Metaproteomics of marine viral concentrates reveals key viral populations and abundant periplasmic proteins in the oligotrophic deep chlorophyll maximum of the South China Sea.

Viral concentrates (VCs), containing bioinformative DNA and proteins, have been used to study viral diversity, viral metagenomics and virus-host interactions in natural ecosystems. Besides viruses, VCs also contain many noncellular biological components including diverse functional proteins. Here, we used a shotgun proteomic approach to characterize the proteins of VCs collected from the oligotrophic deep chlorophyll maximum (DCM) of the South China Sea. Proteins of viruses infecting picophytoplankton, that is, cyanobacteria and prasinophytes, and heterotrophic bacterioplankton, such as SAR11 and SAR116, dominated the viral proteome. Almost no proteins from RNA viruses or known gene transfer agents were detected, suggesting that they were not abundant at the sampling site. Remarkably, nonviral proteins made up about two thirds of VC proteins, including overwhelmingly abundant periplasmic transporters for nutrient acquisition and proteins for diverse cellular processes, that is, translation, energy metabolism and one carbon metabolism. Interestingly, three 56 kDa selenium-binding proteins putatively involved in peroxide reduction from gammaproteobacteria were abundant in the VCs, suggesting active removal of peroxide compounds at DCM. Our study demonstrated that metaproteomics provides a valuable avenue to explore the diversity and structure of the viral community and also the pivotal biological functions affiliated with microbes in the natural environment.

Quantitative proteomic analysis of cell cycle of the dinoflagellate Prorocentrum donghaiense (Dinophyceae).

Dinoflagellates are the major causative agents of harmful algal blooms in the coastal zone, which has resulted in adverse effects on the marine ecosystem and public health, and has become a global concern. Knowledge of cell cycle regulation in proliferating cells is essential for understanding bloom dynamics, and so this study compared the protein profiles of Prorocentrum donghaiense at different cell cycle phases and identified differentially expressed proteins using 2-D fluorescence difference gel electrophoresis combined with MALDI-TOF-TOF mass spectrometry. The results showed that the synchronized cells of P. donghaiense completed a cell cycle within 24 hours and cell division was phased with the diurnal cycle. Comparison of the protein profiles at four cell cycle phases (G1, S, early and late G2/M) showed that 53 protein spots altered significantly in abundance. Among them, 41 were identified to be involved in a variety of biological processes, e.g. cell cycle and division, RNA metabolism, protein and amino acid metabolism, energy and carbon metabolism, oxidation-reduction processes, and ABC transport. The periodic expression of these proteins was critical to maintain the proper order and function of the cell cycle. This study, to our knowledge, for the first time revealed the major biological processes occurring at different cell cycle phases which provided new insights into the mechanisms regulating the cell cycle and growth of dinoflagellates.

Comparative proteomic analysis reveals proteins putatively involved in toxin biosynthesis in the marine dinoflagellate Alexandrium catenella.

Alexandrium is a neurotoxin-producing dinoflagellate genus resulting in paralytic shellfish poisonings around the world. However, little is known about the toxin biosynthesis mechanism in Alexandrium. This study compared protein profiles of A. catenella collected at different toxin biosynthesis stages (non-toxin synthesis, initial toxin synthesis and toxin synthesizing) coupled with the cell cycle, and identified differentially expressed proteins using 2-DE and MALDI-TOF-TOF mass spectrometry. The results showed that toxin biosynthesis of A. catenella occurred within a defined time frame in the G1 phase of the cell cycle. Proteomic analysis indicated that 102 protein spots altered significantly in abundance (P < 0.05), and 53 proteins were identified using database searching. These proteins were involved in a variety of biological processes, i.e., protein modification and biosynthesis, metabolism, cell division, oxidative stress, transport, signal transduction, and translation. Among them, nine proteins with known functions in paralytic shellfish toxin-producing cyanobacteria, i.e., methionine S-adenosyltransferase, chloroplast ferredoxin-NADP+ reductase, S-adenosylhomocysteinase, adenosylhomocysteinase, ornithine carbamoyltransferase, inorganic pyrophosphatase, sulfotransferase (similar to), alcohol dehydrogenase and arginine deiminase, varied significantly at different toxin biosynthesis stages and formed an interaction network, indicating that they might be involved in toxin biosynthesis in A. catenella. This study is the first step in the dissection of the behavior of the A. catenella proteome during different toxin biosynthesis stages and provides new insights into toxin biosynthesis in dinoflagellates.

