Effect of salinity on growth, pigmentation, N2 fixation and alkaline phosphatase activity of cultured Trichodesmium sp

Trichodesmium sp. isolated from the Great Barrier Reef lagoon was cultured in artificial seawater media containing a range of salinities. Trichodesmium sp. actively grew over a wide range of salinities (22 to 43 psu) and hence can be classed as euryhaline. Maximum growth occurred with salinities in the range 33 to 37 psu. Chl a content and alkaline phosphatase activity were found to increase with salinity over the range 22 to 43 psu, but the N-2 fixation rate was reduced at salinities below and above the range for maximum growth. Growth in media exhibiting maximum growth was characterised by well-dispersed cultures of filaments, while significant aggregations of filaments formed in other media. It is proposed that the tendency for Trichodesmium filaments to aggregate in media with salinities outside the range for maximum growth is an opportunistic response to a deficiency of cellular nitrogen, which results from the reduced N-2 fixation rates, and the aggregation occurs in order to enhance the uptake of combined N released within the aggregates and/or the N-2 fixation within the aggregates.

Comparison of the particulate organic carbon and permanganate oxidation methods for estimating labile soil organic carbon

Forty-four soils from under native vegetation and a range of management practices following clearing were analysed for ‘labile’ organic carbon (OC) using both the particulate organic carbon (POC) and the 333 mm KmnO4 (MnoxC) methods. Although there was some correlation between the 2 methods, the POC method was more sensitive by about a factor of 2 to rapid loss in OC as a result of management or land-use change. Unlike the POC method, the MnoxC method was insensitive to rapid gains in TOC following establishment of pasture on degraded soil. The MnoxC method was shown to be particularly sensitive to the presence of lignin or lignin-like compounds and therefore is likely to be very sensitive to the nature of the vegetation present at or near the time of sampling and explains the insensitivity of this method to OC gain under pasture. The presence of charcoal is an issue with both techniques, but whereas the charcoal contribution to the POC fraction can be assessed, the MnoxC method cannot distinguish between charcoal and most biomolecules found in soil. Because of these limitations, the MnoxC method should not be applied indiscriminately across different soil types and management practices.