Microbial Secondary Production from Saltmarsh-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
persistence 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 10-20% 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-m2 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. Rates of fungal production per m2
standing-decaying-cordgrass marsh have been provisionally found to
be 10 times greater in winter than in summer (3652 mg per
m2 per day; æ = 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
30ø to 44øN. 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
saltmarsh-fungal 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.
Key words: ascomycete ecology; bacterial productivity; fungal
productivity; gastropod shredders; lignocellulose digestion;
saltmarsh; Spartina alterniflora.
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