Quantitative proteomic analysis of differentially expressed proteins in the toxicity-lost mutant of Alexandrium catenella (Dinophyceae) in the exponential phase.

Alexandrium catenella is a widely spread dinoflagellate species which can produce potent neurotoxins and result in paralytic shellfish poisoning. To date, the molecular mechanisms regulating toxin biosynthesis remain unclear. This study compared protein profiles of a toxicity-lost mutant of A. catenella, ACHK-NT and its wild-type, ACHK-T in the exponential phase, using two dimensional differential gel electrophoresis and MALDI-TOF-TOF mass spectrometry. Morphological analysis showed that both subcultures were morphologically identical with the distinctive taxonomic characteristics of A. catenella. Sequence analyses of ITS1, 5.8S, ITS2 and 18S demonstrated that the mutant and wild subcultures were genetically identical for these markers. 90 differentially expressed protein spots were identified from ACHK-NT, of which 34 were down-regulated and 56 were up-regulated. Using a multilayered strategy for de novo protein sequence analysis, 67 proteins assigned to different functional categories were identified. Among them, 25 involved in bioluminescence, secondary metabolite biosynthesis, protein modification and toxin biosynthesis were down-regulated, while 42 participating in carbon fixation, stress response, transporter and protein folding were up-regulated. This study indicated that the strengthening of certain biological processes coupled with the depression of essential reactions upstream or downstream of the toxin biosynthetic pathway might have blocked toxin production and resulted in the loss of toxicity in the mutant A. catenella.

The role of cytochrome P4501A activity inhibition in three- to five-ringed polycyclic aromatic hydrocarbons embryotoxicity of marine medaka (Oryzias melastigma).

The mode of action of PAHs that causes fish developmental malformations is unclear. The embryotoxicity of marine medaka (Oryzias melastigma) was investigated after individual exposure to three- to five-ring PAHs Phe, Py, and BaP or co-exposure with α-ANF for 18 days. We found that the relationships between EROD induction and developmental deformities of embryos showed a various pattern under different exposure scenarios of Phe, Py, and BaP, which suggested possibly different modes of action in determining the developmental toxicities. As for co-exposure scenarios of each PAH combined with ANF, it showed potentially synergistic effects. The inhibited CYP1A mediated enzyme activity by ANF after co-exposure did not effectively alleviate developmental toxicity of embryo. It showed potentially synergistic effects after co-exposure of marine fish embryos to CYP1A inhibitors and PAH-type CYP1A inducers. Heart deformities in the early life stages of marine medaka were recommended as a biomarker for indicating the extent of PAH pollution.

Seasonal variation for the ratio of BaP to BeP at different sites in Great Xiamen Bay.

From March 2008 to February 2009, PM(10) samples were collected and analyzed for polycyclic aromatic hydrocarbons (PAHs) at eight sampling sites in Great Xiamen Bay, China. Analyses of the seasonal and spatial variations of these compounds revealed the following results. Significantly high levels of PAHs were found in the winter compared to the summer, sometimes exceeding 100 ng m(-3), and the spatial variations were influenced most by the sampling site surroundings. Composition profiles of PAHs of an urban and a rural site were shown to be very similar with a positive correlation coefficient larger than 0.9 at the 0.01 level of significance for the same season. Diagnostic ratios, together with principal component and multiple linear regression analysis, showed that more PAHs were from grass/wood/coal combustion in winter than in other seasons. The ratios of benzo[a]pyrene to benzo[e]pyrene (BaP-BeP) in winter and fall were 0.6-1.7 times higher than those in spring and summer, suggesting the importance of local emissions of PAHs. The BaP-BeP ratios in Kinmen were generally lower than those in Xiamen, indicating that the aging degree of PAHs was higher in Kinmen than in Xiamen. The external input of PAHs from upwind urban and industrial areas was one of the key factors causing high levels of PAHs in PM(10) in Great Xiamen Bay in winter.

Quantitative proteomic analysis of okadaic acid treated mouse small intestines reveals differentially expressed proteins involved in diarrhetic shellfish poisoning.

