First account of apochlorotic diatoms from mangrove waters in Florida.

From January to December 2004, monthly samples were collected from the mangrove salt marsh of J.U. Lloyd State Park, Florida, to examine the spatial and temporal distributions of heterotrophic diatoms in this subtropical region. Four non-pigmented (i.e. apochlorotic), heterotrophic diatom morphotypes were isolated from mangrove neuston and plankton sites. Colorless diatoms were numerically significant in neuston during the fall and winter months and counts peaked at 174 x 10³ cells/L in December. Heterotrophic diatoms were most prevalent in mangrove plankton in October and averaged 55.5 x 10³ cells/L. Valve morphology was detailed for mangrove morphotypes (I, II, V, and VI). However, only one type (I) was named with confidence (Nitzschia leucosigma). The other three types failed to conform closely to published diagnostic features suggesting that either characters are more variable than accepted or these types were new species. Growth responses to changes in salinity and irradiance were detailed in the laboratory. All morphotypes were euryhaline, tolerating salinities between 10% and 40%; N. leucosigma was most salt tolerant and grew from 10% to 60%. Maximum growth was achieved between 20% and 40%. Growth was not significantly different for heterotrophic diatoms during light and dark treatments or for diatoms grown with or without bacteria.