detritivore's nutritional requirements are met by the microbial component of detritus (for review see Lopez & Levinton 1987). Hence, bacteria function in mediating the recycling of nutrients and may be important in regulating the flow of energy from seagrass detntus to consumer organisms (Fenchel 1977, Klug 1980, Robertson et al 1982.…”

“…In subtidal sediments bacteria and bacterial consumers are an important intermediary in the transformation of organic matter to organic detritus (Fenchel 1977, Klug 1980. Bacteria are capable of metabolizing dissolved organic compounds released from living and detrital seagrass (Moriarty & Pollard 1982, Robertson et al 1982, Kenworthy & Thayer 1984, Moriarty et al 1986). Bacteria also have enzymes for degrading refractory particulate organic compounds and are capable of altering the physical composition of detntus (Klug 1980).…”