Okadaic acid (OA) is a principal diarrhetic shellfish poisoning toxin produced by marine dinoflagellates. This study compared protein profiles of mice small intestines at four time points (0, 3, 6 and 24 h) after a single oral administration of 750 µg/kg OA, and identified the differentially expressed proteins using 2-D DIGE and MALDI-TOF-TOF mass spectrometry. The results showed that the toxin content of the intestines reached its peak 3h after oral administration and then decreased rapidly. OA remarkably inhibited the intestinal PP activity but it recovered to the normal levels within 6 to 24 h. Electron microscope revealed the collapse of the villous architecture and the intestinal microvilli fell off at 3 h, but were repaired within 24h. Notable damage to the intestinal ultrastructure was observed after oral administration. Comparison of the small intestine protein profiles at four time points revealed that 58 proteins were remarkably altered in abundance, and these proteins were involved in macromolecular metabolism, cytoskeleton reorganization, signal transduction, molecular chaperoning and oxidative stress, suggesting that OA toxicity in mouse intestines was complex and diverse, and that multiple proteins other than PP were involved in the diarrhetic process. Villin 1 and hnRNP F might be the key triggers inducing diarrhea in the mouse small intestines.

Homology-Driven Proteomics of Dinoflagellates with Unsequenced Genomes Using MALDI-TOF/TOF and Automated De Novo Sequencing.

This study developed a multilayered, gel-based, and underivatized strategy for de novo protein sequence analysis of unsequenced dinoflagellates using a MALDI-TOF/TOF mass spectrometer with the assistance of DeNovo Explorer software. MASCOT was applied as the first layer screen to identify either known or unknown proteins sharing identical peptides presented in a database. Once the confident identifications were removed after searching against the NCBInr database, the remainder was searched against the dinoflagellate expressed sequence tag database. In the last layer, those borderline and nonconfident hits were further subjected to de novo interpretation using DeNovo Explorer software. The de novo sequences passing a reliability filter were subsequently submitted to nonredundant MS-BLAST search. Using this layer identification method, 216 protein spots representing 158 unique proteins out of 220 selected protein spots from Alexandrium tamarense, a dinoflagellate with unsequenced genome, were confidently or tentatively identified by database searching. These proteins were involved in various intracellular physiological activities. This study is the first effort to develop a completely automated approach to identify proteins from unsequenced dinoflagellate databases and establishes a preliminary protein database for various physiological studies of dinoflagellates in the future.

Identification and Characterization of Cell Wall Proteins of a Toxic Dinoflagellate Alexandrium catenella Using 2-D DIGE and MALDI TOF-TOF Mass Spectrometry.

The cell wall is an important subcellular component of dinoflagellate cells with regard to various aspects of cell surface-associated ecophysiology, but the full range of cell wall proteins (CWPs) and their functions remain to be elucidated. This study identified and characterized CWPs of a toxic dinoflagellate, Alexandrium catenella, using a combination of 2D fluorescence difference gel electrophoresis (DIGE) and MALDI TOF-TOF mass spectrometry approaches. Using sequential extraction and temperature shock methods, sequentially extracted CWPs and protoplast proteins, respectively, were separated from A. catenella. From the comparison between sequentially extracted CWPs labeled with Cy3 and protoplast proteins labeled with Cy5, 120 CWPs were confidently identified in the 2D DIGE gel. These proteins gave positive identification of protein orthologues in the protein database using de novo sequence analysis and homology-based search. The majority of the prominent CWPs identified were hypothetical or putative proteins with unknown function or no annotation, while cell wall modification enzymes, cell wall structural proteins, transporter/binding proteins, and signaling and defense proteins were tentatively identified in agreement with the expected role of the extracellular matrix in cell physiology. This work represents the first attempt to investigate dinoflagellate CWPs and provides a potential tool for future comprehensive characterization of dinoflagellate CWPs and elucidation of their physiological functions.

[Parallel factor analysis for excitation emission matrix fluorescence spectroscopy of dissolved organic matter from a reservoir-type river].

The fluorescent components and their distribution and variation of dissolved organic matter (DOM) were examined using excitation emission matrix fluorescence spectroscopy-parallel factor analysis technique (EEMs-PARAFAC) for samples collected during June, 2008 from Minjiang River, a typical subtropical reservoir-type river ecosystem. Three separate fluorescent components were identified by PARAFAC, including two dominant humic-like components (C1: < 250, 325/424 nm; C2: 270, 395/482 nm) and one protein-like component (C3: < 250, 280/358 nm), of which humic-like components were dominant. Fluorescence analysis provided a 'fingerprint' technique to trace the mixing of DOM between three tributaries in the upstream of Minjiang River. The nearly constant concentration and composition in the main stream of Minjiang River reflected the effect of dam construction Correlation and principal component analysis (PCA) revealed that humic-like components were principally derived from flushing of watershed soils, while protein-like component was formed from in-situ production which could be used as a proxy of the concentration of total dissolved nitrogen (TDN). Multi-linear regression of fluorescent components C2 and C3 can be used to trace the variation of dissolved organic carbon (DOC) concentration. This study demonstrates that Minjiang is a typical subtropical reservoir-type river which still keeps relatively 'unpolluted' aquatic environment.

