Microbial Secondary Production from Salt Marsh-Grass Shoots, and Its Known and Potential Fates

Abstract

Several lines of evidence (direct microscopy, index biochemicals) point to predominance of eukaryotic decomposers in natural decay of dead shoots of smooth cordgrass (Spartina alterniflora). Recent research shows that this is also true for black needlerush (Juncus roemerianus). Ascomycetous fungi are the major initial secondary producers based on the dead shoots. There is no overlap between the species of the cordgrass (e.g., Phaeosphaeria spartinicola) and needlerush (e.g., Loratospora aestuarii) fungal-decay communities. Even when conditions in the marsh are manipulated in directions that would be expected to favor prokaryotes (extra water and nitrogen), the ascomycetes accumulate maximum organic masses in standing-decaying shoots hundreds of times larger than prokaryotic masses. Rates of fungal production are not increased by raising duration of high water availability, probably due to fine-tuned fungal adaptation to periodic dryness, but nitrogen does limit fungal productivity in decaying cordgrass. Content of living-fungal mass can be 10 to 20% of total system (= microbes + remaining plant) mass, depending on nitrogen availability, rates of invertebrate mycophagy, and probably several further factors yet to be determined. Standing crops of living fungi in cordgrass marshes in Georgia (per-m(2) basis) have been calculated to be equal to 3% (summer) to 28% (winter) of living-cordgrass standing crop. This is calculated to be about 50 to 100% of total (non-cyano) bacterial crop; the great bulk of bacterial crop is sedimentary. Fungal productivity per m(2) standing-decaying-cordgrass marsh has been provisionally found to be 10 times greater in winter than in summer (3652 mg per m(2) per day; mu = 0.07 day(-1)). Total bacterial productivity per m2 was calculated to be about x2 fungal in summer, and x0.07 fungal in winter. High yields of fungi (on the order of 50%) from cordgrass shoots may be part of the explanation for high rates of animal secondary production in saltmarsh ecosystems. Cordgrass-fungal standing crops and productivities (per unit leaf mass) do not show pronounced variation (in autumn) along a south-north latitudinal gradient from 30 degrees to 44 degreesN. One major known fate of saltmarsh-fungal secondary production is output to shredder gastropods (periwinkles, Littoraria irrorata). Other potential substantial fluxes are to amphipods (especially Uhlorchestia spartinophila) and other gastropods (especially Melampus bidentatus), and fluxes as sexual propagules (ascospores) and as remnant hyphal wall/sheath mass in fallen, decayed fragments, Key opportunities for saltmarsh-ecological research lie: in learning the details of the life histories of the more important saltmarsh-fungal producers; in determining the biotic and abiotic controls on saltmarshfungal productivity; and in investigations of impacts of fungal activities, such as the probable role that saltmarsh ascomycetes have in release of dimethylsulfide to the atmosphere.