[Preliminary study on linking land use & landscape pattern and water quality in the Jiulong River watershed].

Geospatial analysis and statistical analysis were integrated to link land use & landscape pattern and water quality in 2002 and 2007 at the entire watershed and buffer zone scale in the Jiulong River Watershed. Results show that the relationships between land use & landscape pattern and water quality in 2002 and 2007 were basically consistent, namely: (1) Percentage of built-up area was positively correlated with BOD5, NO3(-)-N, NH4(+)-N and permanganate index, and negatively correlated with DO; percentage of woodland area was positively correlated with NO3(-)-N, NH4(+)-N and permanganate index; percentage of cropland area was negatively correlated with NO3(-)-N, NH4(+)-N and permanganate index. (2) SHDI was positively correlated with permanganate index, TP, NH4(+)-N, and negatively correlated with DO at the entire watershed and buffer scale; LPI was negatively correlated with BOD5, permanganate index, TP and NH4(+)-N, and positively correlated with DO at the entire watershed and buffer zone scale; PD was positively correlated with BOD5, TP and NH4(+)-N; Most of the landscape pattern metrics was not the good predictors for water quality in study watershed. (3) Water quality parameters in buffer zone area have more significant correlations with percentage of land use type areas and landscape pattern metrics, because most water quality parameters in the buffer zone can be better explained with greater adjusted coefficient of determination (Adjusted R2). (4) Compared to landscape pattern metrics, percentage of land use type area can predict water quality better because most water quality parameters have more stable correlations.

Occurrence of polycyclic aromatic hydrocarbons (PAHs) in seawater from the Western Taiwan Strait, China.

Seawater samples (including surface water and bottom water) were collected from the Western Taiwan Strait (WTS) during June 24-25, 2009; polycyclic aromatic hydrocarbons (PAHs) in dissolved phase and particulate phase were analyzed, respectively. The results showed that the total concentrations of PAHs in the dissolved phase and particulate phase were ranged from 12.3 to 58.0 ng L(-1), and 10.3-45.5 ng L(-1), which showed a low-middle contamination level in the China Seas. The spatial variability of PAHs may be related to the complicated currents of WTS, especially the Min-Zhe coastal current. PAHs diagnostic ratios suggested that PAHs mainly originated from the inputs of pyrolytic (combustion) sources, which might be contributed to land-based atmospheric deposition. The particle-water partition coefficients of individual PAH showed that partitions were not correlated with suspended particulate matter content, dissolved organic carbon or salinity, similar to the Yangtze coastal area.

[Study on the linkage between urban built-up land and water quality in the Jiulong River watershed].

Band grouping indices combined with single band characteristic were used to extract urban built-up land based on satellite image in the Jiulong River Watershed. Landscape ecology method and statistical analysis were employed to explore the relationship between urban built-up land and permanganate index, NH4(+) -N, TP concentrations. There were significantly positive correlations between the proportion of urban built-up land and permanganate index, NH4(+) -N, TP (r = 0.701, 0.695, 0.789). It indicates the proportion of urban built-up land areas in the sub-watershed could be an effective indicator of water quality. The largest patch index (LPI) was positively correlated to permanganate index, NH4(+) -N, TP concentrations in the water (r = 0.555, 0.643, 0.722). The landscape shape index(LSI) was positively correlated to permanganate index and TP concentrations in the water (r = 0.564, 0.553). These means the impacts of urban built-up land on water quality are influenced not only by urban built-up land areas but also by spatial patterns. The seasonally linear correlation results show that water quality deteriorates quickly with urban built-up land during the flood season and dry season, and the water is susceptible to eutrophication in both flood and dry seasons. The water quality in most sub-watersheds are impacted by urban built-up land, while the urban built-up land areas of Longmen stream, Su stream and Xiao stream located in headstreams are intensive, which need to be adjusted and controlled to protect the water quality.

[Preliminary results concerning summer-time denitrification in the Jiulong River Estuary].

Denitrification is an important process mitigating nitrogen (N) pollution in aquatic systems. Water samples in 13 sites throughout the Jiulong River Estuary were collected in July, 2010 in a preliminary investigation of the denitrification rate in this area. As end-products of denitrification, dissolved N2 was measured by determining N2 : Ar ratios using MIMS (HPR-40), while the concentration of nitrous oxide (N2O) dissolved in water was determined by Purge and Trap-Gas Chromatography. The results showed significant spatial variance of net increase of dissolved N2 (ranging between - 9.9 and 66.8 micromol x L(-1)) and N2O (ranging between 4.3 and 31.5 nmol x L(-1)) in the Jiulong River Estuary. The net increase of dissolved N2 and N2O declined gradually from river sites to sea sites. Dissolved N2O was supersaturated by 170%-562%. The air-water fluxes of N2 ranged between -2.9 and 53.2 mmol x (m2 x d) (-1), and N2O between 5.2 and 23.9 micromol x (m2 x d)(-1). The N2O yield shared only 0.03% - 1.2% (average 0.25%) of total N air-water flux. The results suggested that water temperature and nutrient (N and P) were the key factors influencing denitrification. The denitrification rate is controlled by nitrate level at fresh-water sites with salinity < 0.5%. However, in salty waters, net increase in N2 and N2O mainly originated from denitrification occurring upstream of the estuary, and was dominated by the salinity gradient due to tidal mixing.

[Resolving excitation emission matrix spectroscopy of estuarine CDOM with parallel factor analysis and its application in organic pollution monitoring].

The distribution and estuarine behavior of fluorescent components of chromophoric dissolved organic matter (CDOM) from Jiulong Estuary were determined by fluorescence excitation emission matrix spectroscopy (EEMs) combined with parallel factor analysis (PARAFAC). The feasibility of these components as tracers for organic pollution in estuarine environments was also evaluated. Four separate fluorescent components were identified by PARAFAC, including three humic-like components (C1: 240, 310/382 nm; C2: 230, 250, 340/422 nm; C4: 260, 390/482 nm) and one protein-like components (C3: 225, 275/342 nm). These results indicated that UV humic-like peak A area designated by traditional "peak-picking method" was not a single peak but actually a combination of several fluorescent components, and it also had inherent links to so-called marine humic-like peak M or terrestrial humic-like peak C. Component C2 which include peak M decreased with increase of salinity in Jiulong Estuary, demonstrating that peak M can not be thought as the specific indicator of the "marine" humic-like component. Two humic-like components C1 and C2 showed additional behavior in the turbidity maximum region (salinity < 6) and then conservative mixing behavior for the rest estuarine region, while humic-like components C4 showed conservative mixing behavior for the whole estuarine region. However, the protein-like component C3 showed nonconservative mixing behavior, suggesting it had autochthonous estuarine origin. EEMs-PARAFAC can provide fluorescent fingerprint to differentiate the DOM features for three tributaries of Jiulong River. The observed linear relationships between humic-like components and absorption coefficient a (280) with chemical oxygen demand (COD) and biological oxygen demand (BOD5) suggest that the optical properties of CDOM may provide a fast in-situ way to monitor the variation of the degree of organic pollution in estuarine environments.

[Occurrence and distribution of organotin compounds in Thais clavigera from Xiamen coast].

Occurrence and distribution of 6 organotin compounds including butyltin and phenyltin species were detected in Thais clavigera which were collected from 9 coastal areas sites around Xiamen Coast, by pentylized derivatization, GC-FPD. Results indicated that all Thais clavigera samples were contaminated with organotin compounds. The concentrations in Thais clavigera soft bodies varied from 0.3 to 70.6 ng x g(-1) with a mean value of 28.8 ng x g(-1) for butyltin compounds, and from nd to 18.8 ng x g(-1) with a mean value of 7.9 ng x g(-1) for phenyltin compounds, respectively. MBT and TPhT were high levels in butyltin compounds and phentyltin compounds, respectively. In addition, butyltin compounds were the dominant contaminates in all samples with high percentage from 74.3% to 96.8%. There was a significant correlation between TBT and TPhT (R2=0.7109, p<0.01). This result showed that both TBT and TPhT came from antifouling paints for ships or for mariculture nets. Compared with those data reported from the other regions around southeast coast of China, present study reveals that contaminated level of organotin compounds in Thais clavigera are relatively lower in Xiamen Coast. But it is higher than those in 2002.

[Over one hundred year sedimentary record of polycyclic aromatic hydrocarbons in the Andaman Sea, Malaysia].

Polycyclic aromatic hydrocarbons (PAHs) in a sediment core collected from Langkawi Island of the Andaman Sea, Malaysia were determined by GC/MS, the vertical variations of concentration and distributions of PAHs were investigated. In combining with 210Pb-dating, the PAHs sedimentary record in the last 100 years was reconstructed and their possible sources were also discussed. The sigmaPAH concentration ranged from 13.2-60.1 ng x g(-1) in the whole sedimentary section (0-56 cm) with the dominant compounds of phenanthrene, naphthalene and perylene. The sediments contaminated to a lesser extent comparing with the surrounding waters. Before the 1920s, the concentrations of PAHs were considered to be the background level, which was implied from the natural inputs. The historical records of PAHs in the core showed that two distinct peaks which represented the input time of 1960s and 1980s, respectively, inferred that there were some relatively dramatically land-based inputs, and human activities leaded a clear impact to these waters during these periods. Furthermore, PAHs diagnostic ratios indicated that PAHs in the core sediments were mainly of pyrolytic origin (combustion), accompanied with minor petroleum origin. These were related with agriculture, industry, ocean import and export, and shipping activities in the surrounding regions. Meanwhile as the vital communication line, the marine transportation of the Strait of Malacca had influenced the environmental quality of the Andaman Sea. Meanwhile, based on the sedimentary record, PAHs concentrations were found to correlate positively with humanism activities and socioeconomic development (Gross Domestic Production) in the surrounding regions.

[Effect of surfactant on the volatilization of organochlorine pesticides from still artificial seawater].

The effect of sodium dodecylbenzene sulfonate (SDBS) on the volatilization of 17 organochlorine pesticides (OCPs) from still artificial seawater has been examined. The results show that SDBS at low concentration can retard the volatilization of OCPs compared to "clean" surface. The retarding influence is more pronounced for high level of SDBS than it is for low level of SDBS. A good positive correlation between the gas-seawater partition coefficients and the measured volatilization rates of OCPs is observed. Although the SDBS films present no significant, direct resistance to transfer, the films can absorb more OCPs from the above atmosphere with respect to the "clean" surface and reduce the net volatilization flux from the still artificial seawater. The effect of the film is more pronounced for volatilization of more volatile OCPs than it is for less volatile OCPs.

A vast-range speed control microchip for retention of all cell types.

This Technical Note is the first description of a large-scale logarithmic flow-rate damping system designed to retain cells of different adherence, different suspensibility and different motility. The chamber, which can easily retain and cultivate many types of cells, including high-motility cells and swimming cells, via a series of "speed bumps", readily facilitates cell retention for complex heterogeneous cultures. Yeast cells, red blood cells, rabbit bone marrow aspirate and dinoflagellate swimming cells were introduced into the chip for multi-cell retention, multi-cell culture and observation. Here, we show that the chamber creates a flow field with a ratio of end/start speeds as low as 0.01. The logarithmic distribution of flow-rate within the chamber is controlled precisely by pressure, all of the cell types that we tested were retained easily within the chamber. Many cell-cell interactions were observed, predicting a high potential for the success of on-chip heterogeneous cell experiments.

Influence of marine phytoplankton, transition metals and sunlight on the species distribution of chromium in surface seawater.

The photoreduction of Cr(VI) to Cr(III) by marine phytoplankton (diatoms, red and green algae), with or without the presence of transition metals (Fe(III), Cu(II) and Mn(II)) was studied. The direct influence of marine phytoplankton on the photochemical reduction of Cr(VI) was confirmed for the first time, and two kinds of mechanisms were suggested to be responsible for the species transformation: (a) Cr(VI) in excited state could be reduced by the electron donor in its ground state via photo produced electrons; and (b) the solvated electrons reduce the CrO(4)(2-) anions in their ground state. The conversion ratio of Cr(VI) to Cr(III) increased with increasing algae concentration and irradiation time. Different species of marine phytoplankton were found to have different photo-reducing abilities. The photochemical redox of transition metals could induce the species transformation of chromium. After photoreduction by marine phytoplankton and transition metals, the ratio of Cr(VI) to Cr(III) was in the range of 1.45-2.16 for five green algae (Tetraselmis levis, Chlorella autotrophica, Dunaliella salina, Nannochloropsis sp., and Tetraselmis subcordiformis), and only 0.48 for Phaeodactylum tricornutum (diatom) and 0.71 for Porphyridium purpureum (red alga). The species distribution of chromium in the sunlit surface seawater was greatly affected by combined effects of marine phytoplankton (main contributor) and transition metals; both synergistic and antagonistic effects were observed. The results provided further insights into the species distribution and the biogeochemical cycle of chromium, and have significant implications for the risk assessment of chromium in the sunlit surface seawater.